Importance of considering body weight change in response to dietary protein deficiency in lactating dairy cows

Abstract

Our objective was to quantify the contribution of body weight (BW) change to the overall response of lactating dairy cows to a shortage of dietary protein. Lactating Holstein cows (n = 166; 92 primiparous, 74 multiparous) with initial milk yield of 41 ± 10 kg/d were fed high and low-protein diets in 7 blocks. Blocks were repeated in the same crossover design with periods of 28 to 35 d. Production of 69 of the 166 cows (42 primiparous, 27 multiparous) was also measured in late lactation. Low-protein diets were 14% crude protein (CP) in peak lactation and 13% CP in late lactation and were formulated to contain adequate rumen-degradable protein to maintain rumen function but inadequate rumen undegradable protein for the average cow in this study. High-protein diets were 18% CP in peak lactation and 16% CP in late lactation and contained extra expeller soybean meal to meet metabolizable protein requirements. Body weight changes were used to predict body energy and protein changes, which were added to milk components to calculate total captured energy and protein. Fixed effects of diet, parity, treatment sequence nested in each block, treatment period nested in block, interaction of diet and parity, and the random effects of block and cow nested within block were included in the model to compare cow responses to diets within each lactation stage. In peak lactation, reducing protein from 18 to 14% resulted in estimated daily losses of 2.9 Mcal of milk energy, 2.2 Mcal of body energy, 127 g of milk protein, and 16 g of body protein. Therefore, BW loss accounted for 43% of the decrease in captured energy and 11% of the decrease in captured protein when cows were fed deficient protein. In late lactation, BW loss accounted for 51% of the decrease in captured energy and 14% of the decrease in captured protein when cows were fed deficient protein. We suggest that BW change should be considered when assessing cow responses to changes in dietary protein.