Short communication: Interactions of milk, fat, and protein yield genotypes with herd feeding characteristics.

Data from 879 Holstein cows from 11 tie-stall herds in Pennsylvania were analyzed to determine the effects of nutritional management practices on the level of genetic expression for milk, fat, and protein yields. Environments were defined according to the amount of dry matter refusals at the end of 24h for the average cow (DMR), diet crude protein percentage (CP), and diet NE(L) concentration. Sire predicted transmitting ability (PTA) was available for all cows, whereas 775 cows were genotyped and received a molecular breeding value (MBV) for milk, fat, and protein yields. Milk, fat, and protein yields were regressed on sire PTA and cow MBV independently in addition to combined breeding values (CBV) of sire PTA and cow MBV. Four-trait animal models with fat-corrected milk yield in high and low environments plus either body weight or body condition score in high and low environments treated as separate traits were also evaluated. Regressions on sire PTA (0.31 for fat yield to 0.54 for milk yield) were significantly lower in the 5 herds that had the lowest average DMR than in the 6 herds with highest average DMR (0.82 for fat yield to 1.11 for protein yield). The regressions of milk and protein yield on CBV were also significantly lower in the 5 herds with low NE(L) concentration in the ration than in herds that had high NE(L) concentration. Genetic correlations from animal models showed that large cows were more affected by low DMR, CP, and NE(L) concentration than smaller cows. Efforts to minimize feed wastage must ensure that cows receive adequate nutrient intake to avoid suppression of genetic potential for yield, particularly for larger cows.