Tips for managing lambs on greenfeed crops
We know from experience and copious trial data that weaned lambs will not grow much over 150grams/day on perennial pasture. Growth rates on typical hill country pasture are often more like 50-70g/d. These growth rates force producers to keep lambs too long and conflict with other stock requirements.
A preferable strategy is to provide specialist finishing feed for lambs or to sell them on to those who do.
‘Greenfeed' brassica crops (eg ‘Pasja'/'Hunter' or Rape) are a popular choice for providing quality summer feed for growing lambs, and once they are fed off, provide an opportunity to replace old worn-out grass swards with pasture that will give much better performance in the long-term.
However, these crops can give somewhat variable lamb performance compared to say, a Chicory/Red clover sward. This can be due to a number of factors, which are described below, with some possible solutions.
Get the preparation right - greenfeed crops and the subsequent pasture renewal are a big investment. Work ahead (the season before) to choose the paddocks and plan their preparation (soil tests, sprays, winter management, method of cultivation, sowing date, seed treatment, nitrogen application...). There is often little difference in establishment cost between a 4 tonne crop and an 8 tonne crop, but the higher yielding crop is clearly going to be more profitable.
Graze at the right time - Pasja and Hunter in particular become less palatable the more mature they become, so getting the timing of sowing and grazing right is important. They can be grazed from 45 days. Rape seems to suffer less from the ‘poor palatability' phenomenon.
Allow for rumen adaptation time - when lambs are first introduced onto a brassica crop, the balance of rumen microbes take some days to adjust from a grass diet to one that can efficiently digest the brassica. In this time lambs may grow very little or even lose some weight. It is important to allow 7-10 days for this adaptation as this can affect the economics of grazing some lambs on these feeds. If you were only planning to have them on the crop for a short time, it may be better not to put them on at all.
Limit sulphur levels in the crop - high or ‘luxury' levels of sulphur in the soil that the crop is to be planted in are known to reduce the palatability of brassica crops.
Don't let worms be an undetected problem - it is good practice to drench lambs before they go onto a greenfeed crop, to prevent any worms on board at the time from limiting production once the lambs are grazing the crop. Although it is unusual for crops to develop significant challenge levels of worms, we have occasionally seen cases where lambs have become heavily parasitised when grazing a crop, usually from grazing out the grassy edges of the paddock while adapting to the crop. The best way not to be ‘caught out' by this phenomenon is to do a faecal egg count on the lambs 3-4 weeks after drenching onto the crop.
Provide extra fibre - leafy Brassica crops can be low in dietary fibre. Lambs need a certain amount of this in their diet for cudding and correct rumen function. Try providing a small amount of hay or straw in a corner of the paddock - if lambs need fibre, they will pick at it.
Graze to the right level - a series of well designed, replicated trials on summer brassicas at the Kimiha research station in Canterbury have clearly shown that the best lamb growth performance and profitability per hectare is achieved when these crops are grazed to 65-70% utilisation. This means leaving more crop behind than would be the norm. Visually most of the green leaf is eaten, but a significant amount of the white stalk plus the odd bit of green is left. It seems to be important that some stalk is eaten; where lambs were stocked at a rate where only green leaf was consumed, individual and (not surprisingly) per hectare growth rates were lower. The stalk portion contains more fibre and, importantly, carbohydrates than the leaf, so some stalk consumption is probably important for good rumen function and hence good growth rates.
If you would like more information on feeding lambs, Totally Vets offers advice which is specifically tailored to your own property. Please contact us today by email, in person or give us a ring - we will be delighted to help.
