Abstract：
The milk-fed calf has a requirement for essential amino acids (EAA) instead of crude protein (CP). However, most milk replacers (MR) are still formulated to a CP concentration, and although limited amounts of Lys or Met may be added, these MR are not formulated solely on an AA basis. Previous work has demonstrated that feeding a modified MR balanced for specific EAA concentrations of a 24% CP MR, but reducing CP to 22%, resulted in improved growth performance in calves compared with a 24% CP MR. The 56-d objective was to determine if the hypothesis that an EAA MR formulated to the specific EAA concentrations of a 24% CP MR, but reducing CP to 22 (22AA) compared with a standard 22% CP MR would result in similar or enhanced growth performance, while reducing feed costs. Two 56-d studies using forty 3- to 5-d-old Holstein bull calves per study received in 1 lot starting on May 8, 2018 (late-spring cooler) and June 22, 2018 (summer with heat stress) were blocked by body weight (BW) and randomly assigned to 1 of 2 MR treatments. Milk replacer treatments consisted of a standard 22:20 (CP:fat; 22CP) and a 22:20 MR with greater EAA concentrations of a 24% CP MR (22AA). All MR, containing decoquinate and diflubenzuron, were fed at 0.57 kg/d per calf split into 2 feedings at 0630 h and 1800 h for 0 to 14 d via bucket, increased to 0.85 kg/d through 35 d split in 2 feedings, and fed once per day at 0.41 kg/d in the morning with weaning after 42 d. Calves were housed in straw-bedded hutches with ad libitum access to water and 25% CP pelleted calf starter (CS). All data were analyzed as a randomized complete block design with block within study considered random and week as a repeated measurement. Initial BW was similar across all treatments (39.9 ± 2.32 kg). Average daily temperature (i.e., heat stress intensity) tended to be greater for calves in study 2 compared with study 1 (20.5 and 22.9°C for study 1 and 2, respectively). The interaction of study and MR effects were nonsignificant. Calf BW, BW gain, average daily gain, CS intake, total dry matter intake (MR plus CS), feed conversions, and EAA intake (MR plus CS) were similar for calves fed both MR. Study 2 (summer heat stress) resulted in more scours incidences compared with study 1, and 22AA MR formulation tended to increase scours possibly due to synthetic AA addition or greater carbohydrate (lactose) inclusion when AA replaced protein sources. Feed costs as MR and total feed costs were lower for calves fed 22AA MR compared with calves fed 22CP MR. The increased Leu, Lys, Met, Thr, and Val concentrations in the 22AA MR compared with the 22CP MR resulted in similar calf growth performance, but the feed cost was lower for calves fed the 22AA MR, which achieved similar growth performance at a lesser cost under current study conditions, but not enhanced as hypothesized.
摘要：
饲喂牛奶的小牛需要必需氨基酸（EAA）而不是粗蛋白（CP）。然而,大多数牛奶替代品(MR)仍被配制成CP浓度，尽管可以添加有限量的Lys或Met，这些MR并非仅根据机管局制定。以前的工作已经证明，喂养一个改良的MR平衡的特定EAA浓度的24％CPMR，但将CP降低到22%，与24%CPMR相比，小牛的生长性能有所改善。56-d的目标是确定EAAMR配制成24%CPMR的特定EAA浓度的假设，但与标准22%CPMR相比，将CP降低至22(22AA)将导致类似或增强的增长表现，同时降低饲料成本。从2018年5月8日（春末凉爽）和2018年6月22日（夏季热应激）开始，在1批次中接受了两项56天的研究，使用40只3至5天的荷斯坦公牛进行了两次56天的研究。体重（BW），随机分配到2种MR治疗中的1种。牛奶替代品处理由标准22:20(CP:脂肪;22CP)和22:20MR组成,具有24%CPMR(22AA)的更高的EAA浓度。所有MR,含有脱草铵和二氟苯并隆，以每只小牛0.57千克/天的速度饲喂，在0630小时和1800小时通过桶分为2次饲喂0至14天，通过两次饲喂的35d分割增加到0.85kg/d，每天早晨以0.41kg/d的速度喂食一次，42d后断奶。小牛被安置在草床的仓房中，随意取水和25％CP颗粒小牛发酵剂（CS）。将所有数据作为随机完全区组设计进行分析,其中研究内的区组被认为是随机的,并且周作为重复测量。所有处理的初始BW相似(39.9±2.32kg)。日平均温度(即，与研究1相比，研究2中的小牛的热应激强度）倾向于更大（研究1和2分别为20.5和22.9°C）。研究与MR效应的交互作用不显著。小牛BW,BW增益,平均每日收益，CS进气,总干物质摄入量（MR加CS），feed转换,饲喂两种MR的小牛的EAA摄入量（MR加CS）相似。与研究1相比，研究2（夏季热应激）导致更多的冲刷发生率，并且22AAMR制剂倾向于增加冲刷，这可能是由于AA替代蛋白质来源时添加了合成AA或包含了更多的碳水化合物（乳糖）。与饲喂22CPMR的小牛相比，饲喂22AAMR的小牛的MR和总饲料成本较低。增加的Leu,Lys,Met,Thr,和Val浓度在22AAMR与22CPMR相比导致类似的小牛生长性能，但是饲喂22AAMR的小牛的饲料成本较低，在目前的研究条件下，以较低的成本实现了类似的增长表现，但没有像假设的那样增强。
