Performance of lambs and the incidence of staggers and heat stress on two perennial ryegrass (Lolium perenne) cultivars using a leader-follower rotational grazing management system

A summer lamb finishing experiment at Palmerston North. New Zealand compare d the performance and health of weaned lambs grazing Aries HD or Yatsyn 1 p erennial ryegrass (Lolium perenne). Aries HD ryegrass was originally select ed for increased digestibility over summer and early autumn. Two groups of Romney x Suffolk lambs (15 per group) were rotationally grazed on each cult ivar in a leader-follower sequence to assess both the nutritive value of th e pastures and the alkaloid challenge imposed by the respective cultivar/en dophyte Neotyphodium lolii) associations. Herbage from leader pastures cont ained 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 o f leader pastures was greater (0.62 v. 0.55 +/- 0.024, P < 0.05) and the ne utral detergent fibre content lower (592 v. 631 +/- 11.4 g/kg, P<0.005) tha n follower pastures. Consequently, the leader lambs had faster liveweight g ains (92 v. 53 +/- 10.6 g/day, P<0.0005) than the follower lambs. Aries HD acid 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 l ambs 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 lam bs grazing follower swards (181 v. 120+/-18.0 ng/ml, P < 0.005). Ergovaline concentration in Aries HD herbage was about half the concentration detecte d in Yatsyn 1 herbage (0.25 v. 0.49 +/- 0.050 mg/kg. P < 0.001). In additio n. respiration rate, which can be an indicator of heat stress, was higher i n 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 stagg ers. Staggers score was highest in Yatsyn 1 followers (4.4) and lowest in A ries HD leaders(3.1). Both grazing sequence and cultivar affected the alkal oid challenge imposed on lambs, resulting in differences in staggers severi ty, heat stress and performance of the lambs over the summer and autumn per iod.