NE161: Association of Fertility with Temporal Changes in Ovarian Function of Domestic Ruminants
(Multistate Research Project)
NE161: Association of Fertility with Temporal Changes in Ovarian Function of Domestic Ruminants
Duration: 10/01/1996 to 09/30/2001
Statement of Issues and Justification
Reproductive insufficiency is a major cause of reduced productivity for both dairy and beef cattle. Reproductive loss occurs primarily through fertilization failure and early embryonic mortality. The resolution of these problems necessitates more knowledge on: regulation of gonadotropin secretion, follicular growth and maturation, corpus luteum (CL) formation and development, gamete transport, utero-embryonic interactions, and fetal-uterine regulation of CL lifespan.
Related, Current and Previous Work
Information about the processes involved in follicular development in a variety of species has been accumulating. In dairy cows, the presumptive ovulatory follicle can be identified as the largest follicle in the ovaries only a few days prior to estrus (Dufour et al., 1972). However, follicular growth and atresia occur throughout the luteal phase (Rajakoski, 1960; Spicer and Echternkamp, 1986). The most recent studies of follicle dynamics in cattle have shown that a large follicle emerges from a cohort of smaller follicles about once every 7 to 10 days and apparently is dominant over the small follicles (Fortune et al., 1988; Sirois and Fortune, 1988; Ginther et al., 1989; Savio et al., 1990).
Early workers reported that follicles were larger (Ulberg et al., 1951; Zimbelman 1963; Zimbelman and Smith, 1966), persisted (Trimberger and Hansel, 1955), or became atretic (Guthrie et al., 1970) when cows received minimal doses of progestogen required to suppress estrous behavior and ovulation. Conversely, follicles were smaller when doses of progesterone were increased (Nellor and Cole, 1957). With the application of ultrasonography to examine patterns of follicular development in cattle, persistence of follicles during conditions of low progesterone or progestogens has been documented in several studies (Sirois and Fortune, 1990; Cupp et al., 1992; Savio et al., 1993b).
Lowered fertility in dairy cows has been associated with lower concentrations of progesterone in milk (Meisterling and Dailey, 1987) or blood (Folman et al., 1973). Several recent studies have confirmed that low fertility is common under conditions of low progesterone, increased frequency of secretion of LH, prolonged preovulatory increases in estradiol, and with persistent follicles (e.g. Wehrman et al., 1993; Savio et al., 1993a, b; Stock and Fortune, 1993). During the current funding period, this cooperative regional research project has completed an experiment to compare fertility in cows that ovulate a growing vs. a persistent follicle. Persistent follicles were induced using the progesterone releasing, intravaginal devices, CIDR-B. The study was conducted at 7 different experiment stations and included a total of 187 cows. Fertility was significantly lower in animals that ovulated a persistent follicle (pregnancy rate = 23.6%) than those that ovulated a growing follicle (pregnancy rate = 58.2 %). Further, fertility was higher in CIDR-treated heifers than in CIDR-treated cows. Thus, although ovulation of a persistent follicle results in lower fertility in both cows and heifers, the effect is most dramatic in lactating cows.
From this study and the work of others, it is clear that hormonal manipulation of the estrous cycle, as is often used in estrous synchronization schemes, may alter patterns of follicular development. This alteration can lead to reduced fertility. It is now of interest to determine if naturally-occurring differences in patterns of follicular development also lead to greater, or reduced, fertility. In cattle, the development of large antral follicles occurs in a wave-like fashion; a cohort of follicles is recruited contemporaneously to grow larger than 4 to 5 mm in diameter. One follicle is selected to be the dominant follicle, but this follicle will undergo atresia with the rest of the cohort if the corpus luteum has not regressed. Most cows have two or three waves of follicular development within an estrous cycle (Sirois and Fortune, 1988; Savio et al., 1988; Knopf et al., 1989). It is unknown whether fertility differs in cows with two vs. three waves of follicular development, or if individual cows tend to have the same pattern of follicular development in subsequent cycles. Therefore, the first objective of the proposed project will be to evaluate fertility after two vs. three waves of follicular development. It is essential that this experiment be performed as a regional research project, so that sufficient numbers of two- and three-wave cows are obtained for statistical analysis. The study will also determine if the duration of the last follicular wave, rather than the number of waves affects subsequent fertility
Differentiation of dominant follicles is a two-step process: recruitment of healthy gonadotropin-sensitive follicles and selection of the follicles) that becomes dominant while the others undergo atresia (Driancourt and Fry, 1988). Both endocrine and paracrine controls are involved in the differentiation of the follicles toward ovulation. Recruitment of each follicular cohort is preceded by an elevation in plasma FSH. Selection is correlated with a decrease in FSH concentrations together with a change in balance between the paracrine stimulators and inhibitors found in follicular fluid. Dominance of ovulatory follicles is maintained perhaps by high estradiol production linked to the stimulatory action of paracrine regulators and by the high sensitivity of the large dominant follicle to gonadotropins.
