Grazing green cover crops

Most commonly grown green cover crops are suitable for grazing by livestock, though some, like Phacelia are not particularly palatable. There are however a few simple rules to follow if the crop is going to deliver as a cover crop and a forage source.

One of the main issues around destroying the cover crop by grazing is that of compaction. Significant sub-surface compaction can occur, especially when ground conditions are wet and particularly in high traffic areas such as around ring feeders or water troughs.

Grazing smaller blocks with sufficient animals present to complete the grazing within 4-5 days is ideal and so electric fencing should be utilized to achieve appropriate paddock sizes. If this is not possible then strip grazing using a back fence is the next best scenario. Using portable water troughs linked to a bowser or moveable pipeline will prevent animals from creating tracks to and from the water supply. Mineral buckets should ideally be moved daily, and ring feeders moved each time they are refilled.

Many cover crop species are highly digestible and contain high levels of rumen degradable protein and though this has the potential to support high growth rates in livestock it also poses certain nutritional and metabolic challenges, along with a host of other potential issues including;

• Nitrates and Nitrite poisoning
• Photosensitive substances (Buckwheat)
• Haemolytic anaemia factors
• Glucosinolates (goitrogen precursors, which can induce iodine deficiency)
• Sulphur and molybdenum (which can induce copper deficiency)

Nitrate poisoning

Nitrate poisoning in livestock is a noninfectious condition that can result in death. Although uncommon, poisoning occurs when livestock eat forages stressed from severe environmental conditions such as drought. The stress disrupts normal plant growth and may cause the plants to accumulate too much nitrate.

Understanding the causes, symptoms and treatments for this disease can help producers prevent losses. Though sampling and testing can indicate when forages pose a danger to livestock most producers only become aware of the issue following the death of several animals.

Nitrate is present to some degree in all forages, and technically, nitrate poisoning is better described as nitrite poisoning. When livestock consume forages, nitrate is normally converted in the rumen from: nitrate to nitrite to ammonia to amino acid to protein. When forages have an unusually high concentration of nitrate, the ruminal process cannot complete, and nitrite accumulates.

Nitrite is absorbed into the bloodstream directly through the rumen wall or epithelium and converts hemoglobin (the oxygen carrying molecule) in the blood to methemoglobin, which cannot carry oxygen. The blood turns a chocolate brown colour rather than the usual bright red. An animal dying from nitrate (nitrite) poisoning effectively dies from asphyxiation. Nitrate poisoning usually does not occur rapidly, but over time, depending on how high the nitrate level in the forage is and how much is consumed.

Nitrate poisoning can occur when: Forage consumed contains high levels of nitrate; The diet changes rapidly or suddenly; Parasitism or other conditions cause anemia; Livestock consume supplements containing urea or excessively high-protein feeds along with forage containing moderate levels of nitrate.

Acute nitrate toxicity symptoms generally include death, blue mucous membranes (lack of oxygen), fast breathing, high pulse rate, weakness, nervousness, excessive salivation, frequent urination and dilated and bloodshot eyes. If nitrate poisoning is suspected diagnosis should be confirmed by post mortem.

Plants need nitrogen for growth and development. However, when drought prevents them from converting the nitrogen they absorb into new growth, nitrate levels may rise and accumulate. Although this can occur in almost any plant we are most likely to see it when feeding brassicas, especially if high levels of Nitrogen fertilizer have been applied.

Nitrates do not normally accumulate when there is normal rainfall or irrigation. Under those conditions, nitrate nitrogen absorbed by roots and moved into the plant is rapidly transformed into plant proteins. However, under dry conditions, plant roots continue to absorb small amounts of nitrogen, but the plant has too little water to keep growing. Nitrate accumulates and is stored in lower leaves and stems, ready for the plant to mobilize and use when normal growing conditions resume. Nitrate levels can change from day to day and even from morning to evening. In cloudy periods, plants continue to absorb nitrogen from the soil but lack the photosynthetic activity to convert the nitrogen into proteins.

Because stock tend to graze the upper leaves first it can take several days or even weeks for the condition to show, as the animals are forced to eat the lower parts of the plant where concentrations are at their highest.

Photosensitivity

The toxic effect of grazing Buckwheat with livestock comes in the form of photosensitization of non-pigmented skin.

The blood vessels in areas of non-pigmented skin can completely die off and cause cell death in those areas resulting in sloughing of the skin in the affected area.

All grazing livestock including horses are affected. In principal, cattle of solid colour such as Aberdeen Angus, Welsh Black, Lincoln Red etc. would not be affected as there is no non-pigmented skin.

Affected skin is swollen and often oozes serous fluid which dries to form yellow crusts. Clinical signs in sheep are usually confined to wool-free un-pigmented areas of the face, ears and limbs, but in some sheep breeds the skin of the midline of the back at the parting of the fleece is also affected. Severely affected animals don't eat and rapidly lose body condition.

