Effects of Variation in Nutrient Composition of Diets on Lactating Dairy Cows

The nutrient and ingredient composition of diets vary over time. A survey found that within farm coefficients of variation (CV) based on monthly sampling of the TMR over a year averaged about 5% for the concentrations of maj or components (dry matter, fiber, and protein). Based on ingredient variation found in that survey, the day-to-day CV is likely similar to the month-to-month CV. We conducted a series of 3 experiments to examine the effect of variation in diet composition on lactating dairy cows. In one experiment, substantial variation over 4 day cycles in dietary concentration of long chain fatty acids (from corn oil) reduced intake and milk yield of dairy cows but moderate variation had no effect. In a second experiment, transient reductions in silage dry matter (DM) concentrations (similar to what might occur when uncovered silage was rained upon) did not negatively affect cows if adequate feed was provided to the cows each day. In the third experiment, extreme daily variation in forage quality (fiber and protein) did not affect average milk yield or average intake by cows. Cows appeared to be able to compensate for days when the diet had lower quality forage, and intake was depressed by mobilization of body energy and by consuming additional feed on days when the diet was better (i.e., lower in fiber). Overall, these data indicate that cows can handle short term (a few days) variation in many nutrients.