Duration: 10/01/2000 to 09/30/2006

Administrative Advisor(s):

NIFA Reps:

Non-Technical Summary

Statement of Issues and Justification

Growers tell us that weed control systems using reduced herbicide inputs are needed to lower production costs and environmental impacts. Crop rotation, cover crops and residue from dead cover crops have demonstrated potential for reducing herbicide inputs while preserving soil quality. Problems with integrating these weed management methods into production systems persist. Reduced tillage with high crop residue systems have demonstrated value for soil conservation. But the crop residue management systems currently in use require high herbicide inputs. The multi-state project proposed here addresses several important questions relating to use of cover crops, reduced tillage and crop rotation for weed management by building on findings of previous NE-92 projects and on other work by project participants.

Effective weed management is and will continue to be an integral component of a profitable, competitive, and sustainable U.S. agricultural sector. Bridges (1992) estimated annual losses from weeds for 46 major commodities in the U.S. to be $4.1 billion in 1991 despite the use of available control strategies. Losses would have amounted to more than $19.6 billion had herbicides not been available for use in these crops. Herbicides have been relatively inexpensive, convenient, and more effective in controlling weeds than most other strategies. However, public concern over the impact of herbicides on human health and the environment has stimulated interest in finding alternative methods of weed management.

A second general problem in agriculture is soil conservation. Croplands in the U.S. loose an average of 17 tons/ha/year of soil (Pimentel et al. 1995). A loss rate of this magnitude potentially reduces corn productivity by 7% over a 20 year period. Lost productivity due to erosion often can not be made up through fertilization because soil water-holding capacity is reduced (Frye et al. 1982). In addition, soil quality may be lost through excessive working of the soil and low organic matter input. Besides these on-farm problems, sediment damage to water bodies from erosion of cropland costs a minimum of $1.0 billion per year in the U.S. (Clark, 1985).

Mechanical weed management in high residue. Despite the conservation advantages of reduced tillage, about 50% of crop land in the Northeast is clean-tilled (<15% residue), and in New York and New England, about 75% of the acreage is clean-tilled (Radke and Honeycutt 1994). Present alternatives are reduced tillage and no-till. Most reduced tillage options, for example, use a field cultivator or heavy disk to retain some surface residue, but still incorporate most of it. This reduces the ability of the residue to protect the soil, and essentially eliminates the possibility of using the residue to suppress weed establishment. No-till methods retain most residue at the surface, but mechanical resistance of the soil is greater without tillage, and this can inhibit crop growth (Bauder et al. 1981, Cox et al. 1990). It also makes planting and crop establishment difficult. For example, Cox et al. (1992) found lower corn stands in no-till relative to moldboard tillage in 7 out of 12 site-years on-farm trials in New York, and lower yield in 5 of 12 site-years. These results parallel findings of Carter and Barnett (1987) in Wisconsin.

Two tillage options developed in the Midwestern U. S. show the potential for overcoming the problems discussed above. These options need to be evaluated in the Northeast. In zone tillage, a 6 to 8 inch strip is tilled into undisturbed ground either prior to planting or with an attachment on the planter. Blade plows and paraplows have wide, shallowly pitched blades or knives that cut off weeds and break the soil surface with minimal soil turning and thus minimal residue incorporation. The use of these tillage options in conjunction with cover crops has received little research attention.

Both types of tillage potentially retain enough crop residue that use of dead mulch from winter cover crops for weed control in spring seeded row crops is possible. This is an important direction for research since most previous work on use of dead mulches for weed management has either been in no-till or with hand applied mulch in high value crops after conventional tillage and planting. Although previous work has shown weed control benefits from dead cover crop residue, usually the amount of residue that can be produced during the winter is not adequate to provide full control, and herbicides have been needed. Approaches that integrate mechanical weed management with mulch need to be explored. These include concentration of the mulch in bands over the row or, alternatively, in the inter-row, and use of high residue cultivators and rotary hoes. These implements have not been extensively tested in the very high residue levels produced by winter cover crops.

Effects of mulch on soil moisture and weed emergence. Many studies have shown that residues from killed cover crops can suppress weeds (Shilling et al. 1985, Dao 1987, Mohler 1991, Teasdale et al. 1991, Dyck and Liebman 1994). Degree of suppression, however, often depends on the type and amount of residue used, the weed species present, and on prevailing environmental conditions. For instance, in field trials by Mohler and Teasdale (1993), several weed species had greater emergence in plots receiving low rates of rye or hairy vetch (Vicia villosa) residue than in plots receiving none, or high rates. Increased emergence at low mulch rates has been observed in subsequent studies. It is most evident during drought periods, and when the weed seeds are on the soil surface. Soil moisture is higher in mulched plots relative to bare controls (Teasdale and Mohler 1993). Thus, small amounts of residues on the soil surface may provide more favorable soil moisture conditions for germination than bare soil, but lack sufficient soil coverage to appreciably reduce emergence of weed seedlings (Hamrick and Lee 1987, Mohler and Teasdale 1993, Buhler et al. 1996).

