Prospects for improving female fertility in dairy cattle via genetic selection are reviewed. Today's high-producing cows have shorter estrous cycles, fewer standing events, shorter duration of estrus, and more frequent multiple ovulations. Although high milk production is often implicated as the cause of impaired fertility, the impact of inadequate body condition appears to be greater, as the latter has a significant impact on probability of conception, rate of embryonic loss, and proportion of anestrous animals. Genetic improvement of female fertility can be achieved by indirect selection for productive life (PL) or body condition score (BCS), or by direct selection for traits such as daughter pregnancy rate (DPR). Most leading dairy countries have implemented genetic evaluation systems for female fertility in the past decade, but refinement of these systems to account for hormonal synchronization, differences in the voluntary waiting period, exposure to natural service bulls, and other confounding factors is warranted. Recent work has focused on the development of data collection and genetic evaluation systems that will allow selection of bulls that have daughters that are resistant to common health disorders, including mastitis, lameness, ketosis, displaced abomasum, and metritis. Such systems will allow selection of animals that can remain healthy and fertile while producing large quantities of milk.

The challenges associated with achieving pregnancy in modern, high-producing dairy cows have received considerable attention from scientists, veterinarians, and farmers in recent years. Todays dairy cows tend to have lower conception rate, greater days open, and greater likelihood of culling due to infertility than their counterparts from two or three decades ago. Genetic selection programs have led to rapid gains in milk yield and conformation traits; but performance for traits such as female fertility, longevity, and susceptibility to disease has tended to decline. While it is impossible to completely disentangle the effects of selection from simultaneous changes in nutrition, cow care, and reproductive management, it is clear that geneticists failed to pay adequate attention to health, fertility, and longevity traits until the past decade. The magnitude of genetic variation in such traits is surprising, and we are now poised to take advantage of recent research and development efforts regarding the definition, measurement, and genetic analysis of these traits.

The objective of this paper is to review the relationships between female fertility and other economically important dairy traits and to discuss opportunities for improving reproductive performance through direct selection of highly fertile animals or indirect selection of animals that maintain adequate body condition and resist metabolic and infectious diseases during lactation.

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Milk production of dairy cows on modern commercial farms has roughly doubled over the past four decades. First parity cows on large commercial dairy farms typically peak at 40 to 45 kg/d, while second and later parity cows typically peak at 50 to 55 kg/d. Furthermore, each group typically sustains daily milk production of 40 kg/d or more during the first seven months postpartum. Therefore, one might expect differences in the reproduction of high-producing cows, as compared with low-producing cows or yearling heifers.

Lopez et al. (2005) discussed some of the differences between the reproductive biology of lactating Holstein cows and yearling Holstein heifers. In particular, Lopez et al. (2005) noted that lactating cows have shorter duration of estrus (7 to 8 hr vs. 11 to 14 hr), longer and more variable estrous cycles (20 to 29 d vs. 20 to 23 d), larger diameter of ovulatory follicles (16 to18 mm vs. 14 to 16 mm), and greater rates of anovulation (20 to 30% vs. 1 to 2%), multiple ovulation (20 to 25% vs. 1 to 3%), and pregnancy loss (20 to 30% vs. 3 to 5%).

Lopez et al. (2005) also documented differences in these characteristics between lactating cows according to levels of milk production. They (Lopez et al., 2005) used the HeatWatch system (DDx Inc., Denver, Colorado) to monitor the estrous characteristics of 146 high-producing Holstein cows (46.4 kg/d for the 10 d preceding estrus) and 177 low-producing Holstein cows (33.5 kg/d for the 10 d preceding estrus). High-producing cows had shorter duration of estrus (6.2 hr vs. 10.9 hr), fewer standing events (6.3 vs. 8.8), and shorter standing time per event (21.7 sec vs. 28.2 sec). Duration of estrus decreased linearly from 14.7 hr for cows milking 25 to 30 kg/d to 2.8 hr for cows milking 50 to 55 kg/d. In addition, the percentage of cows with multiple ovulations increased from 0.0% for cows milking between 25 and 30 kg/d to 51.6% for cows between 50 and 55 kg/d.

