Effects of cattle breed and corn processing on ruminal pH and volatile fatty acid concentrations, and apparent digestibility

Objectives were to evaluate the effects of cattle breed, Holstein or Angus, and corn processing methods on total tract digestibility, and ruminal pH and volatile fatty acid concentrations (VFA). Six Holstein (BW = 548 +/- 6.3 kg) and 6 Angus steers (BW = 596 +/- 7.6 kg) previously fitted with rumen cannulae, were assigned to a 2 x 3 factorial arrangement of treatments. Factors included breed, Holstein or Angus, and corn processing method, whole shelled corn (WSC), dry rolled corn (DRC), or finely ground corn (FGC). Steers were fed diets containing 815 g/kg corn grain, 100 g/kg grass hay, 65 g/kg soybean, and 20 g/kg supplement. Steers were fed in a replicated 3 x 3 Latin square split-plot design. Each period consisted of 14 d diet adaptation followed by 7 d of sample collection. On d 1, rumen contents were collected. Ruminal pH was recorded at 0, 3, 6, 12, and 18 h after feeding and a portion of the strained ruminal fluid was subsampled at 0 and 3 h for VFA analysis. From d 2-7, feces, feed, and refusals samples were collected and analyzed for DM, NDF, and starch. Data were analyzed using the MIXED procedures in SAS (v9.4 SAS Inst. Inc., Cary, NC). Repeated measures were used to analyze changes in ruminal pH and VFA over time. There was no interaction of breed x diet (P >= 0.16) on diet intake and digestibility. Holstein steers had a greater (P <= 0.05) intake than Angus steers, regardless of diet. There was no effect (P >= 0.81) of breed on diet digestibility. Digestibility of DM, OM, and starch were reduced (P <= 0.03) when corn grain was fed whole, regardless of breed. There was an interaction of breed x diet (P = 0.02) on ruminal pH. Angus steers fed FGC had the most acidic ruminal pH, while ruminal pH in Holstein steers fed FGC was similar to ruminal pH Holstein steers fed WSC or DRC. There was no interaction of breed x diet (P = 0.28), nor were there main effects of breed (P = 0.21) or diet (P = 0.92), on total VFA concentrations. Although Holstein steers ate 22 % more DM than Angus steers, they were able to sustain a greater (P = 0.04) ruminal pH. Compared to feeding WSC, starch digestibility improved by 5% when processed corn grain was fed, regardless processing method.