A summer lamb finishing experiment at Palmerston North, New Zealand compared the performance and health of weaned lambs grazing Aries HD or Yatsyn 1 perennial ryegrass (Lolium perenne). Aries HD ryegrass was originally selected for increased digestibility over summer and early autumn. Two groups of Romney×Suffolk lambs (15 per group) were rotationally grazed on each cultivar in a leader-follower sequence to assess both the nutritive value of the pastures and the alkaloid challenge imposed by the respective cultivar/endophyte (Neotyphodium lolii) associations. Herbage from leader pastures contained a higher content of ryegrass (600 v. 474±27·3 g/kg, P < 0·001) and a lower content of dead material (355 v. 479±34·2 g/kg, P < 0·001) than follower pastures. The in vitro organic matter digestibility coefficient of leader pastures was greater (0·62 v. 0·55±0·024, P < 0·05) and the neutral detergent fibre content lower (592 v. 631±11·4 g/kg, P < 0·005) than follower pastures. Consequently, the leader lambs had faster liveweight gains (92 v. 53±10·6 g/day, P < 0·0005) than the follower lambs. Aries HD and Yatsyn 1 cultivars were similar in sward composition, in vitro organic matter digestibility, neutral detergent fibre content and nitrogen content. Lambs grazing Aries HD had a higher herbage intake in late January than lambs grazing Yatsyn 1 pasture (0·95 v. 0·85±0·023 kg OM/day, P < 0·005). Overall, the liveweight gains of lambs grazing Aries HD and Yatsyn 1 were similar (71 v. 73±10·6 g/day, P > 0·7).

Grazing sequence and cultivar did not affect the concentrations of lolitrem B and peramine from herbage cut to ground level. However, serum prolactin concentration, which can indicate ergovaline intoxication, was lower in lambs grazing follower swards (181 v. 120±18·0 ng/ml, P < 0·005). Ergovaline concentration in Aries HD herbage was about half the concentration detected in Yatsyn 1 herbage (0·25 v. 0·49±0·050 mg/kg, P < 0·001). In addition, respiration rate, which can be an indicator of heat stress, was higher in lambs grazing Yatsyn 1 than Aries HD (77 v. 74±1·1 breaths per minute, P < 0·005). All lambs on all treatments had clinical ryegrass staggers from 10 February to 9 March. Lambs were removed from the experiment on 9 March. Both grazing sequence and cultivar affected the severity of ryegrass staggers. Staggers score was highest in Yatsyn 1 followers (4·4) and lowest in Aries HD leaders (3·1). Both grazing sequence and cultivar affected the alkaloid challenge imposed on lambs, resulting in differences in staggers severity, heat stress and performance of the lambs over the summer and autumn period.