INFLUENCE OF FORAGE LEVEL AND NALOXONE INJECTION OF FEED-INTAKE, DIGESTION, AND PLASMA-HORMONE AND METABOLITE CONCENTRATIONS IN DAIRY HEIFERS

Four ruminally cannulated Holstein heifers (287 +/- 26 kg) in a 4 x 4 Latin square were used to evaluate the effects of naloxone injection a nd forage level on dietary intake, ruminal fermentation characteristic s, digestibility, and plasma hormone and metabolite concentrations. Tr eatments were arranged in a 2 x 2 factorial with naloxone injection (0 vs .3 mg/kg; saline us naloxone, respectively) and forage level (20 v s 100%; concentrate vs forage) as factors. Stanchioned heifers were al lowed 21 d for adaptation before a 5-d collection period. Plasma sampl es were collected several times on d 1 and daily at 0730. Concentrate- fed heifers consumed more feed (P < .10) than forage-fed heifers. Nalo xone injection decreased (P < .10) feed intake (grams/kilogram BW) at 1 and 2 h after feeding on d 1. On d 3 at 24 h after feeding, naloxone -injected heifers had increased DM (P < .10) intakes compared with con trol (saline-injected) heifers. Concentrate-fed heifers had decreased (P < .10) ruminal pH and increased total ruminal VFA concentration. Ac etate proportion was decreased (P < .10) and propionate proportion inc reased in concentrate-fed heifers. Naloxone-injected heifers had decre ased (P < .10) total VFA concentrations and increased propionate propo rtions. Concentrate-fed heifers had greater (P < .10) DM, OM, and CP d igestibilities as well as increased plasma (P < .10) insulin, urea N, and glucose concentrations but decreased (P < .10) GH and NEFA concent rations. Naloxone injection increased (P < .10) plasma insulin concent ration. Naloxone injection in dairy heifers reduces intake up to 2 h a fter injection, alters ruminal fermentation patterns, and increases pl asma insulin concentration.