BY: DR KEN GEENTY
Realising the genetic potential of your lambs is very dependent on their efficiently converting feed to liveweight gain. This efficiency is underpinned by a good maternal environment during pregnancy followed by astute management during the all-important milk and pasture feeding of lambs pre- and post-weaning.
The management and feeding options discussed here are based on research by this author several decades ago, still as relevant today as originally.
Ewe pregnancy
It is very hard to maintain ewe body condition during mid-late pregnancy because of limitations in space needed for both the rumen and developing foetus(es). For example, a 65kg ewe carrying twins rarely consumes the 1.6-2kg of dry-matter per day required in late pregnancy to maintain a body condition score of 2.5-3. The sheer bulk of up to 8kg of fresh pasture obviously limits how much a ewe can eat during this pregnancy period. Therefore most highly productive ewes lose body condition during pregnancy.
Such loss of body condition should be minimised or avoided to support important growth of the udder and foetus shown in the diagram below. It is unlikely placental growth will be affected but reduction in foetal and mammary growth can seriously impact lamb survival and early growth.
The most effective way of minimising ewe body weight loss during this late pregnancy period is through generous supply of high quality pasture, supplemented as needed with feed having an energy content of at least 10 megajoules of metabolisable energy (MJME) per kilogram of dry matter (kg DM).
High quality hay or balage can do the job in association with available pasture or crop. Attention to animal health to avoid metabolic pregnancy disorders, or known mineral deficiencies, is also important during this pregnancy period.
Lambing to weaning
There is a lot of flexibility in managing lambs to weaning, including different ages weaned. Various factors such as the season and feed supply will determine optimum times to wean. For example, if seasonal feed is in very short supply due to dry conditions it will be very beneficial to wean lambs early onto saved feed and tighten weaned ewes up. Remember that the moment you wean lambs and ewes stop lactating, their feed requirement pretty much halves to under 1kg DM per day. Furthermore, when lambs are weaned they can be given the best quality feed available and are not competing with ewes.
Importantly, as the lamb weaning ages graph below shows, the growth trajectory of lambs is similar whether on milk from their mothers or good quality pasture alone. This is because the nutritional value of high quality spring pasture is similar to that of ewe’s milk. But economically pasture is vastly more efficient than milk, valued on variable costs, at less than 20 cents per kg of dry-matter compared with around $12 per kg of milk solids based on sheep dairy prices. Furthermore, for ewes the feed cost of producing milk is very expensive. Total pasture required by ewes and lambs between five and 10 weeks of age is 40 percent greater compared with when lambs are weaned at five weeks. So it is much more efficient to channel feed directly through lambs from as early an age as possible.
A key requirement for very early weaning of lambs at 4-5 weeks old, under conditions of extreme feed shortage, is a minimum lamb liveweight of 12kg. Use of creep grazing can assist greatly with early weaning, simply by raising a gateway high enough for the lambs to creep ahead of the ewes onto fresh pastures. Lambs need high quality pasture with good nitrogen levels for early rumen development. Leafy lucerne does this job admirably though good clover-dominant pasture is equally effective.
If quality pasture is plentiful lambs can stay on their mothers until 12 weeks old as long as they are growing at around 200g per day to reach at least 24kg at the later weaning age.
In the experiments shown in the graphs ryegrass-clover dominant pastures were used pre-weaning with lucerne post-weaning. Associated lamb growth and development studies showed that lamb rumen development was surprisingly advanced from three weeks of age meaning very early weaning was not a major setback to lambs. If pasture is not in good supply supplements such as lucerne chaff are ideal to promote lamb rumen function in preparation for early weaning.
Artificial rearing
An alternative way of rearing orphan lambs or those from dairy ewes is use of cold ewe milk replacer as demonstrated experimentally in association with the lamb weaning age experiments above. Lambs were offered cold ewe milk replacer in feeders in the paddock with liberal high quality ryegrass-clover pastures. The objectives with chilled milk were to limit intake at each feed and avoid digestive upsets and, at the same time, promote early intake of pasture and rumen development.
Newborn lambs are monogastrics just like us and unable to digest herbage. So early rumen development is crucial for efficient conversion of pasture to liveweight gain as lambs become self-sufficient from their mums.
This innovative lamb rearing system worked extremely well with lambs successfully weaned at 30 days of age weighing 12kg on average. After weaning the lambs grew at a similar rate to those conventionally early- or later-weaned from ewes. Total amount of milk replacer used per lamb was 6-7kg at a cost in today’s prices totalling about $50 per lamb.
If there had been no colostrum to kick the lamb’s immune system into action an artificial form would need to be given at around $14 per lamb.
Post-weaning
The key requirement for lambs after weaning is a good supply of quality pasture or specialist crops such as brassicas, lucerne or herbs, all with a protein content above 15 percent. To achieve growth rates of 200+ g per day lambs will need to consume 13+ MJME per day or 1+kg per day of dry matter, requiring liberal offers of the high quality feed.
In the lamb weaning age experiments referred to above, quality leafy lucerne did the job admirably.
A key factor for lambs post-weaning is good animal health management including effective parasite control and supply of mineral supplements where there are known deficiencies. Lambs will also need to have had earlier immunisation against clostridial diseases.
- Dr Geenty is a primary industries consultant.
