Abstract
Previous research has identified that Holstein-Friesian dairy heifers with positive (POS) genetic merit for fertility traits (FertBV) reach puberty earlier than heifers with negative (NEG) FertBV. The hypothalamus-pituitary-gonadal (HPG) axis is functional in heifers before the onset of puberty, with increased LH release evident as heifers progress toward puberty. We investigated the functionality of the HPG axis in peripubertal Holstein-Friesian dairy heifers with divergent POS or NEG FertBV, hypothesizing that the earlier puberty onset of POS heifers is associated with earlier activation of the HPG axis than in NEG heifers. In experiment 1, we tested the dose responsiveness of POS heifers to an intravenous injection of either kisspeptin [Kiss; 2, 4, or 8 µg/kg of body weight (BW); n = 3 per dose] or a GnRH agonist (buserelin; 5, 10, or 20 ng/kg of BW; n = 3 per dose). The use of these 2 agonists investigates the status of the HPG axis in both the hypothalamus (Kiss) and pituitary (buserelin) glands. Doses of 4 µg/kg BW of Kiss and 10 ng/kg BW of buserelin produced submaximal LH responses and were used in experiment 2, in which previously unused POS (n = 22) and NEG (n = 18) FertBV heifers were challenged with both agonists at 10 and 12 mo of age in a partial crossover design. Heifers were randomly allocated to treatment groups, balanced for age and BW. The LH response to buserelin was greater in POS heifers than NEG heifers at 10 mo of age, with no difference in response at 12 mo. The FSH response to buserelin and the LH and FSH responses to Kiss did not differ between the POS and NEG heifers at either age. These results indicate an association between divergent genetic merit for fertility and the LH release to buserelin at 10 mo of age, supporting the hypothesis that gonadotropin responsiveness to a GnRH agonist is more advanced in POS heifers than in NEG heifers.
Highlights
Dairy cow fertility is a major determinant of farm profitability in dairy systems that are dependent on seasonal calving (Esslemont et al, 2001; Shalloo et al, 2014; Kerslake et al, 2018)
Previous work has indicated that age at puberty is associated with genetic merit for fertility traits and may be a useful phenotype for genetic selection (Meier et al, 2021b)
We assessed whether the earlier onset of puberty in heifers with POS compared with NEG fertility breeding value (FertBV) (Meier et al, 2021b) is associated with earlier activation of the HPG axis
Summary
Dairy cow fertility is a major determinant of farm profitability in dairy systems that are dependent on seasonal calving (Esslemont et al, 2001; Shalloo et al, 2014; Kerslake et al, 2018). Global declines have occurred in both phenotypic and genetic fertility of dairy cows (Berry et al, 2014; Pryce et al, 2014). Many dairy sectors have responded with an increased emphasis on functional, health, and fertility traits in genetic evaluation, in an effort to reduce or reverse this trend (Egger-Danner et al, 2015). Improving the rate of genetic gain in fertility is a challenge because of the low heritability of relevant traits and the greater focus on production traits within breeding objectives (von Keyserlingk et al, 2013). An increased rate of genetic gain for fertility could be achieved by incorporating traits into the calculation of genetic merit that (1) positively correlate with fertility, (2) have a greater heritability than factors currently considered, and (3) can be evaluated early in life. Previous work has indicated that age at puberty is associated with genetic merit for fertility traits and may be a useful phenotype for genetic selection (Meier et al, 2021b)
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