Some research has investigated the effects of mulch on soil moisture retention (e.g., Bristow et al. 1986, Steiner 1989, Sauer et al. 1996), and several authors have constructed models to relate seedling emergence to soil moisture (Weaver et al. 1988, Forcella 1993, King and Oliver 1994). No work has systematically related the effect of soil moisture modification by mulch to weed emergence. A better understanding of the way in which crop residues affect soil water potential and ultimately emergence patterns for different annual weed species will allow extension of the mulch-weed emergence model of Teasdale and Mohler (2000). Better understanding of when and to what extent mulch suppresses weeds will improve use of mulch for weed management.

Cover crop variety trials. Better understanding of variation in the hardiness, productivity and weed suppressive ability among cover crop cultivars would allow choice of the best cultivars for weed management. Rye is the most commonly used cover crop in the Northeast, but most data on its varietal variation for cover crop use are preliminary. Reberg-Horton et al. (2000) showed that rye cultivars differed in both productivity and weed suppressive ability under field conditions. We intend to repeat their experiment at several locations to determine which varieties of rye are best suited for use in local weed management programs throughout the Northeast.

Subterranean clover is a winter annual legume that develops a dense, intertwined mat of low growing stems. Research associated with the earlier NE-92 project showed that subterranean clover (Trifolium subterraneum L.) has excellent potential to suppress weeds (Enache and Ilnicki 1990, Ilnicki and Enache 1992). However, the varieties that are currently available are not winter hardy at most locations in the Northeast. Breeding work at the USDA/ARS Beltsville Agricultural Research Center is developing winter hardy varieties for weed suppression in the Northeast. Importantly, the broad geographic distribution of researchers associated with this project provides a geographically varied set of locations to test subterranean clover cultivars for use in the Northeast.

Weed management in crop rotation. Crop rotation effects on weeds is an aspect of weed management that has received little research. Notably, the length and type of rotations that are best able to reduce herbicide inputs are unclear. Today, with less crop/market diversity, growers are unable, for economic reasons, to rely on long-term forage rotations for weed reduction. Growers choose between crop rotation and monoculture based on agro-climatic and economic factors. Of these two the latter is the most important. The economic pressures faced by todays producers frequently preclude the presumably ideal rotations that balance legumes, grass sod crops, cereals, and row crops. In particular, land-poor vegetable producers in the northeastern U.S. have few economically viable options but to rotate vegetables with vegetables, using interseeded rye cover crops as their only rotation crop. These intensive production systems lead to wind and water erosion and a depletion of soil organic matter with a resulting loss of tilth. There is a significant need to determine the effects of economically feasible rotations and their impact on weed management strategies in field and vegetable crops.

Related, Current and Previous Work

A substantial body of research has shown that residues from winter cover crops can significantly reduce annual weeds in summer row crops (Crutchfield et al. 1985, Liebl et al. 1992, Wallace and Bellinder 1992, Teasdale et al. 1991, Curran et al. 1994, Masiunas et al. 1995, Creamer et al. 1996a, Smeda and Weller 1996). Effects of residues on weeds are often attributed to allelopathy (Shilling et al. 1985, White et al. 1989), but physical effects are also important (Teasdale and Mohler 1992, 2000). The amount and type of residue substantially affects the degree of weed suppression (Mohler and Teasdale 1993, Wicks et al. 1994, Teasdale and Mohler 2000). Weed emergence is sometimes greater with low rates of residue than with no residue at all (Mohler and Teasdale 1993, Wicks et al. 1994, Buhler et al. 1996) In most studies to date, the amount of residue that could be grown in situ (typically 2-4 Mg/ha) was insufficient to fully control weeds, and herbicides were needed to obtain acceptable weed control and crop yield. However, most studies have used no-till systems, since this is the usual way to obtain high rates of surface residue, and weed populations are likely to behave differently if the seeds are partly incorporated into the soil during tillage (Mohler 1993, Mohler and Galford 1997).

Mechanical weed management in high residue. Most reduced tillage systems bury much of the residue. Studies in the Midwest have shown that a single operation with a disk harrow, chisel plow or sweep implement leaves 22-83%, 17-85% and 28-80% cover of residue on the soil surface (Colvin et al. 1986, Johnson 1988, Hanna et al. 1995, Wagner and Nelson 1995). In these studies the amount of residue remaining after tillage increased with narrower and less twisted chisels, lower crowned sweeps, less acute disk angles, shallower operating depths and lower driving speeds. The lower end of the ranges (average 17 to 28%) listed correspond to the usual way the implements are used in the Northeast. Moreover, multiple operations prior to planting are the norm, with each one burying additional residue. The high end of the range listed above for sweep type implements applies to the wide, low crowned sweeps that will be investigated in the proposed study (Johnson, 1988, Hanna et al. 1995). These should have a similar affect on soil and cover crops to the low-pitched horizontal blades in the undercutter developed by Creamer et al. (1995, 1996a). These investigators found that the undercutter effectively killed the most common cover crops, including rye, hairy vetch and crimson clover without incorporating the residue.

