Effect of grinding or pelleting high grain maize- or barley-based concentrates on rumen environment and microbiota of beef cattle

• Acidosis is the most important nutritional disorder in intensively reared cattle.
• Type of cereal in the concentrate and its processing are key factors in acidosis.
• Barley diets negatively affect rumen fermentation, increasing the risk of acidosis.
• Pelleting at 6 mm instead of grinding is not advisable to avoid subacute acidosis.

Eight 5-month old, rumen-cannulated Holstein male calves were used in a three months experiment to study the effect of the cereal in the concentrate (0.53 of maize, M, or 0.67 of barley, B) given either ground (GR) or pelleted to 6 mm (PE) on rumen fermentation and microbiota in intensive feeding conditions. Four calves received M and the other four B, offered either GR or PE in two consecutive 45-day periods, together with straw. On days 41 and 43 of each period, the feed intake pattern was recorded and rumen contents were sampled before the morning feeding (0 h) and after 4 and 8 h to determine rumen pH and total volatile fatty acids (VFA), lactic acid, ammonia and fatty acids (FA) concentration, and to characterise bacterial and protozoal populations. Compared with those given M, calves fed B showed lower rumen pH (5.64 vs. 6.14, P=0.001) and higher total VFA concentration (140 mmol/l vs. 113 mmol/l, P=0.014) 4 h after feeding, and a greater proportion of records of pH below 5.6 (0.27 vs. 0.10, P=0.039) and VFA over 150 mmol/l (0.18 vs. 0.02, P=0.019), benchmarks of subacute ruminal acidosis (SARA). Feeding B resulted in lower (P=0.072) and higher (P=0.059) proportions of acetic and propionic acids, respectively, with sampling time differences for this diet, as well as more variable ammonia concentrations (P<0.001). On the other hand, concentrates processed as PE promoted a lower acetic to propionic ratio, this effect being more evident with M (1.15 vs. 2.39, P=0.003), and a more pronounced pH drop from 0 to 4 h after feeding compared with those given GR (1.01 vs. 0.87 pH units, P=0.059). Only minor effects were observed on either FA concentrations or rumen microbiota. The more abrupt rumen fermentation induced by B may be due to the more rapidly degradation rate of its starch combined with a greater concentrate intake during the first 4 h (2.83 vs. 2.43 kg dry matter, P=0.031), which was not affected by the concentrate processing. The results suggest that barley-based diets would cause a less favourable rumen fermentation that might increase the risk of SARA compared with maize diets, whereas pelleting the concentrates at 6 mm would not affect the feed intake pattern and would not be advisable to avoid SARA compared with grinding