Effect of forage species and supplement type on rumen kinetics and serum metabolites in growing beef heifers grazing winter forage

The objective of this study was to determine the effect of stockpiled forage type and protein supplementation on VFA production, serum metabolites, and BW in yearling beef heifers. Over 2 yr, spring-born, Angus crossbred yearling beef heifers (n = 42; 305 +/- 2.9 kg initial BW) were randomly assigned to 1 of 3 forage pasture types: 1) endophyte-infected tall fescue [TF; Schedonorus arundinaceus (Schreb.) Dumort], 2) a big bluestem (Andropogon gerardii Vitman) and indiangrass (Sorghastrum nutans L.) combination (BI), or 3) switchgrass (SG; Panicum virgatum L.). Each pasture was then randomly assigned to receive either 1 of 2 isonitrogenous CP treatments: 1) 0.68 kg.heifer(-1).d(-1) of dried distiller's grains with solubles (DDGS; 28% CP and 88% TDN) or 2) 0.22 kg.heifer(-1).d(-1) of blood meal and fish meal (BF; 72.5% CP and 69.5% TDN), resulting in a 3 x 2 factorial arrangement of treatments. Treatments were initiated in January and terminated in April in both years of the study. Body weights and blood samples were collected approximately every 28 d from initiation of grazing until the end of the trial. Heifer BW change from January to February and overall BW change were greater (P < 0.01) for TF heifers. However, BW change from March to April was not different (P = 0.84) among forage types. Supplement type did not influence (P = 0.13) BW or BW change from January to February and from January to April; however, heifers fed DDGS had greater (P = 0.03) BW gain from March to April. Heifer BW change from February to March exhibited (P < 0.05) a forage type x supplement interaction, with BF-fed heifers gaining more BW on BI pastures than DDGS-fed heifers. Serum glucose concentrations, ruminal acetate, and the acetate: propionate ratio were greater (P = 0.04) for SG heifers. However, circulating serum NEFA and urea N (SUN) concentrations were not different (P = 0.85) among forage types. Serum glucose and NEFA concentrations were not influenced (P = 0.61) by supplement type. Circulating SUN concentrations were greater (P < 0.01) in BF-supplemented heifers. Ruminal acetate tended to be greater (P = 0.09) and butyrate concentrations were greater (P < 0.01) for BF-supplemented heifers. The acetate: propionate ratio was not influenced (P = 0.15) by supplement type. These results suggest that a compensatory gain period prior to breeding would be needed for these native warm-season species to be a viable opportunity for growing and developing replacement heifers in the southeastern United States.