Residue decreases soil temperatures (Swan et al. 1987, Griffith et al. 1988, Teasdale and Mohler 1993), and this can reduce crop growth in the cooler parts of the Northeast. However, this problem can be greatly reduced by clearing a strip over the row at planting (Cox et al. 1990, Kaspar et al. 1990, Hares and Novak 1992). Recent work (Mohler, 2000) indicates that large seeded crops like corn can emerge through very high rates of residue. Effects on soil temperature of relatively bare soil on either side of a narrow band of mulch over the row have not been measured.

Few studies have directly investigated integration of mulch with mechanical weed management. Wallace and Bellinder (1989) planted potatoes in killed rye and later cultivated to hill around the crop. Rye mulch reduced early emergence of redroot pigweed and lambsquarters in a wet year, but even in a dry year when rye increased early emergence of pigweed, weed densities in the reduced tillage/rye system were equivalent to the conventional tillage with herbicides after hilling. Liebman et al. (1995) found that rye mulch plus one or two inter-row cultivations provided adequate weed control in dry bean provided field margins were mowed to reduce input of wind borne weed seeds.

A search of the Current Research Information System (CRIS) database located a few projects on cultivation in reduced tillage systems. Most of these involve members of the NE-92 technical committee, and the research proposed here compliments and extends projects currently underway. A project at Cornell (NYC-125587) is examining zone tillage and inter-row cultivation, but without cover crops. Two others, also at Cornell (NYC-142417 and NYC-142578) involve cultivation and interseeding cover crops after planting. A recently completed project in Ohio (OHO00717-SS) examined cultivation and dead cover crops for weed suppression but apparently in different treatments.

Effects of mulch on soil moisture and weed emergence. Effects on weed emergence of the interaction between mulch and soil moisture have not been systematically investigated. Several studies have demonstrated that crop residues on the soil surface can increase soil moisture levels by increasing infiltration (McVay et al. 1989) and by reducing water losses due to evaporation (Bristow et al. 1986, Teasdale and Mohler 1993). However, Bussihre and Cellier (1994) found that plant residues having a high leaf area index permitted less rainfall to reach the soil surface and thus resulted in lower moisture availability than residues having a lower leaf area index. The few studies that have assessed the effect of residue amounts on weed emergence have only provided indirect evidence that soil moisture availability may play a key role in the weed emergence patterns observed under field conditions (e.g., Dao 1987, Hamrick and Lee 1987, Mohler and Teasdale 1993, Buhler et al. 1996). For example, field trials by Buhler et al. (1996) in Wisconsin demonstrated that Common lambsquarters (Chenopodium album) emergence was greater during a drought period in plots receiving a low rate of maize residues than in plots receiving no residues. However, soil moisture was not measured, and as in all of the studies to date, the postulated increase in germination due to greater soil moisture was confounded with decreasing emergence due to the physical interference from the mulch.

Development of models to predict when weeds will emerge during the growing season is an area of active research at the present (Weaver et al. 1988, Forcella 1993, King & Oliver 1994, Oryokot et al. 1997, Roman et al. 1999, Shrestha et al. 1999). These models are typically based on the time course of temperature and soil moisture, and allow prediction of the optimum timing for chemical and physical weed control operations (Forcella 1998). So far these models have been designed for clean tilled systems without surface residue. The work proposed here will extend these models to conditions with various amounts of residue.

A search of the CRIS revealed only one project dealing with microclimate effects of mulch on weeds. Project WNP04277 is investigating effects of moisture retention by crop residue on microbial decay of grass weed seeds in no-till systems in eastern Washington. Two projects in Monatana (MONB0023 and MONB00399) are developing models of weed seedling emergence in response to soil moisture, temperature and vertical position of seeds in the soil, but apparently without reference to crop residue. Project OHO000011 in Ohio is working on a similar model but without soil moisture. Researchers at the University of Guelph, Ontario are developing seed germination and seedling elongation models similar to those of Oryokot et al. (1997), Roman et al. (1999) and Shrestha et al. (1999) for several annual agricultural weeds. These models are based on temperature and water potential. However, germination and emergence are evaluated strictly under controlled conditions in petri dishes and not in soil. Also, this research does not involve the use of mulch.

