The objective of this study was to evaluate the influence of different amounts of rumen-undegradable protein (RUP) on intake, N balance, performance, mammary gland development, carcass traits, and hormonal status of Holstein heifers at different physiological stages (PS). Sixteen prepubertal (PRE) heifers (initial BW = 106 ± 7.6 kg; age = 4.3 ± 0.46 mo) and 16 pubertal (PUB) heifers (initial BW = 224 ± 7.9 kg; age = 12.6 ± 0.45 mo) were used in an experiment over a period of 84 d. Four diets with increasing RUP contents (38, 44, 51, and 57% of dietary crude protein) and heifers at 2 PS (PRE or PUB) were used in a 4 × 2 factorial arrangement of treatments in a completely randomized design. Throughout the experiment, 2 digestibility trials were performed over 5 consecutive days (starting at d 36 and 78) involving feed and ort sampling and spot collections of feces and urine. At d 0 and 83, body ultrasound images were obtained for real-time carcass trait evaluation. The mammary gland was ultrasonically scanned at d 0 and every 3 wk during the experiment. Blood samples were taken at d 0 and 84 to determine serum concentrations of progesterone, estrogen, insulin-like growth factor I (IGF-I), and insulin. No interaction between PS and the level of RUP was found for any trait. Apparent digestibility of dry matter, organic matter, and neutral detergent fiber corrected for ash and protein was not affected by RUP level but was lower for PRE compared with PUB heifers. Sorting against neutral detergent fiber corrected for ash and protein (tendency only) and for crude protein was greater for PUB than PRE heifers. Pubertal heifers had greater average daily gain (905 vs. 505 g/d) and N retention (25.9 vs. 12.5 g/d) than PRE heifers. In addition, average daily gain and N retention were greatest at 51% RUP of dietary protein. Mammary ultrasonography indicated no effects of RUP amounts on mammary gland composition, whereas PRE heifers had greater pixel values than PUB, indicating higher contents of fat rather than protein in the mammary glands of PRE heifers. Serum progesterone and IGF-I concentration was affected only by PS, and PRE heifers had greater values of progesterone and IGF-I concentrations than PUB heifers. Serum insulin concentration was unaffected by PS but tended to be higher at 51% of RUP. In conclusion, an RUP level of 51% increases body weight, average daily gain, feed efficiency, and N retention in heifers regardless of the PS. In addition, PRE heifers have a lower sorting ability and reduced intake, total-tract digestibility, and N retention. They also have higher amounts of fat in their mammary glands, even at moderate growth rates.

这项研究的目的是评估不同数量的瘤胃不可降解蛋白质（RUP）对不同生理阶段（PS）荷斯坦小母牛的摄入量，氮平衡，性能，乳腺发育，car体性状和激素状态的影响。在实验中使用了16个青春期前（PRE）小母牛（初始BW = 106±7.6 kg;年龄= 4.3±0.46 mo）和16个青春期（PUB）小母牛（初始BW = 224±7.9 kg;年龄= 12.6±0.45 mo）在84 d的时间内。在完全随机设计中，以4×2因子处理方式使用了4种RUP含量增加的饲料（分别为膳食粗蛋白的38％，44％，51％和57％）和2 PS下的小母牛（PRE或PUB）。在整个实验过程中，连续5天（从第36和78天开始）进行了2次消化率试验，涉及饲料和粪便采样以及粪便和尿液的现场收集。在d 0和83，获取身体超声图像以用于实时car体性状评估。在实验期间，在d 0和每3 wk对乳腺进行超声扫描。在第0和84天采集血样以确定孕酮，雌激素，胰岛素样生长因子I（IGF-1）和胰岛素的血清浓度。没有发现PS和RUP的水平之间有任何相互作用。经校正后的灰分和蛋白质，干物质，有机物质和中性洗涤剂纤维的表观消化率不受RUP水平的影响，但与PUB母牛相比，PRE的表观消化率较低。校正后的灰分和蛋白质（仅适用于趋势）和粗蛋白对中性洗涤剂纤维的分选比PRE小母牛的分选更大。青春期小母牛的平均日增重（905 vs. 505 g / d）和N保持力（25.9 vs. 12.5 g / d）比PRE小母牛更高。此外，日粮RUP为51％时，平均日增重和氮保留量最大。乳腺超声检查显示，RUP量对乳腺组成没有影响，而PRE小母牛的像素值大于PUB，表明PRE小母牛的乳腺中脂肪含量高于蛋白质含量。血清孕酮和IGF-I的浓度仅受PS的影响，PRE小母牛的孕酮和IGF-I的浓度值高于PUB小母牛。血清胰岛素浓度不受PS的影响，但往往较高，为RUP的51％。总之，无论PS多少，RUP水平为51％都会增加体重，平均日增重，饲料效率和氮素在小母牛中的保留。此外，PRE小母牛的分选能力较低，并且摄入量，全道消化率和氮保留量均降低。即使在适度的生长速度下，它们的乳腺中脂肪的含量也较高。