The rate of early embryonic loss in Holstein cows is also a major concern, as noted in several recent studies that have used ultrasound for pregnancy detection at 27 to 31 d after breeding, followed by pregnancy confirmation via rectal palpation at 39 to 48 d after breeding. Reported rates of embryonic loss during this interval ranged from 0.70 to 1.40% per day (e.g., Cartmill et al., 2001; Cerri et al., 2004; Santos et al., 2004). However, estimates of the rate of embryonic loss (particularly those from commercial farms) may be biased upward by false positive diagnoses at the early ultrasound exam, as most veterinarians tend to use caution when declaring cows as non-pregnant in herds that use hormonal resynchronization programs.

On large western dairy farms, mean veterinary-confirmed conception rates of Holstein cows at 75 d after breeding were nearly constant over the first five inseminations (0.30, 0.31, 0.31, 0.29, and 0.28, respectively), while means for Jersey cows declined linearly from the first through fifth insemination (0.42, 0.38, 0.34, 0.29, and 0.27, respectively). Mean conception rate at first service tended to decline with age in both breeds (0.35, 0.29, 0.28, 0.26, and 0.25, respectively, for first through fifth parity Holsteins and 0.44, 0.43, 0.41, 0.39, and 0.37, respectively, for first through fifth parity Jerseys), though the rate of decline was less noticeable for repeat inseminations than for first insemination (Weigel, 2006 (unpublished)). Both breeds have been selected for many generations under similar management conditions, and both have made rapid genetic progress over the past three decades (mean mature equivalent 305 d milk yield increased from 6,904 to 11,608 kg in Holsteins and from 4,461 kg to 8,273 kg in Jerseys from 1970 to 2000). Differences in mean conception rate within the Holstein breed were found among cows at different levels of daily milk yield, but such differences were smaller than one might expect (Weigel, 2005 (unpublished)). Mean conception rates at 75 d after breeding were 0.33, 0.33, and 0.32 for primiparous Holstein cows that averaged < 27 kg/d, 27 to 36 kg/d, and > 36 kg/d, respectively, during the first 3 mo of lactation; whereas corresponding means were 0.28, 0.28, and 0.27 for multiparous Holstein cows that averaged < 36 kg/d, 36 to 45 kg/d, and > 45 kg/d, respectively. In Wisconsin Holsteins, Lopez et al. (2005) found no relationship between the percentage of cows exhibiting anovulatory condition and level of daily milk yield. The percentage of anovular cows was 27.8% for cows that were milking 25 to 30 kg/d and 26.3% for cows that were milking 50 to 55 kg/d (means for 5-kg intervals in between ranged from 21.7% to 35.1%, with no apparent trend). In California Holsteins, Santos et al. (2004) found a weak, nonsignificant relationship between milk yield and rate of embryonic loss between 31 and 45 d after breeding, with rates of 9.7% for cows that were milking 36 kg/d and 12.7% for cows that were milking 52 kg/d. Thus, it does not appear that increased milk yield is solely responsible for the decline in mean reproductive performance.

High milk production, whether achieved through genetic selection, enhanced nutrition, or improved management, is often implicated as the cause of health, fertility, and culling problems on modern dairy farms. However, a complex relationship exists between milk yield, health, and reproductive performance. High-producing cows tend to be more susceptible to metabolic disorders and infectious diseases, and these can lead to impaired fertility. On the other hand, healthy cows tend to have higher milk production and greater reproductive performance than unhealthy cows. Conversely, cows that remain nonpregnant for much of the lactation tend to achieve higher levels of total production because fewer resources are allocated to the developing calf. Thus, one must be cautious when attempting to formulate cause-effect relationships between these traits.

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Impact of Genetic Selection on Female Fertility - eXtension

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