Brassica crops are fed to dairy cattle as a supplement to summer pastures. Brassicas are a cost-effective feed source and can be useful as part of a re-grassing policy by helping to break pasture pest and disease cycles.
Turnips, hybrids and forage rape are commonly used to fill the summer feed deficit and maintain milk production. They can also be a valuable feed source for heifer replacements and autumn-calving dry cows.
Poor performance on brassicas is most commonly due to under-estimation of crop yield and/or over-estimation of utilisation. There are a number of diseases that can occur while on crops. Most problems occur within 10-14 days of introduction. Cattle can take up to 28 days to fully adapt to a brassica diet, partly due to time needed for the rumen microbes to adapt and partly for the animals to get used to eating a new feed if they have not had brassicas previously.
Nitrate poisoning, rumenal acidosis, bloat, photosensitisation, red water (SMCO poisoning), choke, polioencephalomalacia (PEM), diarrhoea and copper and selenium deficiency are all potential diseases that can occur in brassica-fed cattle.
The following brassica feeding management tips reduce the risk of disease:
- Double-fence breaks and maintain adequate voltage on fences to prevent breakouts.
- Introduce animals to crops gradually.
- Offer cattle hay, silage or pasture before they are given their new crop break. There may be slightly more crop wastage but the risk of disease will be reduced as toxins such as nitrate and SMCO are diluted out.
- Milking cows are often put onto brassicas after the morning milking. Cows should be allowed onto the crop simultaneously, preventing dominant cows from reaching the crop first and gorging.
- Test crop nitrate levels prior to feeding.
- Accurately assess the dry matter yield of the crop before it is fed. Overfeeding increases the risk of acidosis, bloat, PEM and red water as leaf is selected over stem by dominant cattle. Underfeeding causes condition loss and reduced milk production and increases the likelihood of breakouts.
- Graze the crop at the optimal stage of maturity for the species/cultivar. The risk of SMCO and nitrate poisoning increases in over-mature crops. SMCO levels in kale are approximately double that of other brassica crops. Feeding immature rape can cause photosensitisation (‘rape scald'). Most cases of photosensitisation in New Zealand are seen on summer turnips, especially if they are under drought stress.
- Take extra care with crops that have had high levels of nitrogen and/or sulphur fertiliser applied. This can increase the risk of PEM, nitrate or SMCO poisoning.
- Milk taint can occur on brassicas. Limiting the brassica intake to a third of the diet on a dry matter basis and allowing access to the crop after the morning milking are standard practices to help prevent taint.