Cover crop variety trials. Most of the studies that have shown substantial weed control from residue produced in situ by a winter cover crop have used rye or mixtures that included rye (e.g., Liebl et al. 1992, Masiunas et al. 1995, Creamer et al. 1996b, Smeda and Weller 1996). Presumably, use of the most productive and weed suppressive cultivars would improve consistency of weed control with rye. Laboratory work by Burgos et al. (1999) indicated that the allelopathic activity of rye varied substantially between cultivars. Reberg-Horton et al. (2000) found that rye cultivars differed in productivity and weed suppressive ability under field conditions in North Carolina. However, Mohler (unpublished) found that the same varieties did not differ under field conditions in New York, but that they did show differences in allelopathic activity in a laboratory bioassay. Differences between field and laboratory results could arise due to effects of roots in the field but use of shoot tissue in bioassays.

Research associated with earlier NE-92 projects, showed that subterranean clover (Trifolium subterraneum L.) has excellent potential to suppress weeds (Enache and Ilnicki 1990, Ilnicki and Enache 1992). Enache and Ilnicki (1990) showed that sub clover could provide better weed suppression than rye. Teasdale (unpublished data) also observed better weed suppression by subterranean clover than by any other winter annual cover crop tested.

The major limitation to adoption of subterranean clover as a cover crop in the northeastern states is its relatively low winter hardiness compared to other winter annual cover crops (Teasdale 1993). Subterranean clover often suffers significant stand loss from freezing temperatures and even the best performing cultivar, Mt. Barker, has shown incomplete ground cover in central Maryland (Teasdale 1993). Although subterranean clover can suppress weeds effectively when stands are good, the inability to maintain consistently uniform stands reduces its potential for suppressing weeds. Consequently, research has been initiated by Tom Devine and John Teasdale at the USDA?ARS Beltsville Agricultural Research Center (CRIS 1275-22000-135) to identify and breed winter-hardy subterranean clover cultivars that can be used as weed suppressing cover crops in the northeastern states.

Weed management in crop rotation. Crop rotation has traditionally been regarded as an important strategy in weed control. Leighty (1938) stated the most effective means yet devised for keeping land clear of weeds is rotation. No other method of weed control, mechanical, chemical, or biological is so economical or so easily practiced as a well-arranged sequence of tillage and cropping? A crop rotation is a planned sequence of crops grown in succession on the same field over an extended time. Rotation prevents continuous and uniform management practices from selecting for weed species adapted to a particular system. Crop sequence in a rotation largely dictates tillage type, herbicide use, timing of tillage relative to weed emergence, and harvest date relative to weed maturity. The more dissimilar the crops and their management practices are, the less opportunity an individual weed species has to become dominant over several years.

Liebman and Dyck (1993) compared studies where at least one rotation had no herbicides. Of 29 rotations with no herbicides, 21 had lower weed densities than the associated monocultures, one had a higher weed density and five had weed densities equivalent to the monocultures. Jordan et al. (1995) simulated soil weed seedbanks in three four-year rotations and found that velvetleaf (Abutilon theophrasti) populations increased dramatically in non-chemically managed continuous corn compared to rotations that included soybean and oat interseeded with clover. The increase was attributed to the poor control options in corn, whereas mowing, delayed planting dates, etc. contributed to control of velvetleaf in the oat/clover-soybean rotation. Other investigators who have compared nonchemical control with varying levels of herbicide inputs (Ball & Miller 1990, Ball 1992, Barbieri et al. 1997, Young et al. 1994b, 1996) have reported that crop rotation was the most important factor influencing weed species composition in the seedbank, but that this was due to differential weed control caused by the herbicides used in each crop. That is, herbicide use in each cropping system produced a shift in the weed seedbank in favor of species less susceptible to applied herbicides.

The interdisciplinary crop rotation study by Young and colleagues (Young et al. 1994a, 1994b, 1996) was noteworthy. Ten to 14 scientists from eight disciplines conducted research focused on conservation tillage, multiple crop rotations, and reduced herbicide input systems. They demonstrated that well-managed conservation tillage rotations could use less herbicides, had a higher average profit, and lower economic risk than traditional conventionally tilled systems.

With herbicide reduction as a major goal of this regional project, it will be essential to determine the relationship between crop rotation, weed populations, and herbicide use. A survey of weeds conducted in 34 New York State beet fields (1150 A) in 1998 (Bellinder, unpublished data) revealed rotation periods of 2 to 11 years between beet crops. Seventeen of the fields (53%), for which there were accurate records, had three or four-year rotations and of these, 12 rotations consisted of cereal grain crop between years of row crops, four fields were continuous row crops, and only three fields had been in legume sod crops prior to beets. A strong relationship between crop rotation sequence and weed populations was not demonstrated in 1998 in beets. Preliminary evaluation of the data suggest that the success or failure of the weed control program in the preceding crop may be a more important determinant of herbicide reduction potential than any specific three or four-year crop sequence. Consequently, optimal rotations for weed control may involve planting crops with highly effective weed management programs prior to crops for which only weak weed management programs are available.

