Reproductive performance of grazing dairy cows genetically divergent in genetic merit for feed intake

Abstract

Feed-related costs represent one of the largest variable costs in dairy herds, and, consequently, many countries now include explicit measures of feed intake or feed efficiency in their dairy total merit indexes. Nevertheless, there is a paucity of research investigating the effects on reproductive performance from genetic divergence on feed intake and feed efficiency within the framework of a total merit index. Given that the repercussions of compromised reproductive performance in pasture-based dairy systems are severe, the objective of the present study was to characterize the reproductive performance of grazing dairy cows stratified on their EBV for energy intake. After edits, 14,451 net energy intake (NEI) records from 5,060 lactations on 2,521 cows were available from 5 separate Irish research farms; reproductive records were available for 2,508 of these cows across 9,788 lactations. The cows were a mixture of Holstein-Friesian, Jersey, Montbéliarde, Norwegian Red, and their crosses. Estimated breeding values for NEI and NEI adjusted for energy sinks (NEI_adj) were derived using phenotypic data from 2 reference datasets based on a selection of years or a selection of farms; this approach was chosen to reflect practical genetic evaluation scenarios where EBVs are estimated from historical data or data from a limited number of farms but applied to animals born in the future or in different herds. Cows in the test datasets based on year of calving or herd were individually stratified, within experimental treatment by year and season of calving, into 4 strata based on their within-breed EBV for NEI, as well as separately for NEI_adj. Subsequently, predicted marginal means for calving interval, interval from calving to first service, and number of services were estimated within each EBV stratum of the test datasets using linear mixed models; logistic regression models were used to generate predicted probability estimates for a series of pregnancy and submission binary traits. In both test dataset scenarios (by year of recording and by farm), differences between NEI and NEI_adj EBV strata were observed for calving interval, interval from calving to first service, 21-d submission rate, and 21-d calving rate. After correcting for multiple testing, however, differences among the strata remained for only 1 of the submission traits; in the test dataset of farm, cows with higher NEI_adj EBVs were more likely to be served within 21 d of the breeding season, provided they had calved at least 30 d earlier. The findings of the present study suggest that, within the genetic variability present in the studied population, selection for NEI or NEI_adj would, in the short term at least, likely have minimal influence on the reproductive performance of grazing dairy cows, with the possible exception of submission rate.