It is very important to maximise lamb growth rates after weaning.
Rapid growth rates allow lambs to be drafted for slaughter as early as possible. Replacement hoggets reach target weights early in the season, so they don't compete with ewes for precious feed around mating.
When animals are growing rapidly, most feed is used for liveweight gain rather than maintenance.
Lambs should be weaned when they begin to compete with their mothers for feed. This varies from year to year, but is generally when average pasture height falls below 3 cm.
If feed is not limiting, lambs will typically be weaned at about 10–12 weeks of age. At this stage ewe milk production will have declined to about 1 litre/ewe/day. This represents around 30 per cent of the total dietary energy requirements of a single lamb (only 15 per cent for a twin).
This assumes the lambs are growing rapidly at about 250 g/day. If they are not growing this fast, then feed is likely to be limiting and the ewe's contribution to the total energy requirements of the lamb is likely to be even less.
High pasture quality is paramount if maximum growth rates are to be achieved.
If lambs are being offered plenty of feed, the major determinant of their growth rate will be the digestibility and metabolisable energy (ME) content of their diet.
Pastures should be clover-dominant and grasses should be mainly new green leaf, with few stems, or old or dead leaf material present.
Such pastures have protein levels of more than 20 per cent and an ME level of 11–12 MJ ME/kg DM.
It also very important to offer the lambs quantity as well as quality. If insufficient pasture is offered – no matter how good it is – lamb growth rates will be sub-optimum.
Conversely, if feed quality is low it won't matter how much is offered. Lambs will only maintain weight or even lose weight if ME values are below 9 MJ ME/kg DM.
Lambs should not be expected to utilise too much feed per grazing, or intakes and performance will suffer. On ryegrass/white clover pastures aim for 25–30 per cent utilisation.
This coincides with a pasture allowance of 4–5 kg DM/lamb/day, which will allow an intake of near 1.2–1.5 kg DM/lamb/day, from pastures that are in the 1500–2000 kg DM/ha range.
Such pastures are 3–5 cm in height. Grazing below this will depress lamb growth rates. If pastures are longer than 10 cm, feed quality will have declined and they should be grazed with other classes of stock.
Weaning paddocks should be prepared in advance for the lambs. They may have been grazed by the ewes and lambs, or cattle. A plus for using cattle grazers is that they reduce sheep parasite levels on the pasture.
In spring, a spell of 2–3 weeks is sufficient to have a good fresh, leafy sward for the lambs to go on to.
The trick is to keep fresh paddocks ahead of the lambs to ensure maximum growth rates continue. The lambs are the priority mob at this time of the year – use the ewes to clean up behind them.
Mechanical topping is also useful for maintaining pasture quality.
Silage or hay aftermath may be good lamb feed, but it may not be if there was high pre-cut pasture mass – due to a low clover content. One of the potential benefits is that aftermath may be lower in parasitic larvae than pasture.
The growth response from feeds carrying large numbers of larvae is less than feeds with equivalent ME and legume content, but with lower larval burdens. This is due to reduced intakes and the cost of the immune response when animals ingest larvae.
In practice it can be very difficult to reduce larval levels, and there is rarely such a thing as 'safe' pasture, but some steps can be taken. For example, some larvae are destroyed when establishing summer brassicas or new feed crops. Also the physical structure of the plants is less hospitable for infective larvae.
The first step is to monitor parasite levels and, if in doubt, discuss the results with your advisors.
Pasture quality deteriorates as the summer advances. So long as they have not gone to seed, forages like chicory, red clover, trefoil, sulla or summer brassicas – such as pasja or rape – can provide plenty of high quality feed with ME values in the 11–12 range. These will keep lambs growing at near maximum.
A guide to improved lamb growth. Chapter 6
A guide to feed planning for sheep farmers. Chapters 5 & 6
Your local farm consultant or veterinarian will also be able to help. Alternatively, contact your local WoolPro extension specialist:
Northern North Island: Sally Hobson tel 07-823 3321 or 025-924 751 Hawkes Bay/East Coast: Lew Willougby tel 06-835 1888 or 025-434 417 Southern North Island: Richard Gavigan tel 06-376 0006 or 025-499 851 Nelson/Marlborough/Canterbury: Alan Marshall tel 03-343 7913 025 329 399 South Canterbury: Julia Mackenzie, tel 03-680 6782 or 025-782 353 Otago: Robert Pattison, tel 03-489 9021 or 025-323 094 Southland: Aaron Meikle, tel 03-203 9071 or 025-846 377