Factors governing reduced reproductive performance in dairy cattle are numerous and often difficult to diagnose. In general, those factors resulting from fertilization failure (e.g., semen handling and AI techniques) are more easily resolved by technician retraining than those related to early embryonic death. Although it may be difficult to diagnose various causes of embryonic death; they are usually related to some source of stress experienced by the lactating cows. Artificial insemination breeding programs are successful when high rates of heat detection and conception are achieved. A continuous effort should be focussed on reducing various stressors, increasing reproductive performance by implementing some of the practices outlined herein, and hoping for a bit of old-fashioned luck.
Efficient reproduction in a dairy herd is the primary determinant of profitability. Pelissier (8) estimated an annual net loss of income of $116 per dairy cow because of poor reproductive performance. More recently, in a case study of a 300-cow Michigan dairy herd, a 5% increase in conception rates alone was predicted to increase yearly net income by $1226 (6). Furthermore, decreasing days to first service from 80 to 60 days, increasing efficiency of heat detection from 50 to 60%, and increasing conception rates from 35 to 50%, combined to yield an increased net income of $18,485. Although the benefits of improving reproduction are apparent, specific causes of poor reproductive performance are not easily identified or resolved. To improve reproductive efficiency, the limiting factors must be identified. In general, there are two major sources of failure, which include, but are not limited to, fertilization failure and embryonic death. Fertilization failure follows procedures or practices that fail to facilitate union of a viable egg with a viable sperm. Once fertilization occurs, embryonic death results from failure of normal embryonic development, recognition of pregnancy, and its normal maintenance.
The greatest limiting factor to successful fertilization is associated with detection of estrus. It is estimated that approximately 50% of the heats go undetected on the average dairy farm in the U.S. There are two important challenges in heat detection. The first is accurately recognizing signs of heat and the second is catching all possible heats in breeding heifers and cows. One might be quite accurate in catching cows in heat, but still have a major heat detection problem because too few heats are observed. The first problem is a lack of diagnostic accuracy (error of commission). The second problem is a lack of efficiently detecting all heats (error of omission).
Heat detection errors of commission are illustrated by a recent study on eight California dairy farms (1). It was reported that as many as 32% of the cows in one of the participating herds were AI bred when not in heat. In contrast, on another dairy farm, only 2% of cows were inseminated when not in true heat. On the average, 13.5% of 831 inseminations were performed in cows notin heat! Proper timing of insemination only occurs when those doing the inseminating have a thorough and complete understanding of the physiological events associated with estrus, ovulation, and fertilization.
Signs of Estrus
Voluntary signs of heat such as mounting and standing in dairy cattle are influenced by many factors. Those factors that are most important on dairy farms are: 1) number of sexually active animals in a group, 2) freedom for sexually active animals to interact, 3) freedom from interfering activities, 4) ambient temperature, and 5) footing conditions. Behavioral signs of heat require that at least two animals interact. Secondary signs such as butting, licking, and head-resting are influenced less by environmental conditions than are the primary signs of heat, such as mounting and standing. Most experienced observers utilize these secondary signs to pick out cows that are most likely to be in heat even when the immediate environmental conditions limit mounting and standing activity.
A cow will not be detected to stand if no other animal is available to mount. Mounting activity is stimulated strongly by estrogen and inhibited by progesterone. Thus, mounting frequency is considerably greater for cows in proestrus or estrus than for cows that are out of heat or in midcycle with a functional corpus luteum. Once there are four or more sexually active animals (proestrus or estrus) in a group, mounting activity will normally be sufficient for maximal efficiency of heat detection (7).
Table 1 indicates relative mounting activity that one might expect to observe in various locations and conditions on dairy farms. These empirical values are based on data from several published and unpublished studies and on casual observations made on many farms. A value of 1.0 is assigned to mounting activity expected to occur on a relatively dry, grooved, concrete alley. A higher or lower index means more or less mounting activity, respectively.
Table 1. Relative Indexes of mounting activity. Location of cows during heat detection Mounting Index
Milking parlor 0.1
Feed bunk while eating 0.2
Holding pen 0.3
Dry concrete alley 1.0
Dry concrete alley + movement 1.1
Dry dirt lot 1.6
Dry dirt lot + movement 1.8
Source: Vailes and Britt (11).
