Data from 879 Holstein cows from 11 herds in Pennsylvania were recently analyzed to determine the effects of nutritional management practices on the level of genetic expression for milk, fat and protein yields. Results from this study were published in the March 2012 Journal of Dairy Science.

To set the stage, environments were defined according to the amount of dry matter refusals (DMR) at the end of 24 hours for the average cow, diet crude protein percentage (CP), and diet NEL concentration. Sire predicted transmitting ability (PTA) was available for all cows; also 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.
Results show the regressions on sire PTA (0.31 for fat yield to 0.54 for milk yield) were significantly lower in the five herds that had the lowest average DMR than in the six 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 five herds with low NEL concentration in the ration than in herds that had high NEL concentration.
Genetic correlations from animal models showed that large cows were more affected by low DMR, CP, and NEL concentration than smaller cows.
Therefore, researchers recommend that management efforts you take to minimize feed waste must still ensure that cows receive adequate nutrient intake so you don’t suppress genetic potential for yield, particularly for larger cows.

Source: April 2012 Dairy Cattle Reproduction Council Newsletter