In addition to gonadotropins, a variety of other endocrine, autocrine and paracrine regulators may be involved in follicular development. Insulin exerts actions on ovarian tissues which are very similar to those of pituitary gonadotropins (Poretsky and Kalin, 1987). Insulin stimulates thecal cell androgen production (Hernandez et al., 1988; Caubo et al., 1989) via high affinity receptors (Hernandez et al., 1988) and increases granulosal cell metabolism (Weber and LaBarbera, 1988). Perhaps most importantly, insulin has been shown to synergize with FSH to facilitate morphological differentiation of granulosal cells and enhance LH receptor binding (Amsterdam et al., 1988). It is not clear whether all the effects of insulin are direct or are mediated by interaction with receptors for insulin-like growth factors (IGF). There is considerable overlap in activities and associations between insulin and IGF-I.
Although the possible relationship with ovarian IGF-I or its receptors has not been established, insulin was more mitogenic than gonadotropins in bovine follicles grown in perfusion culture (Peluso and Hirschel, 1987). Also, insulin enhanced ovulation rate during negative energy balance in heifers (Harrison and Randel, 1986). Ovulation enhancement has also been demonstrated following long-acting insulin treatment in well-fed gilts (Cox et al., 1987). During negative energy balance in early lactation, the rapid increase in utilization of glucose for milk lactose production results in lower plasma concentrations of both glucose and insulin as compared to later stages of lactation (Hart et al., 1978; Smith et al., 1976), after first ovulation has occurred. Plasma IGF-I levels closely parallel insulin during early lactation (Ronge et al., 1988), suggesting a direct relationship.
Despite the clear relationships between insulin and energy balance or follicular function, very little is known regarding the effects of negative energy balance on patterns of follicular development. It has been suggested that severe negative energy balance may affect initial follicular growth in early postpartum cows (Britt, 1992). Energy balance does seem to be related to rate of follicular growth prior to initiation of estrous cycles in postpartum cows (Lucy et al., 1991) and is correlated negatively with the interval to first ovulation (Butler et al., 1981). Miyoshi et al (1995) found that plasma insulin could be elevated in postpartum cows by daily drenching with propylene glycol. Although animal numbers were insufficient in this trial to produce significant differences in fertility, conception rates tended to be greater in the propylene glycol treated cows (57.1 % vs. 33.0%). Therefore, the second objective of the proposed project will be to determine if alterations in plasma insulin during negative energy balance affect patterns of follicular development and fertility.
Examine naturally-occuring patterns of development and function of the preovulatory follicle for their effects on oocyte maturation, fertilization and embryo survival. Compare naturually-occurring and induced differences in metabolism for their effects on oocyte maturation, fertilization and embryo survival.
Examine how differences in luteal function affect preovulatory follicle development and subsequent fertility.
MethodsObjective 1: Examine naturally-occurring patterns of development and function of the preovulatory follicle for their effects on oocyte maturation, fertilization and embryo survival.
Our collaborative project (NE-161, Relationships Between Patterns of Follicular Development and Fertility) showed that low levels of progesterone delivered by a Controlled Internal Drug Releasing (CIDR) device during the first follicular wave in heifers and lactating dairy cows resulted in the development of a persistent follicle (Ahmad et al., submitted). Furthermore, the fertility rate of heifers and cows ovulating a persistent follicle, as opposed to a growing follicle, was greatly reduced (i.e., pregnancy rate: persistent follicle = 23.6%, growing follicle = 58.2%). Previous studies (Savio et al., 1993a, b; Stock and Fortune, 1993; Wehrman et al., 1993) have also concluded that prolonged follicular growth is associated with lowered or impaired fertility. Various naturally-occurring patterns of follicle growth are exhibited in cattle (i.e., most estrous cycles consist of two or three follicular waves, although four waves have been reported; Fortune et al., 1988; Pierson and Ginther, 1988; Savio et al., 1988; Sirois and Fortune, 1988; Knopf et al., 1989). In heifers that exhibit two waves/cycle, the ovulatory follicle is observed approximately 10 days before ovulation, while in those that exhibit three waves/cycle, the ovulatory follicle is observed approximately 5.9 days before ovulation. In four-wave cycles, the ovulatory follicle is observed approximately 7 days before ovulation (Sirois and Fortune, 1988). Thus, it is of interest to determine whether follicles that naturally have a longer period of growth (i.e., in two-wave cows) ovulate oocytes that are less fertile than those with shorter periods of growth (i.e., in three- and four-wave cows).
