Ruminants fed high-forage diets usually have a low feed efficiency, and their performances might be limited by methionine (Met) supply. However, the INRA feeding system for growing cattle does not give recommendation for this amino acid (AA). This study aimed to assess the effects of Met-balanced diets on animal performance and N metabolism in young bulls fed high-forage diets formulated at or above protein requirements. Four diets resulting from a factorial arrangement of two protein levels (Normal (13·5 % crude protein) v. High (16·2 % crude protein)) crossed with two Met concentrations (unbalanced (2·0 % of metabolisable protein) v. balanced (2·6 % of metabolisable protein)) were tested on thirty-four fattening Charolais bulls for 7 months before slaughter. Animal growth rate was greater in Met-balanced diets (+8 %; P = 0·02) with a trend for a greater impact in High v. Normal protein diets (P = 0·10). This trend was observed in lower plasma concentrations of branched-chain AA only when Met supplementation was applied to the Normal protein diet (P ≤ 0·06) suggesting another co-limiting AA at Normal protein level. Feed conversion efficiency and N use efficiency were unaffected by Met supplementation (P > 0·05). However, some plasma indicators suggested a better use of AA when High protein diets were balanced v. unbalanced in Met. The proportion of total adipose tissue in carcass increased (+5 percent units; P = 0·03), whereas that of muscle decreased on average 0·8 percent units (P = 0·05) in Met-balanced diets. Our results justify the integration of AA into dietary recommendations for growing cattle.

中文翻译：

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蛋氨酸平衡日粮通过改变氮代谢提高饲喂高草料日粮的年轻公牛的育肥性能

饲喂高草料日粮的反刍动物通常具有较低的饲料效率，并且它们的表现可能受到蛋氨酸 (Met) 供应的限制。然而，用于生长牛的 INRA 饲喂系统并未推荐这种氨基酸 (AA)。本研究旨在评估 Met 平衡日粮对饲喂高草料日粮的年轻公牛的动物性能和 N 代谢的影响，该日粮配方为蛋白质需求量或高于蛋白质需求量。由两种蛋白质水平（正常（13·5 % 粗蛋白）的阶乘排列产生的四种日粮v. 高（16·2 % 粗蛋白））与两种 Met 浓度（不平衡（2·0 % 可代谢蛋白）交叉）v. 平衡（2·6 % 的可代谢蛋白质））在屠宰前 7 个月对 34 头育肥夏洛莱公牛进行了测试。Met 均衡饮食中的动物生长率更高（+8 %；磷= 0·02)，在高位有更大影响的趋势v. 正常蛋白质饮食（磷= 0·10)。仅当在正常蛋白质饮食中添加 Met 时，支链 AA 的血浆浓度较低时才观察到这种趋势。磷≤ 0·06) 表明在正常蛋白质水平上存在另一种共同限制性 AA。饲料转化率和氮素利用效率不受添加 Met 的影响（磷> 0·05)。然而，一些血浆指标表明当高蛋白饮食平衡时更好地使用 AAv. Met 中的不平衡。胴体中总脂肪组织的比例增加（+5% 单位；磷= 0·03)，而肌肉平均下降 0·8 个单位 (磷= 0·05) 在 Met 均衡饮食中。我们的结果证明了将 AA 整合到养牛的饮食建议中是合理的。