Effects of complete substitution of dietary grain and protein sources with by-products on the production performance of mid-lactation dairy cows fed diets based on barley silage under heat-stress conditions

This study evaluated the effect of replacing cereal grains and soybean meal with by-products (BY) on production performance, nutrient digestibility, ruminal fermentation, nutrient recovery, and eating and chewing behavior of moderate-producing dairy cows under heat stress conditions. Twelve multiparous Holstein cows (116.7 ± 12.01 d in milk; 42.7 ± 5.06 kg/d of milk yield; 665 ± 77 kg of body weight; mean ± SD) were used in a replicated 3 × 3 Latin square with 28-d periods (21 d for diet adaptation and 7 d for sampling and data collection). Cows were fed a total mixed ration containing a 39.2:60.8 ratio of forage to concentrate throughout the experiment. All diets were formulated to be isoenergetic and isonitrogenous, with different concentrates. Diets were 1) control diet based on cereal grains (ground corn and ground barley, plus SBM; CON), 2) sugar-rich BY (beet pulp, citrus pulp, and liquid molasses, plus canola meal; S-BY-CM), 3) cereal grain BY (rice bran, corn germ meal, wheat bran, barley sprout, and broken corn; CG-BY). Our results showed that replacing grains with BY increased neutral detergent fiber intake and digestibility, whereas starch intake, human edible energy, and human edible protein decreased. Milk yield and dry matter intake decreased in cows fed with the CG-BY diet compared with the other 2 treatments. In contrast, there were no significant differences observed between the CON and S-BY-CM diets in terms of milk yield and dry matter intake. The S-BY-CM diet increased energy-corrected milk production compared with the CG-BY diet (36.2 vs. 34.3 kg/d) but CG-BY enhanced feed conversion efficiency compared with the other 2 treatments. Although the S-BY-CM diet prolonged the eating and sorting of small particles, neither of the dietary treatments affected chewing activity or ruminal pH 4 h after feeding. Furthermore, both diets containing BY contributed to an increase in milk fat content in comparison to the CON group. Additionally, the CG-BY and S-BY-CM diets demonstrated greater performance than the CON diet in terms of human-edible feed conversion efficiency for protein and energy. The results indicated that S-BY-CM can completely replace barley and corn grain in the diet of mid-lactating dairy cows exposed to heat stress conditions without any negative effect on production and ruminal pH. However, the inclusion of CG-BY impaired dry matter intake, milk yield, and digestibility of nutrients and is not recommended during heat stress conditions.