Schadt I, Mertens D R, Van Soest P J, Azzaro G, Licitra G
CoRFiLaC, Regione Siciliana, 97100 Ragusa, Italy.
Mertens Innovation & Research LLC, Belleville, WI 53508.
J Dairy Sci. 2014 Dec;97(12):7995-8007. doi: 10.3168/jds.2014-7987. Epub 2014 Oct 3.
The influence of stage of lactation and corresponding diets on rates of protein degradation (kd) is largely unstudied. Study objectives were to measure and compare in situ ruminal kd of crude protein (CP) and estimate rumen CP escape (rumen-undegradable protein; RUP) of selected feeds by cows at 3 stages of lactation fed corresponding diets, and to determine the incubation times needed in an enzymatic in vitro procedure, using 0.2 units of Streptomyces griseus protease per percent of true CP, that predicted in situ RUP. Residue CP was measured after in situ fermentation for 4, 8, 12, 24, 36, 48, and 72 h of 5 protein sources and 3 total mixed rations, which were fed to the in situ cows. Two nonlactating (dry) cows and 2 cows each at 190 (mid) and 90 (peak) days of lactation were used. Each pair of cows was offered free-choice diets that differed in composition to meet their corresponding nutrient requirements. Diets had decreasing proportions of forages and contained (dry matter basis) 11.9, 15.1 and 16.4% CP and 54.3, 40.3 and 35.3% neutral detergent fiber, for dry, mid, and peak TMR (TMR1, TMR2, and TMR3), respectively. Intakes were 10.3, 21.4, and 23.8kg of dry matter/d, respectively. Kinetic CP fractions (extractable, potentially degradable, undegradable, or slowly degradable) were unaffected by treatment. Lag time and kd varied among feeds. The kd was faster for all feeds (0.136/h) when incubated in dry-TMR1 cows compared with mid-TMR2 (0.097/h) or peak-TMR3 (0.098/h) cows, and no differences in lag time were detected. Calculated RUP, using estimated passage rates for each cow based on intake, differed between dry-TMR1 (0.382) and mid-TMR2 (0.559) or peak-TMR3 (0.626) cows, with a tendency for mid-TMR2 to be different from peak-TMR3. Using the average kd and lag time obtained from dry-TMR1 to calculate RUP for mid-TMR2 and peak-TMR3 cows using their passage rates reduced RUP values by 6.3 and 9.5 percentage units, respectively. Except for that of herring meal, in vitro residue CP at 6, 12, and 48h of enzymatic hydrolysis was correlated (r=0.90) with in situ RUP of peak-TMR3, mid-TMR2, and dry-TMR1, respectively. Although confounded within treatments, stage of lactation, diet, and intake appeared to affect CP degradation parameters and RUP. Using kd from nonlactating cows, or the RUP calculated from them, may bias diet evaluation or ration formulation for lactating cows. In addition, enzymatic in vitro predictions of RUP should be measured using incubation times that are appropriate for lactating cows.
泌乳阶段及相应日粮对蛋白质降解率（kd）的影响在很大程度上尚未得到研究。研究目的是测定和比较奶牛在泌乳三个阶段采食相应日粮时粗蛋白（CP）的瘤胃原位kd，并估算所选饲料的瘤胃CP逃逸量（瘤胃不可降解蛋白；RUP），以及确定在体外酶解程序中，使用每百分比真蛋白0.2单位的灰色链霉菌蛋白酶预测原位RUP所需的孵育时间。对5种蛋白质来源和3种全混合日粮进行原位发酵4、8、12、24、36、48和72小时后，测定残余CP，这些日粮饲喂给原位奶牛。使用了2头非泌乳（干奶期）奶牛，以及泌乳190天（中期）和90天（高峰期）的奶牛各2头。每对奶牛自由选择不同组成的日粮，以满足其相应的营养需求。日粮中粗饲料比例逐渐降低，干奶期、中期和高峰期全混合日粮（TMR1、TMR2和TMR3）的粗蛋白含量（干物质基础）分别为11.9%、15.1%和16.4%，中性洗涤纤维含量分别为54.3%、40.3%和35.3%。采食量分别为10.3、21.4和23.8千克干物质/天。动力学CP组分（可提取的、潜在可降解的、不可降解的或缓慢降解的）不受处理影响。不同饲料的延迟时间和kd有所不同。与中期TMR2（0.097/h）或高峰期TMR3（0.098/h）奶牛相比，在干奶期TMR1奶牛中孵育时，所有饲料的kd更快（0.136/h），且未检测到延迟时间的差异。根据每头奶牛的采食量估算其通过率来计算RUP，干奶期TMR1奶牛（0.382）与中期TMR2奶牛（0.559）或高峰期TMR3奶牛（0.626）之间存在差异，中期TMR2奶牛与高峰期TMR3奶牛之间有差异趋势。使用从干奶期TMR1获得的平均kd和延迟时间，根据中期TMR2和高峰期TMR3奶牛的通过率来计算RUP，分别使RUP值降低了6.3和9.5个百分点。除了鱼粉外，酶解6、12和48小时后的体外残余CP分别与高峰期TMR3、中期TMR2和干奶期TMR1的原位RUP相关（r = 0.90）。尽管在处理中存在混淆，但泌乳阶段、日粮和采食量似乎会影响CP降解参数和RUP。使用非泌乳奶牛的kd或由此计算出的RUP，可能会使泌乳奶牛的日粮评估或日粮配方产生偏差。此外，体外酶解对RUP的预测应使用适合泌乳奶牛的孵育时间来测定。