Objectives

1. Mechanical weed management in high residue. Evaluate pre- and post-planting tillage options for retaining high rates of cover crop residue on the soil surface and maximizing the effectiveness of residue for weed control.
   
2. Effects of mulch on soil moisture and weed emergence. Evaluate (i) the effects of soil moisture on the germination of six species of annual weeds, (ii) the effects of rye residue levels on soil water potential, and (iii) the integrated effects of soil moisture and physical interference on weed seedling emergence to provide data for improvement of seedling emergence models.
   
3. Cover crop variety trials. Evaluate productivity and weed suppression by commercially available cultivars of winter rye, and to evaluate the winterhardiness and weed suppressive capabilities of subterranean clover cultivars developed by the USDA-ARS breeding program at Beltsville.
   
4. Weed management in crop rotation. Determine the impact of different crop rotations on weed abundance, weed population shifts, and herbicide reduction potential in field and vegetable crops.
   
5. 
   

Methods

Mechanical weed management in high residue. NY(C&SS), NC, OH, ARS (Beltsville), The experiment will use a split plot design with pre-planting tillage as the main plots, and weed management regimen as the sub-plots. The crop will be corn. All treatments will be planted with a mixture of rye and hairy vetch the previous summer or fall. Primary tillage treatments will include (1) chisel plow and disk (conventional practice), (2) zone tillage of a strip 6 to 8 in wide for the crop row, with residue removed to the inter-row area, (3) blade plow or undercutter with residue removed to the inter-row area, (4) zone tillage with residue moved back to the row area after planting, and (5) blade plow or undercutter with residue moved back to the row area after planting. The cover crop will be mowed before tillage or cut off at ground level by very light disking. Weed management subplot treatments will include (i) a herbicide treatment of 1.3 kg ha-1glyphosate burndown of the cover crop before planting plus 2.2 kg ha-1 of metolachlor plus 0.56 kg ha-1 atrazine after planting, (ii) 1.3 kg ha-1 glyphosate burndown of the cover crop before planting and mechanical weed management using a high residue rotary hoe and high residue cultivator, and (iii) as for treatment ii but without glyphosate. Comparison of treatments ii and iii will allow us to determine whether primary tillage combined with post-planting cultivation is sufficient to prevent competition from regrowth of the cover crop. Data collection will include biomass of the cover crop before and after tillage, residue cover measured by the beaded string method after planting and again after cultivation, crop height and visual estimate of percentage canopy closure at each cultivation, corn population count after last cultivation, weed biomass at silking, and crop yield. Parallel experiments will be performed at the four stations, except that NC will use only weed management subplot treatment (iii). Replication of the study at different sites will allow comparison across a range of climates, soils and specific machinery. After two years, treatment procedures will be reviewed, and new treatments added or substituted as necessary to meet the objective. Effects of mulch on soil moisture and weed emergence. ARS (Beltsville), ME, NY(C&SS), Guelph(Kemptville). The six annual species that will be used in these trials are widespread, troublesome weeds throughout the Northeast: velvetleaf (Abutilon theophrasti), common lambsquarters (Chenopodium album), redroot pigweed (Amaranthus retroflexus), common ragweed (Ambrosia artemisiifolia), barnyardgrass (Echinochloa crus-galli), and yellow foxtail (Setaria glauca). Seed germination of these species will be examined in a progressive series of experiments from controlled laboratory conditions to field conditions to determine how the modification of soil moisture by mulch affects weed seedling emergence. (a) Germination of weed seed in different osmotic solutions. Germination experiments in various osmotic solutions in the laboratory will provide insight into the processes taking place prior to emergence of seedlings from the soil. This baseline information has been previously determined for common lambsquarters (Roman et al., 1999), redroot pigweed (Oryokot et al., 1997), and common ragweed (Shrestha et al., 1999). Similar data on velvetleaf, barnyardgrass, and yellow foxtail will be obtained in this study. Seeds will be collected from agricultural fields at each location and dry-stored at 40C. Seed germination will be evaluated in aqueous solutions of polyethylene glycol (Michel, 1983) at osmotic potentials of 0, -30, -100, -300, -1000 kPa under optimal germination temperatures and photoperiod for each of the species. Fifty seeds of a species will be placed on a moistened filter paper in plastic petri dishes containing 2 g of vermiculite and 25 ml of the appropriate osmotic solution. Germination (visible radicle) will be monitored daily for a period of 21 days. (b) Emergence of weeds in sealed pots. For each of the six weed species, emergence from soil will be determined in a growth chamber at soil matric potentials of -30, -100, -300, and -1000 kPa. To establish the appropriate soil water potential, soil collected previously from the site where field trials will be carried out will be sieved, moistened and a water release curve determined using a pressure plate apparatus (Klute, 1986). Thirty seeds will be planted at 1 cm depth in 10 cm-diameter plastic pots in soil previously moistened to the assigned moisture potential. The pots will then be tightly wrapped in a plastic bag, sealed and placed in a growth chamber at temperatures and photoperiods that are optimal for each species. Pots will be uncovered periodically for 21 days to count and remove emerged seedlings, and to allow gas exchange. This trial will be carried out three times for each of the six weed species. (c) Emergence under mulch in pots. The goal of this trial is to assess the ability of different levels of rye mulch to maintain initial soil water potential over a 21-day drying period, and