Effect of hay substitution on intake and digestibility of forage rape (brassica napus) fed to lambs.

Intake and digestibility trials were conducted with sheep to evaluate the effect of adding various levels of a typical fibrous grass forage (neutral detergent fiber, NDF = 68%) to a high quality, low fiber (NDF = 22%) brassica forage. Four forage rape:orchardgrass hay diets (0, 40, 70, 100% rape content on a dry matter basis) were fed to groups of six Polled-Dorset crossbred growing wether lambs (39.6 kg) individually housed in metabolism crates. After a 7-d ad libitum intake period, a 7-d digestibility trial was conducted at 90% of the observed ad libitum intake level. Digestible dry matter intake (DDMI) per unit metabolic body weight increased as rape in the diet was increased from 0 to 70%, with increases in both dry matter intake and dry matter digestibility (DMD). However, DDMI was similar for lambs fed the 70 and 100% rape diets, with DMI decreasing to the same relative extent as DMD increased. Digestibility of the cell wall fraction of the two intermediate diets (40% rape and 70% rape) was lower than predicted from component forage digestibilities. This observation suggests the existence of an associative effect similar to that often reported when forage and concentrates are fed in various ratios. Forage brassicas appear to be utilized in the ruminant in a manner more typically resembling a concentrate than a forage.