Activities or conditions that restrict interactions among cows influence whether cows show heat. Cows that are eating or are crowded in holding pens or alleys do less mounting. Cows that are on slippery alleys, frozen ground, or any surface that makes footing tenuous show less mounting activity (9). Cows in heat are more likely to mount one another if the other cows are loose rather than tied. Perhaps this indicates that freedom to interact before mounting is important for maximum expression of mounting activity. Cows that have foot problems, regardless of whether the problem is structural, subclinical, or clinical, apparently show less mounting activity. Many of the foot problems that affect mounting activity can be alleviated by proper foot care (foot baths, dry cows on dirt, regular hoof trimming, etc.).
No firm experimental evidence shows that high levels of milk yield influence mounting or standing activity. Evidence exists that energy balance during the early postpartum period can influence whether a cow is detected in heat at the beginning of the first postpartum cycle. Apparently, cows experiencing a severe negative energy balance can produce enough estrogen to elicit an LH surge without causing them to show heat (10). Once cycles have begun, energy balance does not seem to affect intensity or duration of heat, but might affect level of fertility.
Extremes in temperature affect intensity of heat. Mounting activity is lower on very "hot" or "cold" days than on days when the temperature is near the thermoneutral zone of the cow (30 to 50F). Heats may appear to be shorter when the temperatures are extreme, but it is unclear whether this is because of less mounting activity or because of less willingness to stand.
The process by which the egg is released from a mature follicle is called ovulation. It occurs 24 to 30 hr after the onset of heat (average of 27 hr) and is triggered by the hormonal events that also caused the cow to display estrus. Once the egg is ovulated, its viable life is less than 12 hr unless it becomes fertilized. Figure 1A illustrates the events associated with estrus and ovulation. Secondary signs of estrus may be visible for up to 40 hr before and up to 20 hr after the onset of heat.
Viable Sperm Life
If frozen-thawed semen from most bulls is properly handled, it is thought to have a viable life span of approximately 24 hr in the female reproductive tract (Figure 1B). Sperm are not capable of fertilizing the egg immediately upon thawing and deposition into the uterine body of the female. They must traverse the uterine horns to the uterotubal junction, enter the oviduct, and complete a maturation process known as capacitation. In general, it requires about 6 hr for normal, motile sperm to reach the lower portion of the oviduct, during which the process of capacitation is completed. The subsequent 16 hr represents the period of maximal fertile life of the sperm, followed by rapidly declining motility and fertility.
Timing of Insemination
The key to proper timing of insemination and maximizing fertilization rates is to inseminate cows at a time to allow ovulation to occur when there are adequate numbers of motile sperm in the oviduct. Figure 1A illustrates the combination of factors related to estrus, ovulation, and insemination timing when cattle are inseminated according to the AM-PM, PM-AM rule. Based on a twice-daily heat detection program, when cows should be observed at intervals of 12 hr, cows submitted for insemination should be inseminated about 12 hr after first detected in heat. Because we do not know the exact hour when heat begins, we can estimate that on the average, the female detected in heat at either daily observation period has been in estrus for about 6 hr. So when inseminated 12 hr after first detection, the female is actually bred about 18 hr after the onset of heat or approximately 6 to 12 hr before ovulation. This breeding scheme allows ample time for sperm transport, capacitation, and allows a synchronized overlap of the fertile life of both the egg and the sperm, even if the timing is off by as much as 6 hr.
Figure 1. Time scale summary of physiological events associated with estrus. Proper insemination timing (A); fertile life of sperm (B); improper insemination timing (C); and proper insemination timing following improper semen handling procedures (D; 2).
Errors in the timing of insemination are common. One such error is illustrated in Figure 1C. Inseminations based on secondary signs of heat such as mucous discharge, muddy flanks, activated heat mount detectors, or smudged tail chalk, could result carrying out inseminations too early relative to ovulation. In such cases, the viability of sperm cells is marginal or gone before ovulation. When properly and timely inseminated, the unit of semen should be deposited near the end of standing heat. As a consequence, few females should display standing heat after insemination. If more than 20% of the cows are still exhibiting standing activity 12 or more hr after insemination, it is possible that the inseminations were performed too early in estrus.
