The reproducibility of in vivo estimates of digestibility and voluntary digestible organic matter intake of grass varieties by sheep

Four cocksfoot varieties were harvested at three different growth stages and offered ad libitum to yearling Clun wethers in three 24-d feeding trials indoors over a 4-month period. A further cocksfoot variety conserved in bulk was offered concurrently as a control feed.
The concentration of digestible organic matter in the dry matter (DOMD) and voluntary intake were measured in each feeding period. There was no significant change in the measured DOMD values between the different feeding periods. Voluntary intake of digestible organic matter (DOMI) increased over the 4-month period for all four varieties and for the control feed. The increase in intake was greatest for the most digestible material, i.e. first-cut harvests. Differences in DOMI were statistically significant between feeding periods for all varieties but the differences could be eliminated by correcting intakes with values from the control feed. It is concluded that comparisons of grass varieties involving intake measurements must either be made in the same feeding period or a control feed included and results corrected according to the control response.