Nitrogen partitioning differs between sheep offered a conventional diploid, a high sugar diploid or a tetraploid perennial ryegrass cultivar at two feed allowances

Grazing sheep on ryegrass based pasture with elevated water soluble carbohydrate (WSC) concentration might improve N use efficiency and reduce N excretion into the environment. Diploid ryegrasses with elevated WSC have been bred and released as a high-sugar cultivars (HSG), and tetraploid ryegrass (TRG) generally has elevated WSC compared to conventional diploid ryegrass (CRG). The objective of the current study was to determine N partitioning in sheep fed HSG, TRG, and CRG offered at two feed allowances. Three N partitioning trial periods (Sep 2013, March 2014 and Nov 2014) were conducted with 30 Romney wethers (different sheep in each period) over 5 d of excreta collection measurements per period. For each period, three perennial ryegrass types (HSG, CRG and TRG) were each offered at two allowances (0.7 and 1.0 kg DM/d) (6 treatments × 5 sheep). As a proportion of N intake, urinary-N excretion was greater (P =  0.016) for sheep fed TRG compared with sheep fed CRG, and intermediate for sheep fed HSG. There was an interaction between cultivars and feed allowance levels for N retained as a proportion of N intake (P =  0.04) with retained-N being similar among cultivars at the high feed allowance, while sheep fed CRG had a greater retained-N as proportion of N intake than sheep fed TRG at low feed allowance, with HSG fed sheep intermediate. The ratio of urinary-N:faecal-N was lower (P <  0.001) for sheep fed CRG and HSG compared with sheep fed TRG. The ratio of digestible N to organic matter (OM) was greater (P =  0.003) for sheep fed TRG than sheep fed HSG, with sheep fed CRG intermediate. Forage WSC concentration and WSC:Crude protein and WSC:Neutral Detergent Fibre ratios correlated negatively (r −0.46 to −0.56) with urinary-N as proportion of N intake. Offering a higher forage allowance was, however, a more effective strategy to decrease N excretion and increase retained N, both as a proportion of N intake. In conclusion, urinary-N as proportion of N intake was lower for sheep fed CRG than for sheep fed TRG, with sheep fed HSG intermediate, and it was lower at a the higher feed allowance.