Efficient production by dairy cows depends on optimal protein nutrition. Canola meal (CM) and dried corn distillers’ grains (DDGS) have been primary protein meals on Western USA dairies for >10 years, but a lower relative price of soybean meal (SBM) has created interest in how best to include it in diets containing CM and DDGS. Pens of ∼315 multiparous early lactation cows were fed rations with 145 g/kg CM and 0 g/kg SBM or 65 g/kg CM and 65 g/kg SBM of dry matter (DM). A methionine (Met) diet (SBM+M) was created by addition of ruminally protected Met to deliver ∼4.6 g of absorbable Met/cow/day. Body condition (BCS) was scored, and milk yields recorded for 5 months after treatment assignment at 13.2 ± 0.11 days in milk (DIM). Blood serum was collected for progesterone (P4), E2 and amino acid (AA) analyses, 24 h prior to 1st and 2nd artificial insemination (AI). The DM intake was unaffected by treatment, but milk yield and components had treatment*DIM interactions which indicated CM cows outperformed SBM cows at earlier DIM. The SBM+M cows produced more milk and milk protein than SBM cows, but at earlier DIM, while SBM+M cows produced more milk fat than SBM cows at later DIM. Rumen microbial protein (MCP) outflow was lower when SBM replaced CM, and MCP was lower in later lactation (i.e., 109 versus 51 DIM). Cows culled for clinical mastitis, lameness and physical injury were not treatment impacted. No aspect of 1st or 2nd AI pregnancy was treatment impacted with an overall 1st AI pregnancy rate of 52/100 cows. The E2 levels in serum collected 24 h prior to 1st AI were unimpacted by treatment or subsequent pregnancy outcome. However, E2 levels were lower in cows open at 32 days post 2nd AI compared to those pregnant.
Substituting SBM for CM reduced performance in spite of no apparent Met limitation, with a likelihood that it was metabolizable protein which limited, primarily due to lower MCP outflow. Addition of Met to the SBM diet resulted in improvements, especially to milk protein content and yield, but not to the levels of the cows fed the CM ration. If phyto-estrogens in SBM interfered with E2 and P4 metabolism, it did not occur at a level which impacted pregnancy outcomes, suggesting it is of little concern to dairy farmers when fed to early lactation dairy cows, at least at this feeding level.