la. Do different naturally-occurring patterns of follicle development affect oocyte maturation, fertilization and embryo survival?
To address this question, ultrasound exams will be performed to monitor follicle development in lactating cows. Plasma progesterone concentrations will be determined and pregnancy status assessed. Specifically, seven research stations [Connecticut (CT), Maine (ME), New Hampshire (NH), New York (NY), Ohio (OH), Pennsylvania (PA), West Virginia (WV)] will pool resources to contribute to this objective. Each station will follow the protocol outlined below. Ultrasound exams will be performed in lactating Holstein cows that demonstrate a natural estrus between 45 to 70 days postpartum (n = minimum of 20 cows per research station; assuming a 20% difference in conception rate, a total of 30 cows/station is required to achieve significance at P < 0.05). Following a spontaneous estrus (day = 0), ultrasound exams will commence on day 5 postestrus, and continue every other day until the next estrus. Then, daily ultrasound exams will be performed to confirm ovulation. Following each ultrasound exam, a blood sample will be collected. Semen from bulls (selected for high proof for milk and fertility) will be used to breed cows according to the AM/PM rule. Pregnancy status will be determined on days 30-35 with ultrasonography.
Besides follicle diameters, the size and location (ipsilateral or contralateral to ovulatory follicle) of the corpus luteum will also be noted during ultrasound exams. This information provides the basis for objective 3. Also, each research station will report the incidence of "aberrant" patterns of follicle development, i.e., cystic follicles, failure to ovulate or short cycles. Together, results from this objective will elucidate which naturally-occurring patterns) of follicle development leads to successful oocyte maturation, fertilization and embryo survival.
A preliminary trial was conducted at the New Hampshire and Ohio research stations. Twenty eight lactating Holstein cows were scanned, and the results are summarized below (Table 1). The number of cows exhibiting two waves predominated over those exhibiting three or four waves. Five of 28, or 18 % of cows exhibited "aberrant" waves, i.e., no ovulation, short cycle or cystic follicles. In addition, the pregnancy rate in two wave cows was lower than for three or four wave cows. The data so far appear to support our hypothesis that two-wave cows are less fertile than three-wave cows. However, the low number of threewave cows require the cooperation of seven research stations to pool resources (i.e., increase animal numbers), which will also enable us to make inferences regarding fertility rates.
1b. Ultrasound Exams.
Transrectal ultrasonography will be performed using a 5 or 7.5 MHz probe. Each ovary will be scanned several times and in at least two different planes. Individual follicles (>_ 5mm) will be followed and their diameters recorded. Diameters are determined using a calibrated caliper resident in the ultrasound monitor. These measurements represent the size of the antrum. Since follicles are not perfect spheres, the diameter of the follicle is calculated as the average of the antral length and width. Each research station will provide data on the number of waves, as well as the size of the ovulatory follicle, for their cows.
lc. Progesterone Assays
A blood sample (10 ml) will be collected into heparinized tubes following each ultrasound exam. Following centrifugation, the plasma will be kept frozen at -200C until analysis by radioimmunoassay (RIA). To maintain consistency, all research stations will send their plasma samples to the Connecticut research station for analysis. The extraction protocol and RIA have been previously characterized (Beal et al., 1980).
1d. Data Collection
Objective 2: Compare naturally-occurring and induced differences in metabolism for their effects on oocyte maturation, fertilization and embryo survival.
2a. Initial investigations will utilize treatments known to affect ovarian function/fertility and will determine whether treatment groups differ in secretion or degradation of progesterone and estradiol-17B. This work will be initiated at two stations (NY and WV).
