Effect of nitrogen and energy supplementation on intake, digestibility and rumen fermentation efficiency in sheep fed poor quality Eragrostis curvula hay

Abstract

The overall aim of the study was to study the effects of starch and urea supplementation on roughage intake, digestibility, and microbial nitrogen synthesis (MNS) in sheep fed low-quality Eragrostis curvula hay (< 3% CP, 80% NDF; DM basis). In Trial 1, urea partially substituted the rumen degradable nitrogen (RDN) fraction of sunflower meal (SFM). Neutral detergent fibre (NDF) digestibility was higher in the treatments where urea substituted 45% and 60% of the RDN fraction of SFM. Treatment did not affect roughage intake, rumen ammonia nitrogen (RAN), MNS or efficiency of MNS (EMNS), suggesting that urea could substitute up to 60% of the RDN supplemented by SFM in sheep fed low-quality E. curvula hay. In Trial 2, the quantities of urea and starch supplemented to sheep differed. Urea supplementation did not affect roughage intake and digestibility, however, MNS: available Nitrogen intake (MNS:NI) improved from 2.21 to 0.88 as urea supplemented increased from 10.4 g urea/sheep/day to 32.4 g urea/sheep/day. Microbial N synthesis increased up to the highest level of starch supplemented (280 g/sheep/day). It was concluded that urea supplementation, as high as 26.4 g urea/sheep/day, coupled with starch supplementation, between 240 g and 280 g starch/sheep/day, could be supplemented to sheep (50 kg BW) consuming low-quality E. curvula hay. In Trial 3, RDN and energy supplementation patterns differed in sheep fed lowquality E. curvula hay. Treatment did not affect roughage or N intake; however, roughage digestibility was higher in treatments where starch was supplemented, at least partly, during the morning (08h00) supplementation period. Urinary N excretion, MNS and EMNS were generally higher in the treatments where starch was supplemented twice daily. It was concluded that, while the most optimal rumen pH was achieved in the treatment where both urea and starch was supplemented twice daily, the supplementation frequency of starch was the more important parameter, compared to urea, stimulating roughage digestibility, MNS and EMNS in sheep fed low-quality E. curvula hay. To conclude this research, a meta-analysis was conducted to study the importance of supplemental starch and/or urea on N efficiency in sheep fed low-quality E. curvula hay. Starch or urea supplementation did not affect roughage intake or digestibility. Starch supplementation affected MNS linearly while urea supplementation influenced RAN linearly and MNS:NI inversely, with MNS:NI decreasing as urea supplementation increased. A strong correlation was observed between starch: available CP and RAN, with RAN increasing exponentially as the ratio of starch to available CP decreased below 2:1. It was concluded that urea and starch supplementation, up to 0.5 g urea/kg BW and 2.2 g starch/kg BW respectively, were necessary to optimise N efficiency in sheep grazing low-quality E. curvula hay.