Effects of Grain or Forage-Based Diets on Digestibility and Ruminal pH in Angus or Holstein Steers

Objectives of the study were to evaluate impacts of breed (Holstein vs. Angus) and diet (forage vs. grain) on digestibility of DM, NDF, ADF, and starch; and on ruminal pH over time. Holstein (n=6) and Angus (n=6) steers (initial BW = 450 ± 30 kg; age = 14 ± 1 mo) fitted with rumen cannulae were blocked by diet, 80% forage diet or 80% concentrate diet, and breed, and fed in a switchback design with 2 periods. Each 35 d period had 28 d of diet adaption and 9 d of sample collection. Cattle were fed twice a day to target intake at 2% of BW. On d1 of collection, rumen contents were sampled to measure ruminal pH at 0, 3, 6, 12, and 18 h after feeding. During d 4 to 7 of the collection period, total feces excretion (96 h), feed, and refusals samples were collected, frozen, composited by steer, and analyzed for DM, ADF and 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 over time. There was a treatment by period interaction (P < 0.01) for ruminal pH. Ruminal pH was reduced in period 2 regardless of treatment, but the magnitude of this response was greatest in steers fed grain-based diets. There was a treatment by period interaction (P ≤ 0.05) for DMI and DM, NDF, ADF, and starch digestibility. Cattle that were fed grain-based diets in period 1 had greater (P ≤ 0.01) DM, NDF, and ADF digestibility in period 2, when they transferred to forage-based diets, than cattle that were a fed forage-based diets prior to being fed grain-based diets. Cattle fed grain-based diets in period 2, following forage-based diets, had decreased starch digestibility when compared to cattle fed grain-based diets first. There was no interaction (P ≥ 0.17) of period, treatment, and breed for fiber digestibility. However, a period by treatment by breed trend (P = 0.12) for starch digestibility was observed in steers fed grain-based diets. Feedings Angus steers forage-based diets in period 1 decreased starch digestibility by 10% when compared to feeding Angus steers grain-based diet in period 1, but treatment order did not affect Holstein starch digestibility. Order of treatments in a switchback design affected digestibility results, and these effects were greater in Angus steers than they were in Holstein steers.