the effects of the resulting soil moisture regime on seedling emergence. Pots will be prepared as described in (b), except that pots will be covered with rye and left unwrapped. Rye rates will be 0, 0.3, 1.0 and 3.0 times a base rate of 400 g m-2, and the experiment will be replicated 4 times. To prevent physical interference from the rye, the rye will be enclosed in poultry netting packets that will be suspended above the soil surface by the lip of the pot. To simulate actual field conditions, light will be supplied with fluorescent bulbs at 900 uE/m2/s PPFD and relative humidity maintained at 50%. Time-domain reflectometry and temperature probes will be inserted at a depth of 1 cm in additional pots to monitor changes in soil water potential and soil temperature over the experimental period. These will be connected to a Campbell CR10x datalogger via multiplexers. None of the pots will receive water after set-up of the experiment. Seedlings will be counted and removed at appropriate intervals for 21 days. This trial will be performed twice. To determine the relation between drying at 1 cm depth and drying at other depths, pots will be wired with probes at 1, 2, 4, 8 and 16 cm depths in additional experiments. The substantial expense of the reflectometry probes precludes collecting moisture data at multiple depths, residue rates and initial soil moisture potentials simultaneously. (d) Field trial with surface residue. In mid-May, soil at matric potentials of -30, -100, -300, and -1000 kPa will be placed in 30 cm-diameter pots that are buried in the soil with 3 cm of the lip exposed. Fifty seeds of each of three species will be sown at 1 cm depth in separate sectors of a pot. Freshly cut rye residue will be placed on pots as described in section (c), and the packets held down with large wire staples to prevent disturbance by wind. In an additional series of treatments, the rye will be placed directly on the soil surface. Rye rates will be 0, 0.3, 1.0 and 3.0 times a base rate of 400 g m-2. After initial water potential levels have been established, pots will not receive any supplemental watering except from natural precipitation. Emerged seedlings will be counted and removed twice weekly for two months. During counting, rye will be removed in the suspended rye treatments, but in pots with rye on the soil surface, only seedlings emerging through the rye will be counted. There will be 5 replicates of each treatment combination and trials will be repeated over four growing seasons. Time-domain reflectometry and temperature probes at a depth of 1 cm in additional pots will monitor soil water potential and soil temperature during the experimental period. Additional climatological data will be obtained from an on-field meteorological station. (e) Field trial with incorporated residue. To determine the effects of incorporated residue on soil moisture and seedling emergence, rye, wheat and a wheat/red clover mixture will be incorporated into the soil in separate plots and weed seeds planted at 1 cm. The cover crop treatment will be compared to an unamended control. The wheat/red clover material will decompose faster than the rye and wheat, and the materials are expected to have different water holding action. Subplot treatments will be fineness (based upon soil aggregation) of seedbed preparation which will affect decomposition rate and hence water holding capacity. Seedling emergence will be recorded every two days, and soil moisture and temperature at 1 cm depth will be monitored continuously using a data logger. Analysis and integration of data. Data from the several experiments will be analyzed by analysis of variance and non-linear regression. The several experiments are designed to provide coordinated data that will allow explanation of seedling emergence in field conditions in terms of soil moisture and the physical effects of mulch. Experiments (a) and (b) give information on germination in response to constant soil moisture. Experiment (c) provides information on germination in response to a range of drying soil conditions created by rye residue. Together these will allow construction of a model relating germination pattern to a wide range of soil moisture conditions. This model will then be tested, and possibly refined using the data from the suspended rye treatments in the field trials. The moisture model will then be combined with a previously constructed physical suppression of weed emergence model (Teasdale and Mohler, 2000), and the combined model checked against data from the field treatments in which the rye is on the soil surface. Experiment (e) will provide data to determine whether hydrothermal time emergence models (e.g., Roman et al., 1999) act similarly with and without incorporated residue. Experiment (a) will be done at ME, NY and ARS(Beltsville) to allow comparison of germination responses of local populations on a north-south climatic gradient. Experiments (b) through (d) will be performed by NY, ARS (Beltsville) and Guelph(Keptville). Experiment (e) will be done at ME. Cover crop variety trials. Rye. MD, ME, NC, NY(C&SS), NY(F&VS), OH, Guelph(Keptville). Cultivars to be tested at each location will initially include 'Aroostook', 'Elbon', 'Maton', 'Wheeler', Winter grazer', "Wrens 96', 'Wrens Abruzzi', and the local unnamed variety. Additional cultivars may be added if seed sources can be located. Rye will be drilled in plots at least 3 by 9 m arranged in a randomized complete block design with 4 replications. Rye will be killed by application of glyphosate at approximately boot to early head stage of maturity and then mowed. Measurements will include autumn establishment counts, above-ground biomass shortly after mowing, density of weeds emerging through rye after mowing, and a bioassay of allelopathic activity. Rye for bioassay will be collected from all plots immediately after mowing by bulking grab samples from both ends of a plot. The bioassay will be accomplished by placing into a 9 by 9 by 1.5 cm square petri dish 90 g of screened dry soil, 1 g chopped, dry rye straw, a filter paper, 25 ml of water, and 9 seeds of one of four test species arranged in a row (tomato, turnip, broomcorn millet, Japanese