Flowering brassicas and risk to stock

By Charlotte Westwood BVSc MANZCVS (Animal Nutrition) PhD, Veterinary Nutritionist 

Flowering winter brassicas

With day length stretching out and warmer temperatures underway, most of our remaining winter brassicas are starting to think about elongating and getting close to flowering. For those of you who’ve enjoyed our relatively warm winter and good crop utilisation, chances are many of you will be ending the winter with a surplus of crop. Any issues we need to be thinking about with leftover crops that are now close to flowering, or are actually flowering?

Aside from the drop in utilisation and feed quality on these flowering crops (heavier, lignified stems and loss of leaf), there are potential animal health risks associated with feeding flowering brassicas to stock.

Risks to animal health and well being

With the onset of reproductive development, elongating stems, mini ‘broccoli-like’ growth evolve through to firstly, yellow or white flowers (colour depends on brassica species), and if the crop is left for much longer, seed pods will form.

The elongating top part of the plant including the lengthening stem, the mini ‘broccolis’ that appear before the flowers, flowers and pods are all potentially toxic to sheep, cattle and deer. Risks to the grazing animal depend on the extent of reproductive development of the crop and will be influenced on the amount and types of fertiliser used growing the crop. Use of higher rates of fertilisers that contain nitrogen (N) and sulphur (S) and/or amount of plant available N and S in the soil will predispose crops to accumulate more glucosinolates and SMCO than crops grown in the absence of higher amounts of plant available N and S. More glucosinolates and SMCO increases risk of animal health challenges in grazing animals.

From the animals point of view, risk from reproductive brassica crops depends on how much brassica crop the animals are consuming (as a percentage of the overall diet), the physiological state of the animal (dry, pregnant, lactating) and how long the animals have been grazing crop.

The two main risks we’re most concerned with are:

1) Brassica Associated Liver Disease (BALD)

This condition may affect cattle, but is less likely/unlikely to affect sheep or deer (based on our current knowledge). BALD is caused by progoitrin – this is a glucosinolate that’s found in all brassicas, but especially in the reproductive parts of swedes, forage rape and turnips. Risk of BALD is less likely on kale or raphnobrassica. Under some conditions, particularly if rumen pH is low, progoitrin converts into other compounds that can damage the liver and possibly the kidneys of cattle. The liver damage is very similar to that caused by sporidesmin (= facial eczema). Damage to the liver means cattle can’t excrete the breakdown products of chlorophyll (the green stuff in plants) and these products ‘back up’ in the blood, causing sunburn on white areas of the skin (‘photosensitisation’). Damage to the liver also affects its ability to perform other essential tasks such as converting breakdown products of feed into blood glucose and detoxifying ammonia through into urea.

Although photosensitisation is distressing for the cattle (and for us seeing our cattle looking so sore), it’s actually the reduced ability of the liver to undertake that range of other important functions that is of greater concern to the well-being of cattle than the sunburn alone.

2. SMCO toxicity

Another name for this is condition ‘kale anaemia’, 'red water' or specifically ‘haemolytic anaemia’ which can happen on rape, kale and raphnobrassica (and less commonly on turnips and swedes). Cattle get anaemic (not enough red blood cells to carry oxygen around the body) and can sometimes end up with red tinged urine. Signs are usually seen 3-6 weeks after stock start eating high SMCO crops. Sheep, deer and cattle are all potentially affected by SMCO but it seems cattle are the species most likely to end up with red tinged urine, loss of body condition / liveweight and generally look depressed, lethargic and unwell. Cattle that are deficient in selenium and/or copper may be more susceptible to the effects of SMCO toxicity.

What to do with a flowering brassica crop?

Let’s be honest, if you don’t have much of a flowering brassica crop left it’s better to NOT eat it at all due to health risks. Get rid of it.

If you feel that you must push on and continue to graze a flowering brassica, be VERY mindful of the risks outlined above to grazing stock on flowering brassicas.

The extent of risk of flowering brassica to your animals depends on a range of things:

1. What animal species are you planning to graze on the crop. As mentioned above, cattle are more susceptible to showing clinical signs of both BALD and SMCO toxicity – but for SMCO at least, there are likely sub-clinical (less overtly visible) affects on other species which might show up just as poor feed conversion efficiency / poor weight gains or loss of liveweight.
2. Are your stock pregnant or lactating – particularly cattle? Pregnant and lactating cattle are the most susceptible of all ruminants to getting ill on flowering brassicas compared with non-pregnant cattle, or sheep and deer stock classes. It’s best to NOT let heavily pregnant or lactating cattle near elongating / flowering brassicas at all.
3. How much brassica you are going to feed to your animals (as a % of the total diet). The higher the intake of flowering brassica as a proportion of the total diet, and the lesser amounts of other feeds like pasture, silage etc, the greater the risk of these brassica compounds making the stock unwell. The old expression is “dilution is the solution” – eating less of a risky feed and more of a safe feed.
4. How you feed the brassica. Break-feeding is really important, with enough room along the break for all animals to access the brassica at the same time. Breakfeeding encourages animals to eat a combination of the more risky broccolis, flowers and pods combined with the lesser risk leaf and stem. A real risk remains though that dominant animals will push along the break and selectively eat the tasty broccolis, flowers and pods and not much else – these are the individual animals that tend to become sick, even though in theory we’re "on average" limiting access by break feeding plus other strategies including feeding silage, baleage or pasture.
5. Hungry cattle. If for example when dairy cows drift out of the shed a few at a time and reach the crop face before other members of the herd do - dominant cows eat way too much crop including likely selecting the tasty broccolis and flowers. Add the risk factor of hunger - cattle with the “post-milking munchies” will tend to eat very quickly, increasing risk of low rumen pH and more problems of BALD. Make sure all cows that leave the shed go first to a fresh break of pasture (ideally with some silage or baleage), have a decent feed to knock the edge off their appetite and then, when all cows are full, allow them all equal access to the crop.
6. How long have the animals been on the brassica crop? If the cattle have been on the crop all winter and it's flowering under their feet / ahead of them, the risks are somewhat less than introducing new cattle to a flowering crop that haven't been on brassicas in recent days - this is very high risk!

Let us know how your brassica crops are looking – any flowers yet? Tell us what you’re planning to do with these crops as we head further into spring.