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.