Studies at Cornell will be conducted in lactating and nonlactating cows to assess the effects on steroid metabolism of lactation and associated changes in feed intake. During the first 3-4 weeks of lactation in dairy cows, milk production increases much more quickly than feed intake, resulting in a large negative energy balance. By day 60 of lactation, milk production has passed its peak, feed intake has increased by about one-third from that after parturition and energy balance has been nearly restored. Several metabolic factors have been related to differences in follicular development in postpartum cows (Beam, 1995). In addition the results from the previous regional project suggested that plasma progesterone may be lower in cows than in heifers and that plasma progesterone was lower during the first week after AI in cows that failed to become pregnant. Such differences in plasma progesterone are related to poor embryo development (Maurer and Echternkamp, 1982) and a lower pregnancy rate (Butler et al., 1996). Blastocysts can stimulate progesterone output from luteal cells via interaction with the endometrium (Thibodeaux et al., 1994). Increased progesterone in circulation is important for full inhibitory action of development of the luteolytic mechanism during maternal recognition of pregnancy (Lamming and Mann, 1995). In this latter critical period after AI (d 14-17), higher estradiol concentrations in plasma are negatively associated with pregnancy (Pritchard et al., 1994).
Studies at West Virginia will be conducted in ewes. In earlier studies, short-term flushing with high protein diets has been shown to increase ovulation rate and/or litter size (Thomas et al., 1987; Smith, 1988; Smith and Stewart, 1990). In unpublished work performed in West Virginia, Israel and Sardinia, (S. Landau et al.), corn gluten meal increased ovulation rate, whereas soybean oil meal did not affect ovulation rate, but decreased, or tended to decrease, conception rates. Other studies have shown that higher protein diets (Thomas et al., 1987) or higher intake increased metabolic clearance rate, or reduced circulating concentrations, of progesterone (Williams and Cumming, 1982; Parr et al., 1993; McEvoy et al., 1995) or of estrogen (Payne et al., 1991; Adams et al., 1994). McEvoy et al. (1995) presented evidence that dietary-induced reductions in circulating concentrations of progesterone impaired subsequent development of ova in ewes. Conversely, higher concentrations of estradiol have been predictive of greater early embryonic death in a series of studies in beef cattle (Breuel et al., 1993; Ahmad et al., 1995), and were associated with the persistent follicles in which oocytes undergo premature development (Mihm et al., 1994; Revah and Butler, 1996). Thus it is of interest to determine whether effects of diet on steroid metabolism are generic, or are specific to estradiol or progesterone and whether these effects are mediated in part by changes in blood flow (such as were produced by changes in feed intake in gilts; Prime and Symonds, 1993).
How do changes in steroid metabolism (production and degradation) affect patterns of follicular development, oocyte maturation and embryo survival?
The studies at Cornell will compare steroid metabolism in lactating cows vs. non-lactating heifers consuming moderate or high dietary intakes. On day 12 postpartum (moderate intake), lactating cows (n = 4) will receive an intra-vaginal progesterone-releasing device (CIDR) and two estradiol implants (Compudose, ear implants). This period is chosen prior to formation of the first corpus luteum such that the CIDR provides the only source of plasma progesterone. The estradiol implants are expected (Taft and Deaver, unpublished) to result in turn-over of any large dominant follicles, thus becoming the sole source of plasma estradiol (" 10 pg/ml). The CIDR and estradiol implants will be removed 48 hr after insertion, when plasma steroid concentrations will have reached steady-state equilibrium. Blood samples will be collected at -60, -30 and 0 min before implant removal to establish baseline concentrations and at 15, 30, 45, 60, 90, 120, 180, 240, 300 and 360 min, thereafter, to calculate the disappearance rate (t?, time to 509'a of baseline mean) of both progesterone and estradiol (Hawkins et al., 1995). At 60-70 days postpartum (high dietary intake), the same cows will be studied starting on day 11 of the estrous cycle. Blood sampling catheters will be placed as described below for experiments in ewes to allow calculation of blood flow and production and degradation of progesterone from the corpus luteum on day 12 of the cycle. Upon completion of the 6-hr sampling schedule, a CIDR will be placed intravaginally, two estradiol implants will be placed in one ear, and prostaglandin F2alpha (25 mg Lutalyse im) will be injected to cause luteolysis. After 48 hours all implants will be removed for calculation of progesterone and estradiol disappearance rate as described above. To compare the effects of lactation, a group of non-lactating heifers (14 months of age, n = 4, high intake) will be studied in the same manner as the lactating cows from day 11 of the cycle including calculation of steroid disappearance rate. To complete these studies for the effects of dietary intake, an experiment will be conducted in feed-restricted (moderate intake) heifers (n = 4). The heifers will be fed individually ad libitum from day 2-6 of the estrous cycle and at 66 % of their mean individual intake from days 7-14. On day 12, the treatment protocol for progesterone and estradiol disappearance rate will be followed. The level of feed restriction is chosen to provide a similar relative magnitude difference in intake as for the difference observed in lactating cows between days 12 and 60 postpartum.
