Effect of corn silage particle size and supplemental hay on rumen pH and feed preference by dairy cows fed high-starch diets

The objectives of this experiment were to determine the effects of corn silage particle size and supplemental hay on rumen pH and feed preference in lactating dairy cows experiencing a bout of subacute ruminal acidosis (SARA). In this study, 12 lactating (8 ruminally cannulated), multiparous Holstein cows averaging 91 +/- 40 d in milk and weighing 695 +/- 95 kg (mean +/- SD) were randomly assigned to a replicated 4 x 4 Latin square. During each of the four 21-d periods, animals were offered 1 of 4 diets that were chemically similar but varied in corn silage particle size and supplemental second cutting orchardgrass hay: short corn silage total mixed ration (TMR; ST); short corn silage TMR with 5.6% supplemental hay (SH); long corn silage TMR (L); and long corn silage TMR with 5.6% supplemental hay (LH). Cows were allowed to adapt to this feeding scheme for 14 d, and cannulated cows were then subjected to a rumen challenge to induce a bout of SARA by restricting feed before the challenge and providing 4 kg of ground wheat via the rumen cannula. Although baseline pH was low, the SARA challenge lowered ruminal pH further for all cows regardless of diet. Daily average rumen pH decreased from 5.44 and 5.45 to 5.33 and 5.38 for ST and SH, respectively, and from 5.64 and 5.54 to 5.47 and 5.39 for L and LH, respectively, from baseline to challenge phase. Following the rumen challenge, rumen concentrations of acetate, propionate, butyrate, and valerate increased. Decreasing corn silage particle size significantly increased TMR and total DMI during all phases of the model. Feeding short corn silage TMR increased milk, protein, and lactose yields. Cows fed supplemental hay had increased fat yield and protein concentration in the milk and responded minimally to the effects of particle size selection when challenged with SARA. Cows consuming short corn silage TMR changed feed preference for longer forage par-tidies during the course of the SARA challenge. During the recovery phase, however, cows refused an average of 13.5% of the offered TMR and 78.7% of the supplemental hay. These results indicate that cows did not prefer the supplemental hay regardless of corn silage particle size during a bout of SARA, which may have been due to the possible adaptation of a low ruminal pH throughout the study.