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Genetics are inextricably linked to reproductive physiology and performance of dairy cows. In this context strong associations have been reported for genomic enhanced PTA for fertility and reproductive outcomes (PTA meaning the predicted difference of a parent animal’s offspring from the average, due to genes transmitted from that parent). On reviewing the literature in this context, the authors cited therefore hypothesized that cows with superior fertility potential would have better reproductive performance than cows with inferior fertility potential, regardless of the reproductive management programme used. To test this hypothesis, they designed a large trial with the objective to compare the reproductive outcomes of first-lactation lactating Holstein cows of different genetic merit for fertility. These cows were submitted for insemination with treatments that prioritized artificial insemination (AI) at detected oestrus (AIE) or timed AI (TAI). A further objective was to establish whether subgroups of the fertility cow groups would respond differently to these reproductive management programmes.
Cows from six farms were stratified into high, medium and low fertility groups based on a Reproduction Index value calculated from multiple genomic-enhanced predicted transmitting abilities. Within herd and fertility groups, the cows were randomly assigned either to a programme prioritizing TAI and had an extended voluntary waiting period (P-TAI), or AIE (P-AIE), but used TAI, not AIE. The cows in P-TAI received their first service by TAI at about 84 days in milk after a Double-Ovsynch protocol, were AIE if detected in oestrus after a previous AI, and received TAI after an Ovsynch-56 protocol at about 35 days after a previous AI, if a corpus luteum was visualized at non-pregnancy diagnosis about 32 days after AI. Cows with no corpus luteum visualized at non-pregnancy diagnosis received TAI at about 42 days after AI after an Ovsynch-56 protocol with progesterone supplementation (P4-Ovsynch). Cows in P-AIE were eligible for AIE after a PGF2αtreatment at about 53 days in milk and after a previous AI. Cows not AIE by about 74 days in milk or non-pregnancy diagnosis by about 32 days after AI, received P4-Ovsynch for TAI at about 74 days in milk or about 42 days after AI.
The results showed that pregnancy per AI to first service was greater for cows in the high fertility group (60%) than the medium (54%) and low fertility group (48%), and for the P-TAI (59%) versus the P-AIE (49%) treatment. Overall, pregnancy per AI for all second and subsequent AI combined did not differ by treatment or fertility group. The ‘hazard of pregnancy’ after calving was greater for the P-AIE than the P-TAI treatment, and for the high fertility group compared to the medium and low fertility groups. More cows in the high fertility group (91%) than the medium (88%) and low fertility group (86%) were pregnant at 200 days in milk. Within fertility group, the ‘hazard of pregnancy’ was greater for the P-AIE than the P-TAI treatment for the high and medium fertility groups, but not for the low fertility group.
It was concluded that first-lactation Holstein cows of superior genetic merit for fertility had better reproductive performance than cows of lesser genetic merit for fertility, regardless of the type of reproductive management used. In addition, the effect of programmes that prioritized AIE or TAI on reproductive performance for cows of superior or inferior genetic merit for fertility depended on the outcomes evaluated. This implies that AIE or TAI can be used to invoke particular outcomes wanted.