At West Virginia, studies in ewes will examine effects on steroid metabolism of dietary treatments known to affect ovulation rate and/or fertility. Specifically, three isocaloric dietary supplements will be fed once daily to ewes on a maintenance ration, beginning 7 days before estrus (mating), during the luteal phase. Control ewes will receive ground corn, and treated ewes will receive high protein from either corn gluten meal (high in branched chain amino acids, high rumen bypass protein) or soybean oil meal (lower in branched chain amino acids and rumen bypass protein). On day 3 of supplement feeding, sampling catheters will be inserted in the posterior vena cava via the saphenous vein (Benoit and Dailey, 1991) and into the right ventricle via the jugular vein. A second saphenous vein catheter, in the opposite saphenous vein, will be used for infusion of para-aminohippurate (PAH), so that blood flow and production and degradation rates of steroids can be calculated from concentrations of PAH, estradiol and progesterone in frequent samples. Samples will be collected at 30 minute to 2 hour intervals for 6 hours after feeding, during a period of high progesterone, low estradiol on day 4 of feeding and during a period of low progesterone, high estradiol on day 6 of feeding. These concentrations will be achieved by treating the ewes with prostaglandin F2 on day 5 of the feeding period.
2b. In the early postpartum period, lactating dairy cows enter a state of negative energy balance. Negative energy balance is associated with decreased reproductive performance and with delayed follicular development (Lucy et al., 1991). One characteristic of negative energy balance is reduced plasma insulin concentration. Using daily drenches of propylene glycol to elevate insulin, Miyoshi et al. (1995) demonstrated an increase in cumulative plasma progesterone concentration of the first estrous cycle. This was associated with a tendency toward a longer first luteal phase (13.1 days vs. 7.1 days; treatment vs. control, respectively) and a higher conception rate to first Al (57.1 % vs. 33 %).
This study will confirm and extend the study of Miyoshi et al. As a cooperative regional research project, sufficient animals will be available for a fertility study. This experiment is designed to determine if transient increases in insulin alter the pattern of follicular development and enhance fertility in postpartum cows.
Do propylene glycol or increases in insulin affect the pattern of follicular development, oocyte maturation and embryo survival?
In the study on which this experiment is based, cows were drenched with 500 ml propylene glycol daily from days 7-42 postpartum. Because this is very labor intensive, the practicality of this procedure for on-farm use is questionable. Therefore, a preliminary trial will be conducted to determine if propylene glycol can be administered as a feed additive or if insulin can be administered directly. Also, the duration of treatment will be shorter and will focus on the period during preovulatory follicle development. This preliminary trial will be conducted early in 1996, prior to the initiation of the renewed project.
Four stations (MA, NH, NY, OH) will collaborate on this experiment. At each station, 30 cows (days 40-55 postpartum) will be assigned to one of three groups: control or propylene glycol (treatment begins on day 10 or day 16). Among stations, this will allow for a minimum of 40 control and 80 treated cows, [if we assume there is a 20% difference in conception rates, a total of 132 cows (n =11 cows/group x 3 groups x 4 stations) is then required for the study]. Cows will be observed for estrus twice daily and randomly allotted at spontaneous estrus (day =0) among the three groups.
Beginning on day 5 after estrus, ultrasonography will be conducted and a blood sample collected every other day until the start of treatment. During the treatment period, ultrasonography will be conducted daily and blood samples collected twice daily. Ultrasonography will allow for determination of the growth pattern of the dominant follicle, the size of the follicle prior to ovulation, and the approximate time of ovulation relative to estrus. Size of the corpus luteum will also be monitored. One-half of the cows in the treatment group (n=minimum of 10) will be treated with propylene glycol beginning on day 10. The remaining treatment cows (n=minimum of 10) will be treated with propylene glycol beginning on day 16. Propylene glycol (500 ml) will be administered daily by drenching, unless a suitable alternative means of delivery is determined in the preliminary study. Treatment will continue until ovulation occurs. All cows will be artificially inseminated to a high fertility bull upon standing estrus, according to the AM/PM rule. Pregnancy will be diagnosed on day 30-35 with ultrasonography. Plasma samples will be assayed for progesterone and estrogen, all samples to be assayed at one station. This will provide information about the endocrine environment that exists as the follicle develops in control vs. propylene glycol treated cows. The acute insulin response to the propylene glycol drench is already known (Miyoshi et al., 1995). Twice daily blood samples will also be used for insulin analysis, to provide information on chronic, or baseline alterations in insulin concentration. From previous studies it is known that this short duration of propylene glycol treatment will not significantly alter energy balance.
