The importance of fiber particle size in ruminal health is well known, but there are fewer studies to assess the requirements of physically effective neutral detergent fiber (peNDF) in beef cattle than in dairy cattle. The objective of this study was to establish the optimal peNDF proportion in high-concentrate diets fed to beef cattle, to reduce the risk of subacute ruminal acidosis. The experimental design was a replicated Latin Square 4 × 4, with 4 periods of 21 d. Treatments consisted of 4 diets with different peNDF proportions: 6.4%, 10.4%, 13.6%, and 15.4%, offered ad libitum as total mixed ration, and containing 15% barley straw and 85% concentrate. Diets, which differed in proportions of straw > 4 mm (considered peNDF) and straw < 4 mm, were manually mixed with concentrate. This concentrate was the same for all diets. A ruminal bolus was orally administered to each heifer for pH measurement. Intake, water consumption, intake by particle size, feed sorting, feeding behavior, behavioral activities, and rumen pH were recorded. Chemical composition and particle sizes of diets offered were assessed in the last wk of each period. Data were analyzed using the MIXED procedure of SAS. Orthogonal contrasts determined the linear and quadratic effects of increasing peNDF proportion. T-test procedure determined if heifers carried out sorting behavior. Particles > 4 mm linearly increased (P = 0.001), and particles < 4 mm linearly decreased (P = 0.001) as peNDF increased. Water consumption and feeding behavior were unaffected by treatment. As peNDF increased, intakes of dry matter and neutral detergent fiber linearly decreased (P = 0.001), whereas peNDF intake increased (P = 0.001). Intake of particles > 4 mm linearly increased, whereas intake of particles < 4 mm linearly decreased (P = 0.001) as peNDF increased. Diet 6.4% performed sorting for particles > 4 mm (P < 0.01), and diets 13.6% and 15.4% against particles > 4 mm (P < 0.01). Diet 10.4% tended to sort against particles > 4 mm (P < 0.10). Time spent ruminating linearly increased (P = 0.001) as peNDF increased. Diets did not differ in mean and minimum rumen pH, but time under rumen pH thresholds (5.8, 5.7, 5.6 and 5.5) linearly decreased as peNDF increased (P < 0.05). The results suggested that the diet that best met the requirements of not compromising intake, limiting sorting behavior, and promoting time spent ruminating to reduce the number of hours under rumen pH thresholds, was the 10.4% diet.

中文翻译：

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饲喂牛小母牛的高浓度饲料中增加物理有效纤维的含量会影响摄入量，分选行为，反刍时间和瘤胃pH。

纤维粒度在瘤胃健康中的重要性是众所周知的，但是很少有研究评估物理上有效的中性洗涤剂纤维（peNDF）在肉牛方面要比在奶牛方面好。这项研究的目的是在饲喂肉牛的高浓度日粮中确定最佳peNDF比例，以减少亚急性瘤胃酸中毒的风险。实验设计是复制的拉丁方4×4，有4个21 d的周期。处理方法包括4种不同peNDF比例的饮食：6.4％，10.4％，13.6％和15.4％，按总混合比例随意提供，并包含15％大麦秸秆和85％精矿。将秸秆> 4 mm（考虑到peNDF）和秸秆<4 mm的比例不同的饲料与精矿手动混合。所有饮食中的浓缩物均相同。向每个小母牛口服瘤胃大丸以测量pH。摄入量，耗水量，按粒度摄入的量，饲料分类，饲喂行为，行为活动，记录瘤胃pH值。在每个时期的最后一周评估所提供饮食的化学成分和粒径。使用SAS的MIXED程序分析数据。正交对比确定增加peNDF比例的线性和二次效应。T检验程序确定小母牛是否进行了分选行为。> 4 mm的颗粒线性增加（P = 0.001），并且随着peNDF的增加，<4 mm的颗粒线性减少（P = 0.001）。耗水量和喂养行为不受处理的影响。随着peNDF的增加，干物质和中性洗涤剂纤维的摄入量线性减少（P = 0.001），而peNDF的摄入量则增加（P = 0.001）。随着peNDF的增加，> 4 mm的颗粒的摄入量线性增加，而<4 mm的颗粒的摄入量线性减少（P = 0.001）。饮食6.4％对大于4毫米的颗粒进行了分类（P <0.01），饮食对大于4毫米的颗粒进行了13.6％和15.4％的分类（P <0.01）。饮食中有10.4％的食物倾向于对大于4毫米的颗粒进行分类（P<0.10）。随着peNDF的增加，反省的时间线性增加（P = 0.001）。日粮的平均瘤胃pH值和最低瘤胃pH值没有差异，但是随着peNDF的增加，瘤胃pH阈值（5.8、5.7、5.6和5.5）下的时间线性减少（P <0.05）。结果表明，最能满足不损害摄入量，限制分选行为，延长反刍时间以减少瘤胃pH阈值的小时数的饮食是10.4％的饮食。