Farm Tech

In The Paddock
Seasonal sheep flock management tips December 2001

Prepare lambs well and they'll thrive on forage crops

Key points

• At current lamb prices the use of finishing crops is often profitable
• Specialist forage crops, if grazed at the correct stage of growth, can provide a bulk of high quality feed when the quality or quantity of ryegrass pasture is low
• Risks include possible crop failure, the lag time before the crop can be grazed and potential animal health problems
• Animal health problems can occur with most fodder crops, especially brassicas
• An acclimatisation period is essential when introducing sheep to brassicas.

At current lamb prices, specialist finishing forages are likely to be profitable on many farms, especially where provision of large amounts of high quality pasture is difficult. They can produce a lot of high quality feed in the summer/autumn and winter/early spring pinch periods.

Annual forage crops can have very high nutritive values (85 per cent digestibility, 12-13 MJME/kg DM, 15-25 per cent crude protein) if grazed at the right time. There's also anecdotal evidence that parasitic larvae intakes are lower than on pasture.

If they are not grazed for lengthy periods, all crops tend to go to seed and/or deteriorate markedly in feed quality. At the other extreme, very frequent grazings reduce crop regrowth.

Over-grazing will also reduce the persistence of perennials like lucerne, red clover, timothy, trefoil and chicory. Winter grazing of perennials also damages the crowns and may kill plants.

The best summer options are new or short rotation grasses and the brassicas – especially pasja and barkant turnips, forage radish and rape. Check the recommended time to first grazing for each crop.

Where a perennial summer forage is required, lucerne is best in drier areas, while a mixture of chicory and red clover suits many environments.

Where winter pasture growth rates are too low for satisfactory lamb growth, annual or hybrid ryegrasses are generally better than brassicas or greenfeed cereals.

Animal health

Many farmers have avoided using brassicas because of nutrient imbalances and deficiencies, as well as toxins. While these problems can be overcome, brassica crops are not a panacea for tail-end lambs, which remain vulnerable.

To reduce health risks, stock should be slowly introduced to brassica crops, and provided with a source of hay or straw. Ideally, allow 4-5 weeks for lambs to adjust to brassica diets, giving them 1-2 hours/day access initially and increase it gradually.

Many farmers have a much shorter introductory period – increasing to full grazing within 7-10 days. While this is acceptable with pasja and newer more palatable rape cultivars, with older cultivars it is likely to affect lamb health at a sub-clinical level.

High moisture/low fibre: Fibre levels in brassicas are considerably below the levels recommended for optimum rumen function. Liveweight gains can be improved by around 20 g/day if about 20 per cent of dietary energy comes from hay or straw.

Nitrate/nitrite poisoning: Nitrate poisoning can occur with most rapidly-growing fodder crops. Nitrate levels increase when rapid crop growth follows drought or frost and after applying nitrogenous fertilisers. It can also be a problem in late-autumn brassica regrowth.

Nitrate forms nitrite in the rumen and is absorbed into the bloodstream where it inactivates haemoglobin. Death occurs when 60 per cent of the haemoglobin is immobilised.

As little as 1 per cent nitrate can cause problems. If in doubt, test the crop with a test kit, available from your vet.

To reduce the risk, animals should be gradually introduced to the crop and offered other feed or pasture to dilute dietary nitrate to 0.5 per cent or less. Take special care with sheep in poor condition.

It's also a good idea to transfer lambs directly from good quality pasture onto the crop, to boost carbohydrate levels and to reduce the risk of them eating too much at once.

Kale anaemia & 'red water': Haemolytic anaemia (red water) occurs in ruminants on kale. The anaemia develops over the first 2-4 weeks on the crop and then gradually recovers.

It is aggravated by the scouring and dehydration associated with cold turkey introduction to brassicas, so a slow introduction and a source of fibre should help.

Goitrogenic substances: Brassicas can form substances in the rumen that block the uptake of iodine by the thyroid gland.

Goitre has been noted in 20-30 kg lambs grazing kale. An iodine drench or subcutaneous injection easily overcomes this problem.

Low iodine may also result in still-born or debilitated new-born lambs. Therefore, iodine treatments are worthwhile for pregnant ewes grazed on brassicas.

Rape scald: This is a photo-sensitive condition developed by animals grazing brassicas, especially immature rape. Clinical signs are reddening and swelling of the skin, accompanied by itching. Affected animals seek shade. Animal welfare is the major concern.

Do not graze rape until it turns purplish in colour. Use acclimatisation techniques as above.

More information

Supplementary Feeding: a guide to the production and feeding of supplements for sheep and cattle in New Zealand, New Zealand Society of Animal Production Occasional Publication No. 7. 1980.

Your local farm consultant or veterinarian will also be able to help. Alternatively, contact your local WoolPro extension specialist: