Optimal protein nutrition of dairy cattle is important to support efficient production of high producing lactating dairy cows. Canola meal (CM) and corn dried distillers’ grains with solubles (DDGS) have been primary protein meals on many dairy farms in the Western USA for over 10 years, but decreases in the relative price of soybean meal (SBM) has created interest in how best to include it in lactation diets containing CM and DDGS. Three pens of ˜310 high producing multiparous Holstein cows were fed diets supplying 0, 35 or 70 g/kg SBM on a dry matter (DM) basis, which largely replaced CM, in a way that held the proximate nutrient content of the diets constant. Diets were rotated through the pens in a Latin square design such that each pen of cows experienced each diet once. Milk and component yields were lowest at the highest SBM level (Quad P < 0.01) and body condition score change shifted from a net gain with the 0 g/kg SBM diet to a net loss with both SBM inclusive diets (Quad P = 0.07). Apparent digestibility of neutral detergent fiber, organic matter and crude protein (CP) were highest with the 35 g/kg SBM diet (Quad P < 0.04), but rumen microbial CP output was not treatment impacted. Plasma amino acids were judged to be within normal ranges for all treatments, but there was a 20% decline (Quad P = 0.04) in Met concentrations between the 35 and 70 g/kg SBM diets. When SBM was added to diets with a base level of DDGS (˜75 g/kg diet DM), largely in substitution for CM, there was a reduction in performance, particularly with the 70 g/kg SBM diet.
While no unified hypothesis fully explains all results, the most likely reason for this decline in performance with the highest dietary addition level of SBM was judged to be a reduction in Met as a proportion of metabolized AA, which may have created a Met limitation to animal production.