Zanton G I, Heinrichs A J
Department of Dairy and Animal Science, The Pennsylvania State University, University Park 16802, USA.
J Dairy Sci. 2008 Sep;91(9):3579-88. doi: 10.3168/jds.2008-1210.
Eight rumen-cannulated Holstein dairy heifers [beginning body weight (BW) 340 (+/-5) kg and age 14.5 (+/-1) mo] were fed a high forage diet at 4 levels of intake. Diets were composed of grass silage, grass hay, and corn silage as the forage components and offered at 1.25, 1.50, 1.75, and 2.00% of BW to heifers in a replicated Latin square design. Diets were incubated in situ in heifers receiving all 4 levels of feed. Blood and rumen were sampled at 2-h intervals for 24 h, rumen contents were emptied, and total fecal and urine collection was made. Dietary intake increased in proportion to feed offered until dry matter intake (DMI) was 1.92% BW, after which a statistically determined plateau was evident due to greater refusals when feed was offered at 2.00% BW. In situ degradation of feed was not affected by intake level, which, combined with the greater turnover rate of rumen contents, leads to the inference that rate of passage was increased with increasing intake. Rumen pH decreased and rumen volatile fatty acid concentration and microbial protein flow to the small intestine (estimated using urinary purine derivative excretion) increased as intake increased. Manure excretion increased as DMI increased at a rate 2.54 times greater than increases in DMI; this increase was entirely due to greater excretion of wet feces because urine excretion did not change with intake level. Nitrogen digestibility decreased and N retention increased linearly as the level of feed offered increased. Efficiency of N retention was minimized when feed was offered at 1.25% BW; all levels of feed offered above this level resulted in equivalent efficiencies. From the results of this experiment it can be concluded that when dairy heifers are limit-fed a high forage diet, the efficiency of nutrient utilization is increased as intake decreases, but reducing DMI below 1.50% BW reduced efficiency.
八头安装了瘤胃瘘管的荷斯坦奶牛小母牛（初始体重（BW）340（±5）千克，年龄14.5（±1）月龄）被饲喂四种采食量水平的高粗饲料日粮。日粮由青贮草、干草和玉米青贮作为粗饲料成分组成，按照重复拉丁方设计，以体重的1.25%、1.50%、1.75%和2.00%的量提供给小母牛。日粮在采食所有四种采食量水平的小母牛体内进行原位培养。每隔2小时采集一次血液和瘤胃液，持续24小时，排空瘤胃内容物，并收集全部粪便和尿液。日粮摄入量随着提供的饲料量成比例增加，直到干物质摄入量（DMI）达到体重的1.92%，此后，当以体重的2.00%提供饲料时，由于拒食量增加，在统计学上出现了明显的平台期。饲料的原位降解不受采食量水平的影响，再结合瘤胃内容物更高的周转率，可以推断随着采食量增加，食糜通过率增加。随着采食量增加，瘤胃pH值下降，瘤胃挥发性脂肪酸浓度和流向小肠的微生物蛋白流量（使用尿嘌呤衍生物排泄量估算）增加。随着DMI增加，粪便排泄量增加，其增加速率是DMI增加速率的2.54倍；这种增加完全是由于湿粪便排泄量增加，因为尿液排泄量不随采食量水平变化。随着提供的饲料水平增加，氮消化率下降，氮保留量呈线性增加。当以体重的1.25%提供饲料时，氮保留效率最低；高于该水平的所有饲料提供量导致的效率相当。从本实验结果可以得出结论，当对奶牛小母牛限饲高粗饲料日粮时，随着采食量减少，养分利用效率增加，但将DMI降至体重的1.50%以下会降低效率。
