By Akim Omokanye, PCBFA
From: Peace Country Beef & Forage Association 2016 Annual Report
Cocktail cover crops are often used to further increase the benefits of a one-species cover crop. Cocktail cover crops offer more diversity and allow more goals to be achieved than with one crop. For example, tillage radishes could be added to a mix to reduce compaction, a legume seeded to fix nitrogen and barley added to the mix because it has an extensive root system and will increase the organic matter. In the end, the soil condition is improved and there would be forage available for grazing. There are many options based on a farm’s needs, and to implement a cover-crop program many things must be considered, including the economics, as many of the benefits will be seen over the
long-term, not immediately.
While nitrates (N03) are not very toxic, nitrites (N02) are toxic. Nitrate poisoning occurs when the nitrite level in the rumen exceeds the capacity of the microbes to convert it to ammonia. When this happens, nitrate and nitrite are absorbed through the rumen wall into the bloodstream. It is the nitrite that causes toxicity. Nitrite combines with hemoglobin to form methemoglobin. Hemoglobin carries oxygen to body tissues while methemoglobin is unable to do so. When enough hemoglobin is converted to methemoglobin the animal begins to suffer from oxygen starvation. The change in the hemoglobin (red blood cells) is influenced by: (1) rate of nitrate intake (amount of feed and how quickly it is consumed), (2) rate of conversion of nitrite to ammonia in the
rumen, (3) rate of digestion of feeds and the subsequent release of nitrates and (4) movement of nitrite (feed passage rate) out of the rumen. Chronic nitrate toxicity is a form of nitrate poisoning where the clinical signs of the disease are not observed. With acute poisoning the signs of poisoning are observed and the animal is in critical condition.
Livestock health problems from grazing brassicas are relatively rare, but elsewhere brassica crops have been associated with some animal health problems. These problems can largely be avoided by good agronomic and grazing management. The key to avoiding these problems is to understand how the crop, and particularly
the variety, should be managed. Nitrates accumulate in plant leaves and very high concentrations may cause livestock death. Introduce stock slowly to the brassica crop and never with an empty rumen to minimize problems. This year, several cocktails from producers’ fields (particularly cocktails with brassicas) were analyzed for their nitrate levels in order to determine their safety and any possible risk of high nitrate problems.
Table 2 shows nitrate levels in 21 cocktails and one oat swath sample.
What levels of nitrate are safe to feed? Nitrate levels may be reported in three different ways depending on the analytical procedure used. The results may be reported as nitrate (N03), nitrate nitrogen (N03-N) or potassium nitrate (KN03). Be sure you
know which method was used before trying to interpret the results. Refer to the following table.
Results & Interpretation
Six (6) of the 22 samples tested for nitrates had nitrate levels varying from 0.04 to 0.12% NO3-N, one of which was oat swath sample (Table 2). These 6 samples were within the nitrate levels generally considered safe (see Table 1 above).
One cocktail sample had 0.19% NO3-N. With this level, caution needs to be taken when feeding a cocktail with that level of nitrate, as some subclinical symptoms may appear.
The majority (15) of the cocktail samples analyzed for nitrates showed significantly high nitrate levels. Their nitrate levels varied from 0.63 to 3.75% NO3-N. According to Table 1 above, these 15 cocktail samples would be prone to high nitrate problems. Death losses and abortions can occur.
The high nitrate levels observed for most of the samples analyzed here stresses the need for feed tests in-cluding NO3-N. It is important to consider very strongly the number of brassicas, how much (lbs/acre) and what types to include in cocktails before seeding. High NO3-N problem in crops, and in brassicas as well is largely caused by high soil nitrate levels and dry conditions.
Please note that hungry livestock suddenly introduced to nitrate bearing plants are more exposed to nitrate poisoning.
Livestock health problems from grazing brassicas are relatively rare, but elsewhere brassica crops have been associated with some animal health problems. Here are a few notes on mono-crop brassicas or where large amounts of brassicas are included in cocktails:
The grazing of brassica crops for protracted periods can sometimes result in rumen stasis (rumen stops moving) and constipation.
Affected stock will appear depressed and lack appetite.
Goitre (enlarged thyroid) - This is sometimes a problem in young lambs, where pregnant ewes have been grazing leafy brassica crops. Contact your veterinarian for advice on iodine supplements for lambs or supplements for the pregnant ewes.
Blindness - Occasional outbreaks of the condition that involves blindness, aimless wandering and unpredictable hyperexcitability are observed in cattle grazing brassica crops.
Kale Anaemia - This disorder (sometimes referred to as red water) can occur with all brassica crops, but is more common with kale crops. Anaemia is caused by excess levels of the amino acid compound S-methyl Cysteine Sulphoxide (SMCO) in the plant. SMCO causes a decrease in haemoglobin concentration and a depression of appetite. This condition tends to be worse when soil phosphorous is low and soil nitrogen and sulphur levels are high. Stock should be removed from the crop if they develop this disease.
Respiratory Problems - Grazing brassicas has sometimes been associated with cases of pulmonary oedema (fluid in lungs). Affected animals display respiratory distress.
Pulpy Kidney - Pulpy kidney is most common in young stock. Stock are most at risk when they have been on low quality feed for a period of time, and are then placed onto a highly digestible brassica crop. Vaccination is the best way to guard against this disease.
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