Department of Animal Sciences and Industry, Kansas State University, Manhattan, KS 66506, USA.
Department of Clinical Sciences, Kansas State University, Manhattan, KS 66506, USA.
J Anim Sci. 2022 Oct 1;100(10). doi: 10.1093/jas/skac283.
Six ruminally cannulated Holstein steers (256 ± 14 kg) were used in a 6 × 6 Latin square design to assess effects of guanidinoacetic acid (GAA) supplementation on N retention and methionine (Met) methyl group flux in growing cattle fed corn-based diets. Factorial treatments were two levels of Met (0 or 5 g/d) and three levels of GAA (0, 7.5, or 15 g/d) delivered by continuous abomasal infusion. Periods were 10 d in length and included 6 d of treatment adaptation, 3 d for total fecal and urine collections, and 1 d for blood sampling and flux measurements. Urinary N linearly increased (P < 0.01) with GAA supplementation and decreased (P < 0.05) with Met supplementation. Fecal N excretion was unaffected (P ≥ 0.42) by treatment. Retained N was not affected by GAA supplementation, but it was increased (P < 0.01) by Met supplementation. Use of methionine for transmethylation reactions, as well remethylation of homocysteine, was not affected by GAA supplementation when Met was not supplemented, but tended to be linearly increased by GAA supplementation when Met was supplemented (GAA-linear × Met interaction; P = 0.07), with the increases matching the amount of GAA provided. This response suggests that methylation reactions for compounds other than GAA were reduced by GAA supplementation when Met supply was deficient. Plasma concentrations and urinary excretion of creatine increased linearly (P = 0.03 and P = 0.06, respectively) when GAA was supplemented. There was a linear increase (P < 0.01) in urinary GAA excretion with GAA supplementation. Neither plasma concentration nor urinary excretion of creatinine was affected (P ≥ 0.17) by treatment. No treatment differences (P ≥ 0.13) were observed for plasma haptoglobin concentrations. Plasma urea-N linearly increased (P < 0.05) with GAA supplementation. Concentrations of Met and taurine increased (P < 0.05) when Met was supplemented. Plasma arginine was greatest at the intermediate level of supplemental GAA (quadratic, P < 0.05). The increase in N retention when Met was supplemented demonstrates Met was limiting in the corn-based diet. Supplementation of GAA alone or with Met as a methyl donor did not increase N retention in growing steers, perhaps because creatine production was favored over protein deposition as a use for Met.
六头安装有瘤胃瘘管的荷斯坦公牛（256±14kg），采用 6×6 拉丁方设计，评估胍基乙酸（GAA）对玉米基础日粮下生长牛氮保留和蛋氨酸（Met）甲基化基团流的影响。试验的因子处理有两种 Met 水平（0 或 5g/d）和三种 GAA 水平（0、7.5 或 15g/d），通过持续瘤胃灌注添加。每个周期 10d，包括 6d 处理适应期，3d 粪便和尿液收集，1d 采血和流量测量。尿氮线性增加（P＜0.01）随着 GAA 添加而增加，随着 Met 添加而减少（P＜0.05）。粪便氮排泄不受处理影响（P≥0.42）。GAA 添加不影响氮保留，但 Met 添加增加了氮保留（P＜0.01）。当不添加 Met 时，GAA 添加对转甲基反应以及同型半胱氨酸的再甲基化过程中蛋氨酸的使用没有影响，但当添加 Met 时，GAA 添加有线性增加的趋势（GAA-线性×Met 互作；P=0.07），且增加量与 GAA 提供量相匹配。这种反应表明，当 Met 供应不足时，GAA 添加会减少除 GAA 以外的化合物的甲基化反应。当 GAA 添加时，血浆肌酸浓度和尿肌酐排泄量线性增加（P=0.03 和 P=0.06）。随着 GAA 添加，尿 GAA 排泄量也呈线性增加（P＜0.01）。处理对肌酸酐的血浆浓度和尿排泄量没有影响（P≥0.17）。血浆触珠蛋白浓度无处理差异（P≥0.13）。血浆尿素氮随着 GAA 添加线性增加（P＜0.05）。添加 Met 时，Met 和牛磺酸的浓度增加（P＜0.05）。当添加中等水平的 GAA 时，血浆精氨酸最高（二次，P＜0.05）。添加 Met 时氮保留增加，表明玉米基础日粮中 Met 是限制因素。单独添加 GAA 或添加 GAA 作为甲基供体不能增加生长牛的氮保留，这可能是因为肌酸的生成优先于蛋白质沉积，作为 Met 的利用途径。
