Dairy cattle fertility is determined largely by environmental factors but the genetic component should not be ignored.
The decline in fertility in dairy cattle that has occurred in the last 50 years was caused in part by changes in genetic composition of dairy breeds.
Crossbreeding can result in an improvement in fertility and longevity and a decrease in milk yield.
The profitability of crossbreeding will vary from farm to farm; good estimates of the economic value of crossbreeding have not been made.
A genetic evaluation tool for reproduction now exists in the form of Daughter Pregnancy Rate and one can expect that daughters of bulls with high Daughter Pregnancy Rates will be more fertile that contemporary herd mates.
Today’s US dairy cows produce twice the milk of cows in 1957. This improvement results from genetic selection for milk yield, improved feeding practices and other management changes. Unfortunately, reproductive function has declined steadily for most of that period, with only a slight, recent recovery. The reasons for the decline in fertility are complex and not completely understood. Inbreeding, lack of selection for reproduction, stress of lactation, and changes in cow environment have all been implicated. This paper discusses two effective genetic solutions to the problem of infertility in dairy cattle: crossbreeding and selection for reproductive traits.
Crossbreeding in dairy cattle is generating renewed interest due to concern over inbreeding. The inbreeding coefficient for US Holsteins and Jerseys has increased steadily from 1960 to 2007 (1960 is the ‘base’ year for U.S. inbreeding calculations; the inbreeding coefficient for animals born that year is assumed to be zero).
Cattle have about 21,000 functional genes on 30 pairs of chromosomes. As pairs, the chromosomes carry paired (2) copies of each gene– one inherited from the father and one from the mother. Inbreeding increases the likelihood that the two inherited copies of a gene are identical. The negative effects of inbreeding are seen when identical undesirable genes are inherited from both parents. An inbreeding coefficient estimates the percentage of identical genes pairs inherited by an animal. Mating selection should strive to keep inbreeding coefficients of offspring to less than 6.25%. Average inbreeding coefficient for animals born in 2007 was estimated at 5.1% for Holsteins and 7.5% for Jerseys. Thus, many animals in both breeds surpass the 6.25% threshold.
Crossbreeding takes advantage of heterosis (improved performance of offspring over the average performance of parents). Essentially, heterosis results from the opposite of inbreeding depression; chances are increased that gene pairs include a different copy from each parent. This increases the chance that a ‘better’ version is used by the animal.
Crossbreeding also provides opportunities to exploit superior fertility and health traits of certain dairy breeds. Researchers from the University of Minnesota have been studying the performance of the Holstein crosses of several of these breeds including Normande, Montebeliarde and Scandinavian Red (includes Norwegian Red and Swedish Red breeds). The most recent data are posted by the University of Minnesota at:
In these tests, crossbreeding improved fertility and longevity although milk yield decreased. Days open during the first lactation averaged 19-27 days less for the crossbred groups than for purebred Holsteins. In addition, conception rate at first insemination was significantly higher for Normande x Holstein and Montebeliarde x Holstein than for Holstein. In addition to improvements in reproductive traits, reduced stillbirths and increased cow longevity were also associated with crossbreeding. On the other hand, first lactation Holsteins produced 2,706 lb more milk than Normande x Holstein, 1,317 lb more milk than Montebeliarde x Holstein and 1,050 lb more milk than Scandinavian Red x Holstein. The milk yield advantage for purebred Holsteins increased in second lactation: the 305-day milk yield averaged 26,194 lb for Holstein, 21,863 for Normande x Holstein, 23,547 for Montebeliarde x Holstein and 23,683 for Scandinavian Red x Holstein.
Despite its beneficial effects, there are at least two concerns about crossbreeding. The first relates to the loss of heterosis when breeding crossbred offspring. Loss of heterosis can largely be prevented by the use of a three or four-breed sire rotation. Use of a three-breed rotation allows a herd to maintain 86-88% of the heterosis of the first cross and use of a four-breed rotation maintains 93-94% of the maximum heterosis possible. The second concern with crossbreeding is the loss of milk yield when compared to pure Holsteins. Whether the improvements in fertility and health traits caused by crossbreeding compensate for this loss of milk yield will vary by farm and region. Differences in milk price, feed costs, fertility, etc. mean that crossbreeding may be economical for some operations and not for others.
