Growth Performance and Nutrient Metabolism of Pasture-Finished Beef Steers and In Vitro Fermentation Characteristics of Pasture Forages in Continuous Cultures

A 2-year grazing study was conducted to evaluate the growth performance, ruminal fermentation, carcass characteristics, and fatty acid compositions in subcutaneous adipose tissue of beef steers grazing tall fescue (Festuca arundinacea Shreb.; TF) pastures without or with N fertilization. Nitrogen fertilization increased crude protein concentration of TF pasture and average daily gain of beef steers. Increase in total volatile fatty acids (VFA) and ammonia-N (NH3-N) concentrations were detected in steers grazing fertilized TF. In comparison with steers on feedlot, pasture-finished steers had greater proportions of cis-9, trans-11 CLA and C18:3 n-3, but lower n-6:n-3 ratio in adipose tissue. In the first in vitro study using 2 energy supplements [corn or dried distillers grains with solubles (DDGS)] and 4 pasture mixture forages [TF without or with N fertilizer (TF˗NF or TF+NF), TF-alfalfa mixture (TF+AF), and TF-birdsfoot trefoil mixture (TF+BT)], we found that corn supplementation increased total VFA and propionate concentrations, while DDGS supplementation decreased total VFA concentrations. Lower NH3-N concentration and methane (CH4) production were observed due to energy supplementation, in particular when corn grain was supplemented. Similar NH3-N:VFA ratios were detected in the cultures fed the TF+NF and the TF+BT. This result indicates that the TF+BT had similar fermentation efficiency on in vitro ruminal metabolism compared with the TF+NF. The second in vitro study was performed to investigate the effects of grass-to-legume ratios of 3 different TF-legume mixed diets on in vitro fermentation characteristics in continuous cultures. Propionate concentration increased with the increasing of legume proportion in the mixed diets. The greatest propionate concentration was shown by cultures fed the TF+CM, while the TF+AF and the TF+BT maintained a similar propionate concentration. Increasing legume proportion in the forage diets also increased NH3-N concentration, but decreased CH4 production in the cultures. Further decrease of CH4 production was recorded when the TF+BT was fed to the cultures. Overall results from the grazing study demonstrate that N fertilizer can improve nutrient quality of TF as well as growth performance of grazing steers, while the in vitro studies showed positive effects of grass-legume mixture diets on in vitro microbial metabolism by improving ruminal fermentation and reducing CH4 production