Document Type : Original Research Article (Regular Paper)
Department of Animal Science, College of Agriculture, Isfahan University of Technology, Isfahan 84156-83111, Iran.
This study investigated the effects of replacing soybean meal (SM) with canola meal (CM) and decreasing crude protein (CP) levels in CM based diets on performance of dairy cows when different sources of processed grains and CM were used. Canola meal was untreated or roasted at 130°C for 30 minutes and grains sources (corn and barley) were either ground or steamed-flaked. Eight Holstein cows (2nd lactations; 42 ± 2 d in milk; 600±20 kg body weight) were used in a 4×4 Latin square design with 4 treatments. Treatments were a control diet based on SM containing 16.5% CP (SM), or 1 of 3 diets based on CM as (1) untreated CM + ground grain sources containing 17.7% dietary CP (CM-17.7), 2) treated CM + ground grain sources containing 16.5 % dietary CP (CM-16.5), and 3) treated CM + steam flaked grain sources containing 15.3 % dietary CP (CM-15.3). Diets were iso-energetic, however, the estimated metabolizable protein (MP) and Lys: Met ratio were the highest in the SM diet but both values were gradually decreased with reducing dietary CP in the CM diets. Feeding CM diets led to similar intake, milk yield and efficiency, as well as nitrogen (N) efficiency (milk N/N intake) as compared to SM diet. However, milk fat content tended to be lower in SM diet than in CM diets. Digestibility of dry matter (DM) and neutral detergent fiber (NDF) was lower in cows fed CM-17.7 than those fed either SM, or CM-16.5, CM-15.3 diets. Decreasing dietary CP in the CM diets had no significant effects on DM intake, milk production or milk composition, whereas N efficiency was linearly increased. Different diets or dietary CP level had no significant effect on plasma parameters. In conclusion, the present study indicated that replacing SM with CM resulted in similar milk yield and efficiency. Decreasing dietary CP from 17.7 to 15.3 % by feeding a mixture of treated CM and steam flaked grain sources did not affect milk production but improved N efficiency of dairy cows in early lactation.