How costly are these heat detection errors of commission and omission? The cost in real dollars depends on whether the cow or heifer is receiving her first or a repeat service. Let's assume the average cost of semen is $12/straw and $2/day for each day the cow is open. The costs associated with heat detection errors for first and repeat services are illustrated in Table 2. If a heat is missed for a potential first or repeat service, it will delay AI breeding for another 21 or more days; which is a greater loss than breeding a cow when she is not in heat ($42 versus $12). However, the bottom line worsens when we consider the costs associated with heat detection errors at repeat breedings. Making decisions when to rebreed a cow in heat, which has been previously AI-bred, requires extra good judgment. Table 2 illustrates that when the cow is open and we fail to rebreed her when in heat, the cost is $42 or the same as that for first-service cows. This is also true for cows that we mistakenly AI breed when not really in heat; the loss is equal to the cost of a straw of semen ($12). Realistically, it should be pointed out that the costs/day of an extra day open increase as days in milk increase. Therefore, the losses of delayed rebreeding of cows are likely more costly than those made at first services.
Table 2. Potential heat detection costs for first and repeat services Error Service number Pregnancy status Costs
Error of omission: Failure to AI breed cows in heat 1st or repeat Open 21 d × $2 = $ 42
Error of commission: AI breed cows not in heat 1st or repeat Open 1 straw × $12 = $ 12
Error of omission: Failure to AI breed cows in heat Repeat Pregnant No error = $ 0
Error of commission: AI breed cows not in heat Repeat Pregnant 2 straws × $12 +
42 d × $2 = $108
Source: M. DeJarnette (3).
The most costly error is when the cow is pregnant and we rebreed her (error of commission); the costs are increased 2.5 to 9 times! The increased cost is associated with wasting one straw of semen in the cow that is AI-bred when not in heat plus another straw is wasted because of the potential for aborting the previous conception. In addition, 42 or more days are lost from the previous fertile service until the cow can be rebred at the next heat.
To avoid aborting a previous pregnancy when attempting to AI breed a cow in heat, one should only pass the tip of the AI gun to about mid-cervix and then deposit the semen there. This reduces the potential for aborting a pregnancy, but still gives the cow a chance of settling if she is open and really in heat.
Semen Handling Techniques
Another cause of fertilization failure is improper handling of semen. Failure to follow recommended procedures for retrieving, thawing, and protecting straws until safely inside the cow results in damaged sperm membranes, cold- and heat-shocked sperm, or impaired sperm motility. The final result is an overall reduction in the fertile life of the frozen-thawed sperm in the straw. Figure 1D illustrates how reduced fertile sperm life can narrow or restrict the window of opportunity for proper timing of insemination. To maintain maximum fertilization rates, it is essential that recommended semen handling techniques be followed: 1) when removing straws for thawing, prevent exposure of other straws by keeping them below the frost line of the tank; 2) thaw straws in water at 37C (95F) for at least 40 sec; and 3) once thawed, provide thermal protection to the breeding unit in the French gun by keeping it at near body temperature until the semen is deposited in the female.
Actual insemination technique may or may not be a major factor contributing to failure of fertilization. Improper placement of the semen in the reproductive tract can be a limiting factor when the technician is unsure where the tip of the breeding gun is placed upon deposition of semen. Research demonstrated that fewer numbers of motile sperm gain access to the oviduct when semen is placed in the cervix. The target for insemination is the uterine body, although when in doubt, deposition of the semen slightly into one or both uterine horns is less likely to compromise fertility than when placed only in the cervix. Because approximately 85 to 90% of the inseminate is expelled from the female by retrograde flow, it is critical that all of the semen be placed in the uterus. Graham (5) demonstrated that errors in semen placement are common among professional technicians. Below-average technicians only placed the inseminate in the target site (body of uterus) about one-third of the time compared to 85.7% accuracy by above-average technicians (Table 3). Nearly 25% of the time, semen was not even placed in the uterus by below-average technicians. Few cows will conceive when the semen is placed in the vagina. The difference in semen placement between the right and left horns by below-average technicians is probably due to more right-handed (hold the French gun with their right hand) than left-handed breeders.
Table 3. Site of semen deposition by professional technicians and fertility.
AI technician's ability
Site of deposition Below average, % Above average,%
Vagina-cervix 23.5 0.0
Body of uterus 29.7 85.7
Right horn 42.4 14.3
Left horn 4.4 0.0
Source: E. F. Graham (5).
Fertilization Losses and Embryonic Death
Table 4 illustrates the potential outcome of inseminating 100 lactating dairy cows once each. Within 24 hr of insemination, about 81 cows would have fertile eggs. In other words, 19 potential pregnancies were lost due to some type of ovulation disturbance or fertilization failure. These losses are caused by many factors including lost eggs, ovulation failure, blocked oviducts, breeding too soon or too late relative to the onset of estrus, abnormal eggs, low fertility, poor quality semen, and other unknown causes. Lower fertilization rates generally are observed in older cows than in younger cows. In heifers, fertilization rates may approach 100%.
