Ruminal fermentation characteristics of beef steers grazing grass monocultures versus low- and high-tannin grass-legume mixtures

Abstract Text: Ruminal fermentation characteristics were investigated with 36 Angus crossbred steers grazing 4 treatments: 1) tall fescue (TF) with no fertilizer (TF–NF), 2) TF with N fertilizer (TF+NF), 3) TF-alfalfa mixture (TFALF), and 4) TF-birdsfoot trefoil mixture (TFBFT). Treatments were tested in a randomized complete block design with 3 pasture replicates, 4 paddocks per pasture, and 3 steers per pasture. Replicated 0.47-ha paddocks were grazed with beef steers from May through September in 2013 for total of 16 wk. Steers grazed for 7 d per paddock on a 28-d rotation interval. Pasture forage samples were collected at 4-wk intervals throughout the experiment. Ruminal fluid samples were obtained from all steers using a Geishauser probe at wk 4, 8, 12, and 16 to measure pH and analyze VFA profiles and ammonia-N (NH₃-N) concentration. Concentration of CP in pasture forages was greater (P < 0.01) in mixtures than grass monocultures at wk 4 (12.8 vs. 7.89%), but it was similar after wk 4. In contrast, NDF concentration was lower (P < 0.05) in mixtures compared with grass monocultures throughout the grazing season. Ruminal pH was maintained at 6.54 or higher and did not differ across treatments. Starting at wk 8, total VFA concentration increased (P < 0.05) in steers grazing the mixtures compared to those grazing monocultures. However, the VFA concentration was not different between TFALF and TFBFT. Acetate proportion increased with mixtures at wk 12, but propionate proportion decreased due to grazing mixtures, resulting in increased acetate-to-propionate ratio at wk 12 (P < 0.05). Concentration of NH₃-N was highest with TF+NF at wk 8 and 12, whereas at wk 16, it was highest in steers grazing mixtures followed by TF+NF (P < 0.05). Steers grazing grass-legume mixtures resulted in enhanced ruminal fermentation evidenced by increased VFA concentration likely due to greater concentration of nonfiber carbohydrates, which may have supported increased growth performance compared to those grazing grass monocultures. Therefore, grass-legume mixtures for grazing steers can replace N fertilization of TF, and thus it can be a sustainable approach to improve pasture utilization for finishing beef steers.

Keywords: grass-legume mixtures, grazing beef steers, ruminal fermentation