Risk of subacute ruminal acidosis in sheep with separate access to forage and concentrate

This study aimed to investigate whether sheep offered free-choice intake of forage and concentrate develop subacute ruminal acidosis (SARA) and to identify SARA-associated feeding behavior components. In a crossover design over two 28-d periods, 11 rumen-cannulated wethers received wheat and alfalfa hay in 2 separate compartments. Concentrate and forage were provided for ad libitum access or in a fixed amount corresponding to 80% of ad libitum hay intake with a concentrate:forage ratio of 60:40 on a DM basis. In both diets, sheep were fed 2 equal portions at 0800 and 1600 h. Ruminal pH, voluntary intake, and feeding behavior were recorded continuously from d 1 to 9 and d 15 to 23 in each period. When no measurements were performed, the animals were housed in larger pens with straw bedding. When fed for ad libitum intake, the sheep ingested 1,340 g of DM/d consisting of 49.1% wheat, whereas with the fixed diet they ate 872 g of DM/d consisting of 58.4% wheat. Sheep fed for ad libitum intake spent more time with ruminal pH < 5.6 than when fed in fixed amounts (7.77 vs. 3.05 h/d, P < 0.001). The time spent with ruminal pH <5.6 was mainly linked to the amount of feed ingested and especially the amount of wheat (P < 0.001). Our results suggest that when fed for ad libitum intake with free choice wheat, the achieved concentrate:forage ratio of near 50:50 and a larger hay intake enable sheep to consume more wheat. When sheep were fed for ad libitum intake, feeding bouts were spread evenly throughout the day. Although ruminal pH reached the same minimum level in both diets after main meals, time to reach pH nadir was longer with ad libitum diet (P < 0.001). In addition, after reaching this minimum value, ruminal pH increased more slowly in this diet, inducing a decreased preprandial ruminal pH (P < 0.001). Consequently, the ad libitum diet led to a longer time below pH 5.6. A slow decrease in ruminal pH may enable sheep to consume larger quantities of food. However, free access to concentrate maintains continuously elevated content of ruminal fermentation end products and so requires more time for pH to return to neutral values. Thus, interval between feed distributions should be as large as possible to help resume the preprandial ruminal pH and to limit time spent with pH <5.6.