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

Introduction

Minimising the mortality of weaner lambs is required to optimise production and avoid the perception of poor animal welfare. Mortality has been reported as exceeding the industry benchmark of ≤4% on 50% of Merino enterprises (Campbell et al. 2014), with 80.3% of deaths occurring within 3 months of weaning (Hatcher et al. 2008). In extensive grazing systems, the lightest 25% of weaner lambs are more than two times more likely to die than are heavier lambs if adequate nutrition is not provided (Hatcher et al. 2008). Lighter weaners therefore require targeted management.

Separate management of lighter weaners improves their survival in the grazing context (Campbell et al. 2014), presumably through allocation to better nutrition. Separate management aims to avoid weight loss and slow growth, as these indicate a higher risk of mortality (Hatcher et al. 2008), with even heavy weaners having high mortality if they lose weight (Hatcher et al. 2010). Under poor pasture conditions, Merino weaner lambs may be fed in confinement (pens) to prevent overgrazing. The purpose is to enable retention of animals rather than to grow quickly to slaughter weights and sale, with the recommendation by the DEDJTR VIC (2018) to achieve slow growth rates (minimum 66 g/day or 2 kg/month for lambs of <20 kg) to optimise survival. Achieving survival particularly in the higher-risk light lambs is the primary aim to protect welfare and contribute to later sale income, as these lambs may not be retained for breeding.  

Pen-feeding introduces behavioural factors that may adversely affect the feed intake and growth of light weaners. Studies with feedlot Merino lambs showed no association between liveweight and the proportion of shy feeders (Rice et al. 2016). However, lighter, younger crossbred weaners have a greater chance of being shy feeders than do heavier weaners when fed supplementary grain at pasture (Holst et al. 1997). Similarly, in pen-fed adult sheep (3–4 years), larger individuals tend to be more dominant (Lobato and Beilharz 1979), so may compete more strongly for restricted resources than do smaller sheep age and breed can also influence competitive feeding behaviour (Arnold and Maller 1974).

However, whether the growth rate of light-weight lambs is compromised if they are penned together with heavy lambs rather than separately when fed to a similar slow growth target is not documented. Therefore, it was hypothesised that lighter lambs that are fed in a mixed pen with heavier lambs would have reduced weight gain and wool production associated with more competition than for heavy lambs or those fed separately.

Materials and methods

Results

Discussion

The weight gain and wool production of lighter lambs was not compromised when they were fed in the same pen with heavy lambs, allowing rejection of the hypothesis. This study supports the finding of Hatcher et al. (2010) that weaners of light weight but positive growth can have a high survival rate. Identifying weaners with negative or slow growth, and preferential management of those, rather than light lambs in good condition, may be a more effective strategy to improve weaner survival through achieving adequate growth rates. Our study excluded lambs considered unhealthy and some of those that were <17 kg, which may have been those at a highest risk. However, the study still involved a light cohort where their weight at <20 kg is considered risky (Hatcher et al. 2008), and these lambs did not need to be separated from heavier lambs to achieve similar growth rates.

The slower growth in the Mixed compared to Light treatment may indicate possible underfeeding because the treatments were fed to achieve the same weight gain and there were no feed refusals. Alternatively, the Mixed groups may have been less efficient in feed use than were the Light lambs. The 44.5% more bullying that occurred within the Mixed treatment may have contributed to the lower feed efficiency and weight gain while being pen-fed. The higher weight gains of the Mixed treatment while grazing suggests compensatory growth when nutrition is improved (Thornton et al. 1979), which could include the removal of bullying and competition for feed.

A greater incidence of bullying within pens containing lambs with a larger range in weights was expected because social hierarchy or the incidence of negative social interactions can be influenced by liveweight (Dove et al. 1974), although this relationship does not occur consistently (Rice et al. 2016). It is possible that bullying may have affected the weight gain of lambs in Mixed pens through increased exercise because of more competition for space at the feed trough, but movement was not recorded. The distinction between whether the heavy or lighter lambs were responsible for the bullying was also not made. Instead, only the total number of bullying incidents was recorded, creating a limitation in the behavioural observations.