Haemolytic anaemia

One particularly important toxic substance present in some brassicas including rape and kale but also mustard and radish is a haemolytic anaemia factor, S-methylcysteine sulfoxide (SMCO). SMCO is converted by bacterial fermentation in the rumen to dimethyl disulphide, which causes haemolysis. Severity of the disease is proportional to the SMCO content of the crop. When present in small amounts, the toxin results in poor growth rates. However, when SMCO is present in high concentrations, lambs become anaemic, with red urine, progressing rapidly to death.

SMCO concentrations in plants can be analysed and Brassica crops categorised as low or high potential risk, with most forage brassicas being categorised as low risk. However, SMCO increases with the age of the crop, so even low risk varieties can become potentially hazardous as they reach maturity or if they are fed to excess. To avoid these risks, long-keep store lambs should not be grazed on Brassica crops for prolonged periods and animals should be provided with a pasture run-off or supplementary feed.

When the disease is suspected, animals should be removed from the crop and carefully introduced to supplementary feed. Veterinary examination is needed for an accurate diagnosis.

Goitrogenic glycosides (Glucosinolates)

Glucosinolates present in Brassica crops can cause goitre, reduced growth rates and/or diarrhoea, depending on their composition. Sudden onset blindness in sheep (rape blindness) is also thought to be associated with glucosinolate poisoning. The highest concentration of these glycosides is present in the seeds of mature plants.

Most forage brassicas have low levels of glucosinolates and represent little or no problem however, mustards and fodder radish grown in cover crop mixtures are not specifically destined for animal feed crops and so should be grazed well ahead of flowering to reduce the risk.

Ewes that are heavily in lamb should not be grazed on high risk brassicas as the ensuing deficiency in iodine can cause lambs to be born with a low birth weight and poor thrift resulting in hypothermia, starvation and mis-mothering. Iodine deficiency can also account for high barren rates and low scanning results when ewes are grazed on high risk brassicas at mating.

Trace element deficiencies

Though most commonly used components of cover crops contain high levels of certain minerals and trace elements due to their deep-rooted nature, certain imbalances can occur resulting in poor performance or in some cases high mortality.

High levels of antagonists such as Molybdenum and Sulphur can effectively lock up Copper which is essential for many functions including:

• Assisting in the absorption of Iron and subsequent haemoglobin synthesis
• Being a component of enzymes involved in energy metabolism
• Assisting in regulation of blood pressure and heart rate
• Promoting fertility in breeding animals

Brassicas in particular, tend to be low in cobalt. Cobalt itself is not used by the animal but is used by the rumen microbes for the synthesis of vitamin B12 which is required for energy metabolism. Animals deficient in vitamin B12 have poor growth rates, ill thrift and suffer from a compromised immune system leading to susceptibility to disease and parasitic infestations.

Correct supplementation of minerals and trace elements will in most cases overcome the issues mentioned above. Animals showing signs of poor performance should be blood tested to determine their mineral and trace element status. Forage mineral analysis can be used ahead of grazing to determine the need for supplementation.

Bloat

Bloat is simply a build-up of fermentation gasses in the rumen. This gas is produced as part of the normal process of digestion and is normally lost by belching (eructation). Bloat occurs when eructation is prevented.

There are two types of bloat. The least common type is gassy bloat, which occurs when the gullet is obstructed (often by foreign objects such as pieces of beet) or when the animal can't burp (such as with milk fever or tetanus).

The second type of bloat is frothy bloat, which happens as the result of a stable foam developing on top of the rumen liquid, which blocks the release of the gas. This is by far the most common form of bloat, and unlike gassy bloat, it is highly seasonal with peaks in the spring and autumn. This is because the foam is formed by breakdown products from rapidly growing forages (particularly legumes such as clover). These increase the viscosity of the rumen fluid and prevent the small bubbles of gas formed by rumen fermentation from coming together to form free gas that can be eructed.

Cattle are far more susceptible to frothy bloat than sheep.

Sudden deaths in lambs

Sudden deaths are often seen in lambs within about 2 - 3 weeks of a move onto cover crops, associated with changes in feed rather than specific toxicities. Conditions such as redgut, pulpy kidney, systemic pasteurellosis and cerebrocortical necrosis (CCN) (thiamine; vitamin B1 deficiency), caused by a move onto forage crops.

Management

Care should be taken when introducing stock to any new crop and a gradual introduction is ideal. However, the practicalities of a gradual introduction are limiting, and mitigating management practices should be introduced.

A grass run back allows stock access to more fibrous grazing but if not available then supplementary feeding of hay, haylage or even good quality dry straw will help to overcome most nutritional issues. As a rule of thumb, a green cover crop should never make up more than 70% of the total diet on a dry matter basis.

Stock should never be introduced to a new crop in a hungry condition as gorging exacerbates the situation.

Finally, correct mineral supplementation should be available either though buckets/blocks or boluses. In most cases a general-purpose mineral bucket will provide adequate supplementation, but it is important to remember that when given free access, some will take more than others and some none at all. Boluses, though not totally infallible are a surer way of getting the right amount of supplementation into each animal but should ideally be based on blood sample analysis.