millet). The lids will be taped in place and the dishes incubated in a vertical position at 28?C for 72 hours. Number of germinated seeds will be recorded, and the shoot and radical lengths of each seedling will be measured. This procedure was modified from Weston et al. (1989) and demonstrated differences between cultivars in a preliminary trial in New York (Mohler, unpublished data). All listed stations will participate in the field trials. This will allow good assessment of cultivar by environment interactions. Selected stations will conduct the bioassay (NC, NY(C&SS)), since results are likely to vary less with location. Subterranean clover. CT, MD, ME, NC, NY(F&VS), OH, ARS (Beltsville), Guelph(Keptville). Evaluations have been conducted in 1999 and 2000 at ARS (Beltsville) to identify winterhardy subterranean clover germplasm from approximately 500 selections obtained from Europe and Australia. These evaluations and crosses will continue for approximately three more years. By 2002, selected advanced lines should be available with sufficient seeds for testing at other locations throughout the Northeast. Seeds will be delivered to cooperators at the listed locations in late summer and planted at the recommended time in each location for establishment of cover crops. Subterranean clover lines will be arranged in a randomized complete block design in plots of approximately 10 m2. Subterranean clover will be evaluated for establishment, winter survival, biomass, and phenology of flowering and senescence. Weed suppression by senesced residue will be determined in comparison to control plots without cover crop. Evaluations will be conducted for 2 to 3 years. A profile of cultivar performance relative to weather conditions and weed species at the several test locations will permit recommendations for potential use in cropping systems in the Northeast. Weed management in crop rotation. CT, ME, NY(F&VS), ARS (Beltsville), Guelph(Keptville). Specific crop rotations will be chosen with regard to the perceived needs of the cooperating states. In all experiments, relatively simple rotations within individual states will be compared with more complex rotations (tested at six locations) to determine whether the greater variety of weed management options, cultural conditions and competitive pressures in the complex rotations contributes to overall weed control. The complex rotations will also help to determine whether crops with good weed management options can be used to reduce weed pressure in subsequent crops with relatively poor weed management options. New York. Five-year rotations will evaluate the influence of crop rotation and herbicide use patterns on incidence and persistence of weed species in vegetable crops. Each rotation will include three different herbicide use patterns: mechanical weed control without herbicide, low herbicide input based on IPM strategies, and high herbicide input. A fall seeded rye cover crop will be used throughout. Rotation 1. Field corn - sweet corn - field corn - sweet corn - snap bean Rotation 2. Squash - cabbage - sweet corn -potato - snap bean Rotation 3. Cabbage - sweet corn - potato - squash - snap bean Rotation 4. Soy bean - potato - field corn - squash - snap bean Connecticut. The structure and intent of the study is similar to that in New York, with a comparison of four five-year rotations, each with three weed control programs: a mechanical program without herbicides, a low herbicide input program based on IPM, and a full herbicide program. Rotation 1: Sweet corn - sweet corn - sweet corn - sweet corn - sweet corn Rotation 2: Sweet corn - pumpkins - sweet corn - sweet corn - snap bean Rotation 3: Snap bean - sweet corn - pumpkins - sweet corn - snap bean Rotation 4: Snap bean - pumpkins - snap bean - pumpkins - snap bean Ohio. Three four-year rotations will be compared under organic and conventional management. Rotation 1: Sweet corn - potato - tomato - cucurbits Rotation 2: Wheat - clover- tomato - cucurbits Rotation 3: Cucurbits - tomato - potato - sweet corn In addition, the four possible starting points of a four-year rotation of wheat - clover - tomato - cabbage using organic production methods will be compared to determine the best entry point for conversion to organic practices. The experiment will use a split-split plot design, with initial crop as the main plot factor, amount of compost applied as the sub-plot factor and weed control (cultivation vs. cultivation plus additional treatment to prevent seed return) as the sub-sub plot factor. ARS (Beltsville). The impact of field crops rotations grown organically on weed incidence and persistence will be evaluated. All rotations will include a rye cover crop before soybean. Rotation 1. Two-year: corn - soybean (includes hairy vetch before corn) Rotation 2. Three-year: corn - soybean - wheat (includes a crimson clover cover crop before corn) Rotation 3. Four-year: corn - soybean - wheat - hay Guelph(Kemptville). This experiment will be similar to the study at ARS(Beltsville) except that herbicide and nonchemical weed management will be compared in each rotation. Rotation 1. Two-year: corn - soybean (includes hairy vetch before corn) Rotation 2. Three-year: corn - soybean - wheat (includes red clover cover crop before corn) Rotation 3. Four-year: corn - soybean - wheat - hay In addition, a vegetable rotation trial will parallel those at NY and CT. Maine. The experiment will continue a rotation study begun in 1991 and modified in 1999. A major intent of the Potato Ecosystem Project is to compare the two-year rotation commonly used by Maine potato growers with others that might improve soil quality. All three rotations will be run with and without organic matter amendments (cover crop green manure/manure). Collection of weed data will allow assessment of the effects of soil building rotations on weeds. Rotation 1. Potato - barley Rotation 2. Potato - soybean - potato-barley Rotation 3. Potato - soybean - potato - forage At all locations, data collection will include measures of weed control, cover crop biomass prior to tillage, and crop yields. Density of major weed species in the seedbank will be analyzed in the New York and Maine trials.

