Effect of fescue toxicosis on whole body energy and nitrogen balance, in situ degradation and ruminal passage rates in Holstein steers

This study was designed to examine alteration of ruminal kinetics, as well as N and energy balance during fescue toxicosis. Six ruminally cannulated Holstein steers (bodyweight (BW) = 217 +/- 7 kg) were weight matched into pairs and pair fed throughout a crossover design experiment with a 2 x 2 factorial treatment structure. Factors were endophyte (infected, E+ vs. uninfected, E-) and feeding level (1100 (L) or 1800 (H) kJ/kG BW.75). During each period, after 8 days of feeding level adaptation, animals were ruminally dosed twice daily with ground fescue seed for the remainder of the period. One steer per pair was dosed with ground endophyte infected fescue seed (E+), the other with ground endophyte free fescue seed. In situ degradation of ground alfalfa was determined on Days 13-16. Total faecal and urinary collections were performed on Days 17-21, with animals placed into indirect calorimetry head-boxes during Days 20 and 21. Heat production (HP) was calculated using the Brower equation. Retained energy (RE) was calculated as intakeE -(faecalE + urinaryE + gaseousE + HP). Liquid and particulate passage rates were evaluated using Cr: EDTA and iADF respectively on Days 22 and 23. There was no difference (P > 0.9) in dry matter intake (DMI)/kg(.75) between endophyte treatments, and DMI/kg(.75) was different (P < 0.01) betweenHand L intake by design. Animals on H feeding had higher (P < 0.01) water, N and energy intakes. Energy and N excretion, as well as retained DE, ME, RE, and HP were higher (P < 0.03) for H versus L. There was no difference in retained N, DE, ME, or HP (P > 0.15) between endophyte treatments. Neither rate nor extent of in situ degradation was altered by intake level or endophyte treatment (P > 0.3). DM percentage and DM weight of rumen contents were increased (P < 0.01) by E+ dosing. Particulate passage increased (P = 0.0002) during H intake and decreased (P = 0.02) with E+ dosing. Ruminal liquid passage decreased (P < 0.03) with H feeding, while liquid flow rate tended to be reduced (P < 0.14) with E+ dosing. Total VFA concentration increased with both H feeding (P < 0.01) and E+ dosing (P < 0.0001). Despite these differences, the N and energy balance data indicate that the reductions in weight gain and productivity seen during fescue toxicosis are primarily due to reduced intake.