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

Bridget Longley; Emma Doyle; Susan Robertson

Just Accepted

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

Bridget Longley, Emma Doyle , Susan Robertson

Abstract

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 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 six weeks, and then grazed pasture for a further six weeks. Wool, live weight, 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 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<0.001) after the first week to >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 %/minute, respectively; P<0.001) and declined after the first week of feeding (7.8 ± 0.75 in week 1, 4.7 ± 0.75 %/minute in week 2 and thereafter; P=0.007). Wool fibre diameter and length growth were similar between treatments. Conclusions: This study indicates 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 similar rates to heavier lambs when fed for slow growth in mixed pens, provided that sickly individuals are removed, and the weaners have been exposed to feeding practices prior to weaning.

AN25036 Accepted 07 July 2025