Separation of light weaner lambs may not improve growth rates or wool production when pen-fed for drought feeding

Context. The lightest 25% of weaner lambs have double the risk of mortality, but it is unclear whether separation of lighter from heavier lambs will improve growth rates when pen-fed for slow growth. Aims. This study compared the behaviour and performance of the lightest 25% of weaners when pen-fed in mixed-weight groups or when fed separately. Methods. Merino wether weaners (n = 234; aged 6 months) were allocated to two replicates of the following two treatments: Light, containing only the lightest 25%, or Mixed, containing 25% lighter and 75% heavier lambs, with 58 or 59 lambs per pen. Lambs in both treatments were fed to gain 100 g/day.head for 6 weeks, and then grazed pasture for a further 6 weeks. Wool, liveweight, and behavioural data were collected. Key results. During the pen-feeding period, light and heavy lambs in the Mixed treatment gained similar weight (4.0 ± 0.43 and 3.7 ± 0.24 kg respectively; P = 0.555). Light lambs that were fed separately gained as much weight as lighter lambs that shared a pen with heavier lambs. However, overall, lambs in the Light treatment group gained significantly more weight than did those in the Mixed treatment group (4.5 ± 0.20 and 3.7 ± 0.20 kg respectively; P = 0.007). The percentage of lambs eating at feeding increased (P 98%, but was similar between treatments. The incidence of bullying observed at feeding was lower in the Light than Mixed treatment (3.2 ± 0.59 and 7.3 ± 0.59%/min respectively; P < 0.001) and declined after the first week of feeding (7.8 ± 0.75 in Week 1, 4.7 ± 0.75%/min in Week 2 and thereafter; P = 0.007). Wool fibre diameter and staple length were similar between treatments. Conclusions. This study indicated that the separate pen-feeding of the lightest 25% of weaner lambs did not improve weight gain or wool production but may improve animal welfare through a 44.8% decrease in bullying. Implications. The lightest 25% of weaners can grow at rates similar to those of heavier lambs when fed for slow growth in mixed-weight pens, provided that sickly individuals are removed, and the weaners have been exposed to feeding practices prior to weaning.