Once the eggs are fertilized, the next obstacle is loss of the early embryo that occurs during the cleavage stage of pregnancy. By three days after fertilization, the fertilized egg will undergo at least three cell divisions to produce an eight-cell egg. During this time, the embryo is free-floating in the lower oviduct or isthmus before it enters the uterus at day 3 or 4. Cell divisions continue normally as the morula forms (32 or more cells) and eventually the blastocyst forms by day 7 or 8. It is at this stage (late morula or early blastocyst) where embryos are recovered from donor cows for transfer to recipient females. This is a critical period of development where early losses can occur. Sometime around day 5 to 7, just after the embryo enters the uterus and when the morula is developing into the blastocyst, is where early embryo death (EED) often occurs. These losses, in addition to fertilization failures, are significantly greater in repeat-breeding heifers and cows. Losses of embryos at this early stage of pregnancy generally are not detected because the cow returns to estrus at a regular interval of about three weeks.
Table 4. Fertilization and embryo losses in cattle.
100 cows AI-bred ONCE each
81 cows with fertilized eggs
Calving results: Pregnancy losses:
29 heifers 15 embryos
30 bulls 6 fetuses
1 set of twins 21 lost calves
(61 calves from 60 cows) (25% of cows with fertile eggs)
Source: D. Olds, unpublished data.
The next critical stage is around day 15 or 16 when the embryo must be developed sufficiently to override the spontaneous uterine secretion of prostaglandin F2, which normally causes the corpus luteum to die to initiate another period of heat and a new estrous cycle. This process is called the maternal recognition of pregnancy. Secretion of various substances (proteins and prostaglandins) by the developing embryo and its rudimentary placenta (trophoblast) are involved in maintaining the successful continuation of pregnancy. Losses at this stage often are detected because the cow will generally return to estrus after an extended period (>24 days or longer than a normal estrous cycle). In other words, if the embryo establishes the pregnancy signal but dies, then the next estrus would occur in several days to a week, resulting in what appeared to be an abnormal interestrous interval.
Additional embryonic losses can occur during the period of 25 to 40 days after insemination. These so-called late embryonic deaths (LED) probably occur as a result of some failure in the attachment process of the developing placenta to the uterine wall. These fragile connections of developing placental cotyledons (buttons) to the uterine caruncles of the endometrium are similar to Velcro®-like tissue interdigitations that will accommodate transfer of gases, nutrients, and waste products between the uterus and the developing calf. We (11) have measured LED loss and found that about 13% (16 out 122 cows) of the embryos died between day 28 of pregnancy (using transrectal ultrasonography) and day 40 to 50 when pregnancy was diagnosed by palpation of the uterus.
Various research studies have documented the range of EED to be 7 to 22%, whereas LED were in the 5 to 10% range. In summary, Table 4 shows that approximately 15 embryos (EED + LED) are lost (18%). Following palpation, there may be further fetal losses representing about 6 out of 81 cows (7%). In total, 21 abortions occur, representing about 25% of all conceptions (21 out of the 81 original fertilized eggs). Losses associated with either fertilization failure, embryonic or fetal deaths reduce the potential from 100 to 60 pregnancies. What that leaves is about 61 calves born to 60 cows, depending on the number of twins that occur. Table 4 shows 29 heifers, 30 bulls, and one set of twins. However, twinning may be higher because since this study was done, the twinning rate in Holsteins has increased to about 3 or 4%.
Remember that the 60 cows that calve are the result of only ONE insemination per cow in the original 100 cows. In other words, the potential first-service calving rate is about 60%. This is different from a first-conception pregnancy rate that can be determined at the time of pregnancy diagnosis by palpation because there will be some fetal losses occurring after pregnancy diagnosis. Also remember that many of those cows that either failed to conceive (fertilized egg) or had an early pregnancy loss can be reinseminated to achieve a viable and successful pregnancy. By so doing, the cumulative herd pregnancy rate can reach about 85 to 90%, depending on their culling rate.
