325 Genetic parameters and genomic predictions for milk yield by season in US Holsteins: Assessing genotype by environment interactions under heat stress in Texas and Wisconsin.

Abstract Genotype by environment (GxE) interactions and heat stress on dairy cattle significantly influence milk production and quality, fertility, health, and overall animal well-being. Production data collected in different environments can be used to model the effects of heat stress and understand how genetics respond to different environmental conditions using reaction norm models. This study aims to investigate the extent of heat stress using test-day milk yields in US Holstein cows in the states of Texas and Wisconsin. Data was made available from the National Cooperator’s Database managed by the Council in Dairy Cattle Breeding and the Cooperative Dairy DNA repository in both states from animals born after the year 2000. Data for the first lactation of animals with more than 5 test-day records from the largest herds were used in this study. The variance component for milk yield was estimated using an Average Information Restricted Maximum likelihood algorithm. A total of 499,496 milk test days for 60,059 animals across 15 herds in Texas and 625,626 test days for 67,187 individuals across 31 herds in Wisconsin were included in a single trait repeatability test-day model. To model seasonality associated with heat stress, data were divided into two scenarios based on months of the test day, and records were assigned to summer and non-summer. Animals with less than 2 records in each scenario were removed. Two definitions of summer months were used: May to September and June to August. For single-trait analysis with complete data, the heritability (SE) for milk production in Texas and Wisconsin was 0.34 (0.007) and 0.30 (0.006). In the two-trait approach for Texas, the May to September summer and non-summer months heritability for milk production was 0.37 (0.008) and 0.34 (0.008), respectively, with a genetic correlation of 0.92 (0.005). Similarly, for Wisconsin, the heritabilities were comparatively lower: 0.28 (0.007) for summer and 0.31 (0.006) for no-summer, respectively, with a genetic correlation of 0.94 (0.005). The restricted summer model (June–August) showed higher heritability compared to the normal summer model (May–September) in both states. For Texas, the heritabilities for milk production were 0.42 (0.01) for summer and 0.34 (0.008) for non-summer with a genetic correlation of 0.91 (0.006). Similarly for Wisconsin, the heritabilities for milk production were 0.33 (0.007) and 0.30 (0.005) with a genetic correlation of 0.87 (0.005). This suggests that a two-trait approach can be used to model seasonality associated with heat stress in test day models. The next steps of this study are to calculate a heat load function using weather station data to split the data into records collected into heat stress or thermoneutral conditions. Finally, genomic information will be used to calculate genomic breeding values and to perform a Genome-Wide Association Study.