Intake and digestibility of fresh grass fed to sheep indoors or at pasture, at two herbage allowances

The effect of two feeding systems (indoors and at pasture) on intake and digestion of fresh 28-day regrowth Digitaria decumbens grass was studied at two herbage allowances: a low herbage allowance (1.3 × animal voluntary intake), and a high herbage allowance (1.5 × the low level). Sixteen adult Martinik rams, including eight fitted with rumen cannulae, and weighing, on average, 52.4 (±0.25) kg, were randomly assigned into four groups, according to a 4 × 4 Latin Square design. In vivo organic matter digestibility (OMD) was measured indoors by total collection of faeces. In addition, OMD was estimated indoors and at pasture using the fecal crude protein (CPf) method (OMDCPf). In vivo organic matter intake (OMI) was then estimated from fecal organic matter output, and OMD estimated using the CPf method (OMICPf). Indoors, the CPf method gave precise estimates of in vivo OMD (r = 0.61 and P<0.001) and OMI (r = 0.79 and P<0.001). At high herbage allowance OMDCPf at pasture was greater compared to indoors, whereas, for both herbage allowances, OMICPf and digestible organic matter intake estimated from OMDCPf and OMICPf were greater indoors compared to at pasture. Indoors, OMDCPf increased from the low to the high herbage allowance, whereas it did not vary at pasture. At the same time, OMICPf increased indoors by 8.5 g/kg LW0.75 from the low to the high herbage allowance, whereas it did not differ at pasture. The greater ammonia concentration in the rumen of rams at pasture illustrates the greater selective behavior of the rams and explains this difference in OMDCPf, whereas, the difference in OMICPf was linked to differences in prehensibility of forage between the two feeding systems. This study shows that differences in OMDCPf and OMICPf exist between animals fed indoors and at pasture with the same forage, and that these differences may vary according to the quantity of grass on offer, mainly due to variations indoors.