The present study found no difference in the incidence of shy feeders between the Mixed and Light treatments. In contrast, Holst et al. (1997) suggested that the lightest 25% of the mob were more likely to be characterised as shy feeders when offered oat grain, while grazing pasture post-weaning. Competition for trough space has increased the incidence of shy feeders when length of trough was less than 4 cm per sheep for wethers fed at maintenance (McDonald 1986), but carcase weights were not increased with trough length above 5 cm per sheep (Dundon and Mayer 2015). The length of trough in the present study was well above these minimums, such that competition for feed trough, and hence shy feeding, was not increased in the Mixed treatment. However, the percentage of shy feeders was elevated in the first week of feeding (17.8%) compared with subsequent weeks (≤1.3%), being consistent with reported reductions after the first week of feeding (Bowen et al. 2006). Although no shy feeder lambs needed to be removed during the present study, some sheep may not adapt after even 2 weeks and so continued monitoring is required to allow timely removal to prevent continued weight loss (Robertson et al. 2022). The rapid decline in shy feeding in the present study was probably due to adaptation to the novel pen environment and feed troughs. Savage et al. (2008) reported higher rates of shy feeders for weaners previously exposed to feed, but not feed in troughs. Weaners in the present study had been imprint fed with the same feeds pre-weaning with their dams, and this would have contributed to the rapid adaptation to the pen-feeding environment (Thorhallsdottir et al. 1990).

However, shy feeding in Week 1 was probably also influenced by adaptation to a new group of individuals because changing social group and location (pen) has increased aggressive behaviours in lactating ewes for a week after changes (Sevi et al. 2001). Although weaners may be less aggressive than ewes, the increased incidence of bullying observed in the Mixed treatment in the first week of feeding is indicative of the re-establishment of social hierarchy. A greater number of lambs in each pen may also have altered behaviour and led to potentially different results. However, evidence for effects of mob size where adequate trough length is provided is lacking, and such studies are needed with commercially relevant numbers.

Selection of ration components potentially contributes to differences in weaner performance (Berthel et al. 2024), with more competitive lambs possibly consuming a different ration from that of lambs that are less competitive. Under ad libitum feeding, presentation of the same ration in pelleted, total mixed ration or hay and grain separately has not resulted in differences in the weight gain of sheep (Bowen et al. 2006). Preferential consumption of ration components by heavier lambs did not appear to occur in the present study, as indicated by similar weight gains between lighter and heavy lambs within the same pen. This may have been influenced by the ample trough space, but also by the limited quantity of feed and high quality of both hay and grain and that lambs were familiarised to feeds prior to the study.

The mean condition score of lambs was 2.8 and consistent between treatments. Lighter lambs within a mob might be expected to be either younger, smaller framed, or with lower condition score with a higher representation of twin- rather than single-reared lambs. As the producer excluded most lambs under 17 kg (unfasted) (estimated at 12 lambs or <5% of the flock if a normal distribution of weights) and those considered unhealthy prior to the present study (percentage unknown), some of which may have been in lower condition, it is possible that in this study there was not an extreme tail end of lambs. It may be more challenging to gain weight in the extreme tail end of unhealthy lambs, with an increased risk of shy feeders, bullying, and deaths. Therefore, there may be a greater probability that separate feeding of those lambs will be beneficial. Additionally, the small number of lambs in pens (58 or 59) and spacious pen design in the mixed-weight pens may have limited competition and negative behaviours, which may be more prevalent in larger mobs. In commercial flocks, high-risk lambs may be removed for preferential nutritional treatment, but these would be expected to be a low percentage of the mob. The small size of such mobs may provide reduced competition, more rapid detection and treatment of sickly individuals, as well as improved nutrition, all of which would promote survival.

Hatcher et al. (2010) identified that the tail end with negative growth (−9.7 g/day; 14.1% of the mob), rather than weaners of lighter weight but positive growth, were at greatest risk of mortality. The exclusion of high risk lambs, feeding to the target growth of 100 g/day, combined with best-practice management of imprint feeding, high-quality ration and introduction to grain may have prevented poor performance of light lambs in mixed-weight pens and potential benefits from the separate feeding of lighter lambs in the present study, even though the lightest 25% of weaners were evaluated and the weights of this lightest cohort (mean < 20 kg) was below 22 kg where there is an increased risk of mortality (Campbell et al. 2009). However, further studies are needed to evaluate whether a more prolonged period of feeding in mixed-weight pens would result in differences in growth of a cohort becoming evident.

Heavier weaners could be expected to have greater wool growth than have lighter weaners when fed at maintenance (Masters et al. 1998). In the present study, the length of wool growth and fibre diameter was not increased by separate feeding of lighter weaners and was similar between lighter and heavy lambs, possibly because the condition score of lighter and heavy lambs was similar and, when fed in the same pen, their weight gain was similar. Thus, there were no practical benefits from the separate feeding of lighter lambs and heavier lambs for wool production.

Conclusions

This study indicated that the separation of the 25% lightest lambs based on liveweight may improve animal welfare through a 44.8% decrease in bullying. Because the lighter and heavy lambs in the Mixed treatment gained weight and produced wool at a similar rate, there was no production advantage from the separate feeding of lighter weaner lambs. The exclusion of unhealthy and the extreme lightest lambs from the study may have influenced this result, and further studies are needed to validate the findings.