Ruminal Responses in Cattle Grazing Tall Fescue Pastures Differing in Endophyte Level

Three 1.2-ha tall fescue (Festuca arundinacea, Schreb.) pastures at the Blount Livestock Unit of the Knoxville Experiment Station containing 1) Kentucky-31 endophyte (Neotyphodium coenophialum) infected tall fescue (KY-31/E+), 2) Jesup endophyte-free tall fescue (Jesup/E-), or 3) Jesup infected with a non-ergot alkaloid producing endophyte tall fescue (Jesup/MaxQTM) were grazed. The objectives of this experiment were: 1) to validate endophyte status of the experimental pasture through determining ergovaline levels of masticate samples and animal response using serum prolactin concentrations, weight gain, and rectal temperatures, 2) to compare weight and composition of rumen contents of cannulated beef steers grazing tall fescue pastures that differ in endophyte status, 3) to determine the effect of time of grazing on the in situ and in vitro nutrient disappearance of commonly used supplemental feedstuffs, and 4) to compare nutrient composition of rumen content to masticate sampling of the pastures. Two ruminally cannulated steers were randomly assigned to each pasture treatment in early May 2003. After an 18-day acclimation period, steers were subjected to four 0700 and four 1900 rumen evacuations, where contents were weighed and sub-sampled for DM, NDF, ADF, and CP analyses. After rumens were evacuated and washed, steers were returned to their respective pastures to graze for about 45 minutes. Masticate samples were collected from the rumen for determination of pasture forage quality. Between evacuation days, in situ degradation of dehydrated alfalfa hay, corn grain, corn gluten feed, and soybean hulls was performed by placing five of substrates into 10x20 cm Dacron bags and incubating them in the rumen of steers grazing the forage treatments. Samples were incubated for 12 h, from 0700-1900 and from 1900-0700, to evaluate differences in day and night ruminal environments. Samples incubated in situ were analyzed for DM, NDF, ADF, and CP disappearance. Steers gained less (P \u3c 0.05) weight while consuming KY-31/E+ tall fescue than Jesup/E- or Jesup/MaxQ. The ADG for KY-31/E+ was 0.29 kg/d (P \u3c 0.05), while Jesup/E- and Jesup/MaxQ were 0.65 kg/d and 0.70 kg/d, respectively. Serum prolactin levels were significantly reduced in steers grazing KY-31/E+ compared to steers grazing Jesup/E- or Jesup/MaxQ. Rumen DM contents for steers grazing Jesup/E- or Jesup/MaxQ were greater (P \u3c 0.05) than steers grazing KY-31/E+ and 1900 content weights were greater than 0700 content weights, indicating that steers consume more forage during the day than overnight. Rumen contents were compared to masticate samples to determine if rumen samples could be used to evaluate forage quality. There were significant (P \u3c 0.05) differences between the two sampling methods. Therefore, rumen content sampling should not be used in place of masticate sampling to evaluate forage quality. Steer rectal temperature was higher (P \u3c 0.05) at 1900 in steers consuming KY-31/E+ compared to Jesup/E- or Jesup/MaxQ. In situ DM, NDF, ADF, and CP disappearance of corn grain was greater (P \u3c 0.05) in steers grazing Jesup/E- and Jesup/MaxQ than in steers grazing KY-31/E+. In situ alfalfa hay and corn gluten feed DM, NDF, and ADF disappearance tended (P \u3c 0.15) to be greater in steers grazing Jesup/E- and Jesup/ MaxQ than in steers grazing KY-31/E+. Soybean hull disappearance was not affected by endophyte status of pastures. Degradation of substrates was not affected by time of day of incubation