Measuring Pregnancy Rate
Several factors determine the number of pregnancies or the number of calves born (herd fertility, technician ability, sire fertility, and heat detection rate). Another way to examine the success of the insemination program is to determine the number of cows that become pregnant during each 21-day period after the end of the rest or elective waiting period (EWP). This concept is one that several individuals have suggested to be a new measuring stick for success of AI-breeding. Let's examine the two major factors that are involved in determining this 21-day pregnancy rate.
The pregnancy rate (PR) equation can be simplified to two factors: heat detection rate (HDR) and conception rate (CR). Heat detection rate is the proportion of cows detected in estrus and inseminated that were synchronized for program breeding. Conception rate is the number of cows detected in estrus and inseminated that became pregnant. Pregnancy rate is the proportion of cows that became pregnant that were synchronized or PR = HDR × CR. Conception rate is determined by herd and sire fertility plus inseminator proficiency. Using the simplified method of determining pregnancy rate, one can evaluate the success of the AI-breeding program, including the programmed breeding system used to synchronize heats for first services after calving. Let's assume a 50-day EWP. Because no cows are AI-bred until after 50 days, 100% of the fresh cows are open at that time. By using a programmed breeding system to synchronize heats before first services, a group of eligible cows in the breeding pool are synchronized, heat checked, and AI-bred in the first week after the end of the EWP. Of those inseminated, a certain percentage of cows will conceive and the remaining cows will repeat to estrus for rebreeding in about 21 days. By graphing the percentage open, we can generate a curve that looks similar to those in Figure 2. At 50 days in milk, 100% of the fresh cows are open, and following heat synchronization and first services, and as a result of inseminations at repeat heats, the percentage of open cows decreases with each subsequent estrous cycle or 21-day period.
The downward slope of those generated curves (pregnancy rate) in Figure 2 are determined by the number of cows detected in heat and their resulting conception rate after each AI-breeding. In Table 5, a few examples of various heat detection and conception rates are shown to illustrate their effect on pregnancy rate during each 21-day period. In the first four examples, holding heat detection rate constant at 60%, conception rates are varied from 30 to 60% (low to high). The resulting pregnancy rates range from 18 to 36%, or in other words, 18 to 36% of the cows detected in heat and inseminated became pregnant during each 21-day period. In the last four examples in Table 5, conception rate was held constant at 50% and heat detection rate varied from 40 to 70% (low to high). Resulting pregnancy rates ranged from 20 to 35%. The number of pregnancies achieved during each 21-day period, or after each additional estrous cycle, can be affected by varying rates of conception and heat detection. For example, one herd could achieve a 24% pregnancy rate with better than average heat detection (60%) and an average conception rate (40%), whereas another herd could achieve a similar pregnancy rate (25%) with average heat detection (50%) and better than average conception rate (50%).
Using the first example (first four lines of data in Table 5), four curves are plotted in the Figure 2. The upper curve represents an 18% pregnancy rate for each 21 days. The remaining curves represent 24%, 30%, and 36% pregnancy rates. Draw a horizontal line across the graph at the 50 percentage open mark to approximate the average days open for each pregnancy rate curve.
Table 5. Examples of 21-Day pregnancy rates.
Heat detection rate Conception rate Pregnancy rate
HDR × CR = PR
60 × 30 = 18
60 × 40 = 24
60 × 50 = 30
60 × 60 = 36
40 × 50 = 20
50 × 50 = 25
60 × 50 = 30
70 × 50 = 35
Using this method of evaluating the number of pregnancies established during each 21-day period after the end of the EWP, one can see how improvements in either heat detection rate or conception rate, or both, can increase the number of pregnancies achieved in the AI-breeding program. Because conception rate is easily determined, one can also estimate the herd heat detection rate by dividing the pregnancy rate for each 21-day period by the conception rate during that same period.
This example points out the importance of approaching the challenge of getting cows pregnant in a timely fashion before their days in milk are so far out that establishing pregnancy for another lactation becomes futile. Examine how one can either increase the rates of heat detection or conception. By improving these two measures, a higher 21-day pregnancy rate can be achieved, and as well as fewer days open and future reproductive culls.
Reducing Embryonic Losses
To reduce problems associated with nutritional stressors, feed complete diets with particular attention to energy and protein. Feeding too much digestible intake protein can cause several problems. Excess protein fed above that which is needed by the rumen bugs is converted to ammonia and is belched or absorbed normally into the blood stream. When it reaches the liver, most of the ammonia is converted to urea and contributes to blood urea nitrogen concentrations or BUN (measured in mg/dL). High BUN (>20 mg/dL) and blood ammonia seem to result in changes in various reproductive traits, as discussed below. High blood ammonia may suppress immune function, resulting in less resistance to disease. Excess urea is excreted in the urine or recycled to the rumen by the blood or in the saliva and can be utilized by the rumen microbes to produce amino acids necessary for bacterial cell proteins.
