This study investigated whether the interaction of protein level and grain type can affect milk production, nutrient digestibility and rumen fermentation in primiparous Holstein cows. Four dietary treatments were used: high-protein with barley as the only grain source, HP-B; (2) high-protein with an equal mix of barley and maize, HP-BM; (3) low-protein with barley as the only grain source, LP-B and (4) low-protein with equal proportions of barley and maize, LP-BM. High-protein diets showed no improvement in milk or protein yield compared with low-protein, but barley and maize mix diets increased energy-corrected milk yield and fat yield compared with barley-only diets. The highest total apparent digestibility of dry matter, organic matter and neutral detergent fibre was observed for LP-BM whereas HP-BM showed the greatest crude protein digestibility. Treatment had no effect on total volatile fatty acid concentrations, molar proportion of acetate and propionate and acetate to propionate ratio. The lowest ruminal pH was observed for LP-B. High-protein diets resulted in greater concentrations of ammonia nitrogen (N), urinary N, blood and milk urea nitrogen compared with low-protein diets, whereas low-protein diets showed better nitrogen utilization efficiency. This study showed that primiparous lactating cows do not benefit from high-protein diets with different fermentation rates of grain sources, but barley and maize diets may improve milk production performance, ruminal fermentation and pH under the present dietary conditions. The current results on milk production performance should be interpreted with caution due to the small number of cows used (eight in each treatment).

中文翻译：

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蛋白质水平和谷物来源对初产荷斯坦奶牛产奶量、养分消化率和瘤胃发酵的影响

本研究调查了蛋白质水平和谷物类型的相互作用是否会影响初产荷斯坦奶牛的产奶量、养分消化率和瘤胃发酵。使用了四种饮食处理：以大麦为唯一谷物来源的高蛋白、HP-B；(2) 大麦和玉米等量混合的高蛋白，HP-BM；(3)以大麦为唯一粮源的低蛋白LP-B和(4)大麦和玉米等比例的低蛋白LP-BM。与低蛋白日粮相比，高蛋白日粮在牛奶或蛋白质产量方面没有改善，但与仅大麦日粮相比，大麦和玉米混合日粮增加了能量校正的牛奶产量和脂肪产量。干物质的最高总表观消化率，LP-BM 观察到有机物和中性洗涤纤维，而 HP-BM 显示出最大的粗蛋白消化率。处理对总挥发性脂肪酸浓度、乙酸盐和丙酸盐的摩尔比以及乙酸盐与丙酸盐的比值没有影响。LP-B 的瘤胃 pH 值最低。与低蛋白日粮相比，高蛋白日粮产生更高浓度的氨氮（N）、尿氮、血尿素氮和牛奶尿素氮，而低蛋白日粮显示出更好的氮利用效率。本研究表明，初产泌乳奶牛不受益于不同粮源发酵速率的高蛋白日粮，但在目前日粮条件下，大麦和玉米日粮可能会提高产奶性能、瘤胃发酵和 pH 值。