Genome has good indicator for fertility

For efficient outputs, modern dairies using calving throughout the year are required to have well-functioning reproduction programs that allow for adequate replacement rates, optimal milk yields and a consistent income throughout the year.

Infertility of lactating dairy cows represents a major challenge for dairy production. Decreased reproductive performance is associated with decrease in economic yields due to reduced milk yield per cow per day, lower production of replacement heifers and increased weaning.

Historically, high milk production has been associated with reduced fertility in lactating dairy cows. The decrease in fertility may be due to genetic factors as other traits (eg milk production and build characteristics) were prioritized over fertility in the selection for dairy cattle. Increased individual milk production was achieved in part due to improvements in management as well as intensive genetic selection. It has also been shown that this increase in milk production is associated with reduced reproductive performance, which has become a major concern of farmers and the dairy industry. However, the interpretation made from observed associations between greater milk yield and poor reproductive performance may require critical evaluation, as it is a complex relationship. In addition, inappropriate management practices of high-producing dairy cows can be a significant contributor to the inability of dairy cows to conceive and maintain pregnancy, regardless of milk yield.

Genomic daughter pregnancy rate (GDPR) is calculated using the risk of pregnancy of a bull’s daughters and predicts the genetic improvement in pregnancy rates for a future daughter of a bull. Better pregnancy rates are associated with increased profitability of a herd and reduced open days. An increase of one point in GDPR leads to a reduction of four days open. Improving genomic prediction for GDPR provides the opportunity to use the GDPR as selection criteria to improve fertility of lactating dairy cows. The use of the GDPR in reproductive programs has been reported to reverse the phenotypic decline in fertility in dairy cows.

In addition to conception risk (or pregnancy per AI), embryonic mortality can be considered as a reproductive performance indicator. Indeed, late embryo loss causes economic losses due to delays in establishing pregnancy. Since several reports showed embryonic mortality during the first third of pregnancy (3.2% to 42.7%), it is an important component of dairy cow production and its assessment with an indicator such as GDPR would be interesting.

A study conducted between 2018-2020 on two large herds in Brazil, entitled “Association between genomic daughter pregnancy rates and reproductive parameters in Holstein dairy cattle” was recently published in the Journal of Dairy Science. This study assessed the association between GDPR and pregnancy at first insemination, overall pregnancy per AI, pregnancy losses, and estrus behavior. A total of 12,949 events from 3,499 Holstein cows were included, from a single herd. Hair samples were collected for genetic analysis from the tail switch of each animal, with most samples taken at about 2 months of age.

Lactating cows were selected to receive either conventional or sexed semen through a sailing-fit breeding management program. Embryo transfer or bovine semen was used for cows identified as the poorest 10% of the herd in any of the following categories: GTPI or net merit values, milk production, long days in milk (DIM) or repeated breeders. Heifers were inseminated using sex semen through a paternal breeding management program for first and second inseminations, and thereafter with conventional semen thereafter.

The results of this research showed that as GDPR increased, the chance of pregnancy (at first insemination as well as overall pregnancy) increased and the chance of pregnancy loss decreased, independent of parity (heifers, first-lactation cows as well as older cows with two or more lactations; Figure 1).

Proportion of pregnancy per AI and pregnancy loss according to parity

Interestingly, cows with higher GDPR were more likely to demonstrate estrus on the day of timely AI than cows with lower GDPR. Furthermore, cows inseminated after an estrus event (ie observed in heat, either with tail chalk or an automatic activity monitor) were more likely to become pregnant, both at first insemination postpartum and subsequent inseminations, and reduced pregnancy loss than cows with lower GDPR (Figure 2).

Proportion of pregnancy per AI and loss of pregnancy by estrus on the day of AI

The association of GDPR and fertility was unchanged regardless of the breeding strategy used (i.e., receipt of embryo transfer, or beef, sexes, or conventional semen; Figure 3).

Proportion of pregnancy per AI and loss of pregnancy according to type of breeding management

The relationship between GDPR and reproductive outcomes suggests that the choice for higher GDPR may result in better reproductive performance in a herd. This is one of the first studies that was able to demonstrate and confirm this genetic marker for fertility, indeed leading to greater fertilization risk, lower pregnancy loss as well as better expression of estrus.

Read more about this study, “Association between genomic girl pregnancy rates and reproductive parameters in Holstein dairy cattle”. end point

Augusto ML Madureira

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