Managing lamb growth rates and weaning
For a lamb to thrive, it needs nourishment and protection to survive and grow through to maturity.
Initially all energy comes from its mother’s milk, with milk yields peaking at 2-3 weeks post birth for twins, and 3-5 weeks for singles. By 2 weeks of age lambs begin to graze, and by 3 weeks it can nearly, fully digest the pasture. Because twin lambs are sharing the milk supply, they get less milk each and begin consuming more pasture earlier compared to single born lambs; by 6 weeks twin lambs consume 35% of their energy intake as pasture, whereas single lambs are consuming 22% at the same point (Figure 1). By 8-10 weeks milk is providing only 25-40% of the energy requirement.
Typically, we wean at 12-14 weeks of age when lambs are getting most of their energy from pasture. There will be a spread of ages of lambs weaned depending on how long ewes were joined for. Tighter joining periods enable you to target a narrower range of lamb ages at weaning.
Early weaning of lambs is a useful strategy to target feed ewes and lambs and gain feed efficiency in lean years or when feed comes late. Ewes lose condition during lactation, so weaning 4 weeks earlier (from lamb age 10 weeks) gives ewes more time to recover before their next joining. For every 1 condition score (CS) put on ewes, 20 more lambs per 100 ewes are conceived. Check ewe CS at lamb marking; if they’re in good condition continue with a standard weaning. If CS is low and ewes needs body condition recovery for joining to reach CS 3, consider an earlier weaning and feed her more (will often be twin and triplet bearing ewes) so there is enough time to regain condition. If lambs are too light at the early weaning (eg. <20kg), it’s best to leave them on mum for longer and have a 2-stage weaning.
A weaner survival rate of 95% or better can be achieved with targeted nutrition and health. The standard weight target for weaning is 45-50% of their mature standard reference weight (SRW) (Table 1). In a tough year this can be hard, so will need to push weaner growth rates until they pass the 45% SRW target.
Table 1. Standard weight targets (% of adult liveweight) for replacement ewes
To reach the target weights, calculate growth rates needed and provide the nutrition accordingly.
Required growth rate (g/day) = (target weight – starting weight (kg) ) / time (days) x 1000
For example, for a 70kg adult weight, lambs need to grow at 265 g/day to reach 31.5kg by weaning at 14 weeks (31.5 – 5.5) / 98 x 100 (Table 2).
Table 2. Minimum growth rate targets to reach weight targets at weaning and joining
Manage stress over weaning to minimise the chance of the lambs having a growth rate check. Leaving mum, having new paddock, feeders, water source, loose licks etc imposes stress on lambs, which reduces appetite and nutrient absorption, causes lambs to urinate more and dehydrate, and they may have diarrhoea and suppressed immunity. Introducing lambs to new paddocks or yards and feeding methods before weaning will ease the transition. This year, lambs will have likely benefited from imprint feeding before weaning already.
Manage health by vaccinating for pulpy kidney, ensure safe introduction to starchy feeds to prevent acidosis, and monitor for worms. Even if lambs have been imprint fed on their mothers, feed again with starchy feed at least 3 times after weaning to ensure the rumen fully develops, even when weaned onto good pasture. Leave enough time between marking and weaning for marking wounds to heal; this year if you plan to wean earlier, you may also need to mark earlier.
Weaning in containment works well and enables establishing pastures to grow better feed quantity and quality. Lambs have already bonded with the ewes in the paddock, so ewes with their lambs can be penned in bigger mobs at 200-300 ewes per pen for feeding until weaning time.
Once lambs are weaned, and dry ewes and lambs are fed separately, feeding becomes more efficient and there can be a 30% reduction in feed requirements.
Prioritise which lambs you need to put weight on the fastest, and calculate growth rates needed to reach joining or sale target weights (Table 2). This year you may want to sell lambs earlier as 35kg stores; you could draft off ewe and wether lambs separately, then slow down growth rates of ewe lamb replacements to save on feed, while feeding sale lambs to reach target weights faster.
Webinars by Deb Scammell, Talking Livestock, and Steve Cotton, Dynamic Ag, AWI’s Feed budget tables for Merino Weaners provide useful information to assist planning and calculating feed budgets.