Measurement of Progress and Results

Outputs

• Outputs from Objective 1 will be determination of the : (i) Feasibility of mulch plus mechanical weed control without herbicides and, (ii) Response of mulch plus mechanical weed control systems to climatic gradients in the Northeast.
• The outputs from Objective 2 will be the: (i) Publication of scientific papers that extend current weed seedling emergence models to include the effects of modification of soil moisture on seed germination and seedling establishment.
• The outputs from Objective 3 will include: (i) Determination of the best rye varieties for use as weed suppressive cover crops in the Northeast (ii) Release of subterranean clover varieties with increased winter hardiness and, (iii) Determination of the climatic limit of these new varieties.
• The outputs from Objective 4 will be: (i) Publications with recommendations for optimal sequencing of vegetable production and for reduced-purchased-input field crop systems.

Outcomes or Projected Impacts

• The outcome from the proposed research will be an increased use of high residue cropping systems and crop rotation in the Northeast. This will result in a reduced dependency of herbicides and/or less erosion of topsoil as a result of improved weed management strategies. However, due to the normal lag between research results and farmer adoption, most of the impact of this research is likely to occur after the 5-year life of the project.

Milestones

(2001):<LI>Mechanical weed management in high residue run & data collected <LI>Data from osmotic solution /seed germination studies gathered <LI>Methods for sealed pot experiments verified <LI>Rye variety field trial run; rye variety allelopathy bioassay <LI>Breeding work on subterranean clover continued <LI>Weed management by rotation study initiated

(2002):<LI>Mechanical weed management in high residue repeated, assessment <LI>Data gathered in pot drying studies with rye <LI>Rye variety field trial repeated; rye variety allelopathy bioassay <LI>Breeding work on subterranean clover continued <LI>Weed management by rotation study initiated

(2003):<LI>Mechanical weed management in high residue - modified trial begins <LI>Pot drying studies with rye completed, field trial begins <LI>Construction of model based on moisture experiments <LI>Rye variety field trial repeated; ye variety allelopathy bioassay <LI>Breeding work on subterranean clover continued <LI>Weed management by rotation - continued data collection

(2004):<LI>Mechanical weed management in high residue - modified trial repeated <LI>Publication on rye variety trial prepared <LI>Field moisture trial with rye repeated <LI>Model verification <LI>Field tests of subterranean clover hardiness repeated <LI>Weed management by rotation study continued, data collected

(2005):<LI>Field tests of subterranean clover hardiness repeated <LI>Weed management by rotation study continued, data collected <LI>All data analysis completed <LI>Participation in farmer field days continues, as opportunities are presented <LI>Manuscripts prepared for publication

(0):0

Projected Participation

View Appendix E: Participation

Outreach Plan

Extension representation in this activity through joint appointments and direct participation by extension specialists will favor the deployment of research results as they become available. Additionally, participation in farmer field days will occur as opportunities present themselves.

Organization/Governance

The project will be executed by a Technical Committee made up in the manner described in the revised Multi-state Research Guidelines. The officers will consist of a chair and secretary who are elected by the committee annually. The committee will meet annually in January, in conjunction with the annual meeting of the Northeastern Weed Science Society, and may occasionally have additional meetings as needed, as authorized by the Administrative Advisor.

Literature Cited

Attachments

Land Grant Participating States/Institutions

CT, MD, ME, NC, NY, OH

Non Land Grant Participating States/Institutions

Kemptville College - University of Guelph, USDA-ARS/Maryland