Effects of replacing soybean meal with pumpkin seed cake and dried distillers grains with solubles on milk performance and antioxidant functions in dairy cows

Pumpkin seed cake (PSC), a byproduct of pumpkin seed oil processing, is used in ruminant feed as a beneficial protein source. Experiments were conducted to evaluate PSC as a substitute for soybean meal in the diets of lactating cows based on performance, rumen fermentation, antioxidant function and nitrogen partitioning. Six multiparous lactating cows were used in a replicated 3 × 3 Latin square experiment with 27-day periods. The cows were randomly divided into three treatment groups: group (1) was fed a diet containing no PSC (0PSC), and groups (2) and (3) were fed diets in which soybean meal was replaced with PSC and dried distillers grains with solubles (DDGS) at levels of 50% (50PSC) and 100% (100PSC), respectively. The diets were isonitrogenous and contained identical roughage but different proportions of PSC and DDGS. Replacement of soybean meal with PSC and DDGS did not influence rumen degradation, milk performance, rumen fermentation, DM intake or apparent total tract digestibility, and nitrogen partitioning between milk, feces and urine did not differ in the animals fed the three diets. However, compared with a diet containing no PSC, the total antioxidant capacity (P < 0.05) and antioxidant enzymes (total superoxide dismutase, glutathione peroxidase and catalase) activities (P < 0.05) were increased in the animals that received the 50PSC and 100PSC diets. In contrast, addition of PSC significantly reduced concentrations of aspartate transaminase (P < 0.05), alkaline phosphatase (P < 0.05) and malondialdehyde (P < 0.05) in the plasma. These results demonstrate that PSC can be completely substituted for soybean meal in the diet of dairy cows without any negative impact on milk performance, rumen fermentation or apparent digestibility and that this dietary change improves antioxidant functions and blood parameters in dairy cows, indicating that PSC has the potential for use as a feed source for dairy cows.