Direct administration of insulin will be tested as an alternative to elevating plasma insulin concentrations by propylene glycol drenching. Dr. Nancy Cox (Mississippi State University) has treated lactating beef cows with long-acting insulin (Lente Insulin, Eli Lilly) twice daily for five days. Plasma glucose concentrations fluctuated between 30 and 45 mg/dl. Lactating animals are much more sensitive to insulin than non-lactating animals. Careful dosetitration studies will be performed in lactating dairy cows to establish a dose that enhances plasma insulin levels and glucose uptake by tissues (i.e., lowers blood glucose by 25-35 percentage points) for 12 hours each day. If feasible, this treatment will be used in place of the more laborious propylene glycol drenches in the protocol described above. A preliminary trial will also determine the route of administration (either intramuscular or subcutaneous) and time of treatment (e.g. day 10 of the cycle).
Objective 3: Examine how differences in luteal function affect preovulatory follicle development and subsequent fertility.
Objective 3 will examine how the location and the function of the CL affects concurrent follicular development. Experiments proposed to satisfy objective 3 can be categorized into three broad areas. The first area will examine whether the location of the CL affects follicular development. The second and third areas will examine if increased or decreased output of luteal progesterone affects patterns of follicular development.
3a. Does the location of the CL affect follicle development'!
Ultrasonographic and fertility data obtained in experiments conducted to satisfy objectives 1 and 2 will be analyzed to determine if follicle development is affected when the CL is located either on the ipsilateral or contralateral ovary. Seven research stations (CT, ME, NH, NY, OH, PA, WV) will contribute to this objective. We will determine if the numbers of 2 vs. 3 wave patterns of follicular growth are affected by the local presence of the CL. Additionally, we will determine if fertility and follicle development are affected when the ovulatory follicle is on the same ovary as the CL vs. the contralateral ovary.
3b. Do increased or decreased concentrations of systemic progesterone affect follicle development and subsequent fertility'!
These experiments will utilize two methods to alter luteal output of progesterone to determine the effects on concomitant follicle development and subsequent fertility. The first, twice daily SC injections of oxytocin on days 3 and 4 of the estrous cycle, have been reported (Milvae and Hansel, 1980) to inhibit luteal function and lower concentrations of plasma progesterone. In those experiments the length of the estrous cycle was not affected by oxytocin treatment. In the proposed experiments heifers will be treated with oxytocin (0.33 USP units/kg body wt) or saline on days 3 and 4 of the estrous cycle [n = minimum of 5/group; if we assume there is a 20% difference in conception rates, a total of 80 heifers (n=8 heifers/group x 2 groups x 5 stations) is then required for the study.
In the second experiment, levels of progesterone will be elevated by the insertion of a CIDR on days 8 through 17 of the estrous cycle. This time period reduced progesterone levels in the oxytocin treated group.
Five research stations (CT, MA, ME, NH, .OH) will contribute to this objective. In both experiments, nonlactating heifers will be scanned by ultrasound every other day beginning on day 5 of the estrous cycle. Concentrations of plasma progesterone will be determined in plasma collected at the time of ultrasonography. Animals will be bred by AI with semen from a bull of high fertility at the estrus following alterations in luteal function. Results from these experiments will determine if changes in plasma levels of progesterone alter patterns of follicle development and subsequent fertility.
Measurement of Progress and Results
- Description of how the number and growth period length of naturally occuring waves of follicle development affect fertility, especially in regard to proximity between the CL and the ovulatory follicle.
- Determination of how energy balance may alter follicle wave patterns.
- Methods for manipulating follicle growth and duration.
Outcomes or Projected Impacts
- Increased understanding of follicular development in cattle.
- Development of methods for maximizing fertility.
- Development of new management practices to decrease calving intervals.
- Reduction of calving interval could save producers $200,000,000/year in costs related to calving interval.