Selection for Reproduction – Daughter Pregnancy Rate
Until recently, there has been little interest in using genetic selection to improve reproduction in dairy cattle. The heritability of reproductive traits is relatively low compared to production traits like milk yield, so genetic improvement is more difficult. Heritability is the proportion (percentage) of the observed variation in a trait that is due to genetics. When heritability is low, it is difficult to make much progress in genetic selection because the probability that the best animal is best for genetic reasons is relatively low.
Heritability estimates for reproductive traits are 0.05 or less. In other words, less than 5% of the variation between cows in their reproductive function is caused by differences in genes. This is not surprising because reproduction is greatly affected by non-genetic factors – level of feeding, air temperature, skill of the inseminator, etc. It is a mistake, however, to conclude that low heritability means that there are no genes controlling reproduction. In fact, scientists are now finding specific genes that affect reproduction function.
In 2003, the USDA began estimating the genetic merit of bulls for reproduction. The trait used is Daughter Pregnancy Rate (DPR). This term is calculated from days open and is directly related to the proportion of females eligible to become pregnant in a 21-day period that actually become pregnant (i.e. the 21-day pregnancy rate).
The heritability of DPR is only 0.04 but work is underway to control for other factors affecting days open. This will improve the accuracy of estimates of genetic ability for reproduction. Weigel (2006) reported that the top 10% of Holstein bulls had a DPR 4.9% higher than bulls in the lowest 10%. This difference corresponds to a difference in days open of 20 days (a 1% difference in DPR is equal to a 4 day difference in days open).
Although low heritability makes progress more challenging, reproduction can be improved through genetic selection. The continual decline in DPR for US dairies from 1959 to 1995 was arrested in the latter 1990s and has been increasing since 2000. This increase is likely due to increased management attention to reproduction and the implementation of timed artificial insemination protocols.
Additionally, selection for Productive Life (first evaluated in 1994), which is highly related to reproduction, has likely contributed to recent improvements. Changes in sire and cow breeding value for DPR suggest that the long-term decline in genetic merit for fertility also stopped beginning in the mid-1990s and has since improved slightly.
Alta Genetics, which markets bulls with high breeding values for DPR, has posted data on their website supporting the idea that selection of bulls for DPR can affect cow fertility. In one dairy, fertility of two-year old daughters of Blastoff, the number one ranked Holstein bull for DPR, was compared to fertility of contemporary herd mates. Reproductive function in Blastoff daughters was much better than for contemporary controls. Caution should be used when interpreting these data – number of Blastoff daughters was low and data are not from an independently-conducted, controlled experiment.
Improvements in DPR will pay off economically. De Vries (personal communication) has estimated that a 1-percentage unit increase in 21-day pregnancy rate is worth about $20/cow/year. The value is greater when pregnancy rates are low. The marginal return on 1 lb extra milk achieved by genetic selection is about $0.10/lbs. In other words, improving 21-day pregnancy rate by 1% is equivalent economically to improving milk yield by 200 lb per cow per year.
Take Home Messages
- Dairy cattle fertility is determined largely by environmental factors but the genetic component should not be ignored.
- The decline in fertility in dairy cattle that has occurred in the last 50 years was caused in part by changes in genetic composition of dairy breeds.
- Crossbreeding can result in an improvement in fertility and longevity and a decrease in milk yield.
- The profitability of crossbreeding will vary from farm to farm; good estimates of the economic value of crossbreeding have not been made.
- A genetic evaluation tool for reproduction now exists in the form of Daughter Pregnancy Rate and one can expect that daughters of bulls with high Daughter Pregnancy Rates will be more fertile that contemporary herd mates.
Heins, B.J., L.B. Hansen, and A.J. Seykora. (2006a) Production of pure Holsteins versus crossbreds of Holstein with Normande, Montbeliarde, and Scandinavian Red. J. Dairy Sci. 89:2799-2804.
Heins, B.J., L.B. Hansen, and A.J. Seykora. (2006b) Fertility and survival of pure Holsteins versus crossbreds of Holstein with Normande, Montbeliarde, and Scandinavian Red. J. Dairy Sci. 89:4944-4951.
Weigel, K.A. 2006. Prospects for improving reproductive performance through genetic selection. Anim. Reprod. Sci. 96:323-330.