1 INRA, UMR 1213 Herbivores, Theix, F-63122 Saint-Genès-Champanelle, France.
2 INRA, UMR 1348 PEGASE, F-35590 Saint-Gilles, France.
Animal. 2014 Feb;8(2):275-85. doi: 10.1017/S1751731113002012. Epub 2013 Nov 14.
Nitrogen emissions from dairy cows can be readily decreased by lowering the dietary CP concentration. The main objective of this work was to test whether the milk protein yield reduction associated with low N intakes could be partially compensated for by modifying the dietary carbohydrate composition (CHO). The effects of CHO on digestion, milk N efficiency (milk N/N intake; MNE) and animal performance were studied in four Jersey cows fed 100% or 80% of the recommended protein requirements using a 4×4 Latin square design. Four iso-energetic diets were formulated to two different CHO sources (starch diets with starch content of 34.3% and NDF at 32.5%, and fiber diets with starch content of 5.5% and NDF at 49.1%) and two CP levels (Low=12.0% and Normal=16.5%). The apparent digestible organic matter intake (DOMI) and the protein supply (protein digestible in the small intestine; PDIE) were similar between starch and fiber diets. As planned, microbial N flow (MNF) to the duodenum, estimated from the urinary purine derivatives (PD) excretion, was similar between Low and Normal CP diets. However, the MNF and the efficiency of microbial synthesis (g of microbial N/kg apparently DOMI) were higher for starch v. fiber diets. Milk and milk N fractions (CP, true protein, non-protein N (NPN)) yield were higher for starch compared with fiber diets and for Normal v. Low CP diets. Fecal N excretion was similar across dietary treatments. Despite a higher milk N ouput with starch v. fiber diets, the CHO modified neither the urinary N excretion nor the milk urea-N (MUN) concentration. The milk protein yield relative to both N and PDIE intakes was improved with starch compared with fiber diets. Concentrations of β-hydroxybutyrate, urea and Glu increased and those of glucose and Ala decreased in plasma of cows fed starch v. fiber diets. On the other hand, plasma concentration of albumin, urea, insulin and His increased in cows fed Normal compared with Low CP diets. This study showed that decreasing the dietary CP proportion from 16.5% to 12.0% increases and decreases considerably the MNE and the urinary N excretion, respectively. Moreover, present results show that at similar digestible OM and PDIE intakes, diets rich in starch improves the MNE and could partially compensate for the negative effects of Low CP diets on milk protein yield.
奶牛的氮排放量可以通过降低日粮 CP 浓度来有效减少。本研究的主要目的是测试低氮摄入与牛奶蛋白产量降低相关的情况下，通过改变日粮碳水化合物组成（CHO）是否可以部分补偿。在使用 4×4 拉丁方设计的 4 头荷斯坦奶牛上，以 100%或 80%的推荐蛋白质需求分别饲喂 100%或 80%的推荐蛋白质需求，研究了 CHO 对消化、牛奶氮效率（牛奶氮/摄入氮；MNE）和动物性能的影响。用两种不同的 CHO 源（淀粉日粮，淀粉含量为 34.3%，NDF 为 32.5%；纤维日粮，淀粉含量为 5.5%，NDF 为 49.1%）和两种 CP 水平（低=12.0%和正常=16.5%）配制了 4 种等能量日粮。表观可消化有机物摄入量（DOMI）和蛋白质供应（小肠可消化蛋白；PDIE）在淀粉和纤维日粮之间相似。按照计划，十二指肠中微生物氮流量（MNF），通过尿嘌呤衍生物（PD）排泄来估计，在低和正常 CP 日粮之间相似。然而，MNF 和微生物合成效率（每克表观 DOMI 中的微生物氮）在淀粉日粮中高于纤维日粮。与纤维日粮相比，淀粉日粮的牛奶和牛奶氮分数（CP、真蛋白、非蛋白氮（NPN））产量更高，正常 CP 日粮的产量高于低 CP 日粮。粪便氮排泄在不同日粮处理之间相似。尽管淀粉日粮的牛奶氮产量高于纤维日粮，但 CHO 既没有改变尿氮排泄，也没有改变牛奶尿素氮（MUN）浓度。与纤维日粮相比，淀粉日粮的牛奶蛋白产量相对于氮和 PDIE 摄入量都有所提高。与纤维日粮相比，饲喂淀粉日粮的奶牛血浆中β-羟丁酸、尿素和 Glu 浓度增加，而葡萄糖和 Ala 浓度降低。另一方面，与低 CP 日粮相比，饲喂正常 CP 日粮的奶牛血浆中白蛋白、尿素、胰岛素和 His 浓度增加。本研究表明，从 16.5%降至 12.0%的日粮 CP 比例增加了 MNE，并大大降低了尿氮排泄量。此外，目前的结果表明，在相似的可消化 OM 和 PDIE 摄入量下，富含淀粉的日粮可提高 MNE，并可部分补偿低 CP 日粮对牛奶蛋白产量的负面影响。
