LAUSR

Abstract 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