Excesses of amino acids relative to the amount of energy in the diet may increase negative energy balance and delay first ovulation, first estrus and subsequent insemination, and reduce pregnancy rates. This situation is more likely to occur in high-producing cows in their first lactation, which are still growing, than in older cows. Recent work at Cornell University (4) provides some evidence to substantiate the claims that high BUN concentrations in heifers, fed differing diets of normal (15.4%) or high (21.8%) crude protein for 4 weeks before insemination, reduce fertility. The diets were formulated for 1.75 of daily gain and composed of equal amounts of corn silage, legume or grass hay, and soybean meal, with urea being .5% in the normal and 3.2% in the high protein diet. Both diets met undigestible intake protein requirements and the amount of digestible intake protein for heifers fed the normal protein diet was at recommended levels, but for heifers fed the high protein diet, digestible intake protein exceeded recommendations (by 50%).
Based on these diet formulations, one would anticipate that the high protein diet would produce high concentrations of BUN and reduce pregnancy rates. Pregnancy rates were reduced (61 vs 82%) and concentrations of BUN (23.6 vs 17.5 mg/dL) were elevated in the high protein diet as predicted. It also was demonstrated that uterine pH on day 7 of the estrous cycle (the stage during pregnancy when early embryo loss generally occurs) was decreased (more acidic) regardless of source or degradability of the excess protein fed in the diet. Therefore, feeding excess crude protein, particularly when there may be inadequate fermentable energy supplied to the rumen, alters adversely the uterine environment and may be linked to infertility.
Other tips to prevent nutritional stressors: 1) use forage analyses to allow proper assessment of nutrients derived from forage sources; 2) use body condition scoring to evaluate adequacy of nutrition (cows should score in the 3.5 to 3.75 range at calving time); 3) promote dry matter intake with adequate, frequent feeding of fresh, high-quality feeds.
Provide adequate shade and ventilation for cows at the feed bunk as well as in areas of permanent shelter to stimulate feed intake. Provide plenty of cool, fresh water, free of nitrates. Increase energy density of diets in the summer to compensate for heat- and humidity-induced decrease in dry matter intake. Avoid over-crowding of cows, especially in the holding areas while cows await entry into the milking parlor. Minimize unnecessary physical activities of cows by providing shade, feed, and water in close proximity.
Establish a sound preventive-herd health program with your veterinary practitioner. Vaccinate routinely for brucellosis, leptospirosis, and other diseases prevalent in your geographic area. Isolate new cattle that are brought into your herd until blood tests reveal that they are not carriers for various disease organisms. Avoid using a herd breeding bull because of the potential for him to become infected by sexual contact and then infect other cows. Practice cleanliness and sanitation when assisting cows with dystocia and twins.
Some bull studs and dairy record processing centers rate the fertility of AI sires by reporting nonreturn rates of sires or providing a ranking of fertility. Use above-average bulls during peak periods of heat or other stress. If a herd bull is used during the summer months, they are susceptible to heat stress as well as the cows. Some bulls exposed to even short (48 hr) periods of heat stress (>85F) have markedly reduce semen quality that may last for more than 4 to 5 weeks after the end of the heat-stress period. Long-term heat stress will reduce motility, sperm quality, and sex drive (mounting activity) of the bull. Even when sires at bull studs are maintained in air conditioned facilities or well-ventilated barns during summer, the quality of semen often is compromised to some degree. Known above-average fertility AI sires may help decrease the magnitude of stress associated with declining fertility.
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Smith, M.W. and J.S. Stevenson. 1995. Fate of the dominant follicle , embryonal survival, and pregnancy rates in dairy cattle treated with prostaglandin F2 and progestins in the absence or presence of a functional corpus luteum. J. Anim. Sci. 73:3743-3751.
Stevenson, J.S., G.C. Lamb, D.P. Hoffman, and J.E. Minton. 1997. Interrelationships of lactation and postpartum anovulation in suckled and milked cows. Livestock Prod. Sci. In press.
Vailes, L.D. and J.H. Britt. 1990. Influence of footing surface on mounting and other sexual behaviors of estrual Holsteins. J. Anim. Sci. 68:2333-2339.