Grass Tetany is a common magnesium deficiency disorder in cattle that is caused by a variety of factors such as fertiliser use, soil properties, breed, age, climatic effects and management strategies. The following essay aims to outline the causes of magnesium deficiency as was as point out a number of treatment options and prevention methods available to the producer as well as ways for predicting magnesium deficiency. Improving control of this disorder will result in increased economic benefits in those livestock industries affected.
Causes & Symptoms
There are a number of factors which need to be considered when assessing grass tetany in cattle as there are a number of different circumstances in which can lead to the onset of this disorder. As a result of the disorder the cow or calf’s levels of magnesium in the blood will begin to decline thus showing symptoms of tetany. There are two main forms of grass tetany, firstly the simple form which is caused as a result of a simple deficiency in magnesium and the second complex form is caused by potassium interfering with magnesium absorption within the rumen. A number of factors which can bring on the disorder include the age of the animal in question, the feed they have been grazing on, pH of the soil, use of potassium fertilisers as well as environmental effects such as temperatures, wind and rainfall(Grunes 1979).
The first conformation for farmers of an outbreak of magnesium deficiency in the case of grass tetany is finding dead cattle in which have been frothing at the mouth and nose with sighs that the animals legs have kicked violently. Commonly cows affected by low magnesium levels can go weeks without showing any sign of ill health however conditions of tetany are brought on with the introduction of stress. An example of this is mustering and yarding of cattle increasing stress to include symptoms such as twitching of facial muscles, a wary appearance and a stiff gait (Elliott 2009). In most cases these primary signs of a disorder are missed resulting in furthering the condition.
After the initial sighs of light muscle twitching cattle showing signs of magnesium may begin to have spasms in their front legs whereas their front legs begin to “goosestep,” their tail becomes erect and in some cases cattle can appear to lose vision. From this stage it is possible for cattle to recover however predominately if left untreated in this state the condition of the cattle is likely to deteriorate and they are more likely to die(Grunes 1979). In the most dire form of the disorder cattle will begin to become excited, start staggering, bellowing and galloping. Eventually she will go off her feet and lay on her side and with stretched out legs begin to thrash violently as her mouth froths with saliva. If the animal has survived the attack causing further stress such as approaching her may induce further attacks. Cattle are likely to die within minutes of going down.
Grass tetany usually happens in late autumn and early winter because the chemical composition of pastures changes due to the seasonal changes which affects cows as they are under a high level of stress physiologically late in their pregnancy or with a calf at foot and also environmentally with lowered nutrition in pastured. As described by Elliott (2009) tetany prone pastures have low concentrations of the following in cattle;
•Magnesium (< 2 g/kg DM)
•Calcium (< 3 g/kg DM)
•Sodium (< 1.5 g/kg DM)
And have high;
•Potassium (>20 g/kg DM)
•Nitrogen (> 50 g/kg DM)
Role of Nutrients
Magnesium – Bone formation, cell replication, hormone regulation & nerve impulses Calcium – Clotting of blood, contraction of muscles, functioning of biochemical reactions. Sodium – Maintains acidity levels in body fluids and pressure in body cells. Potassium – Maintains body fluid acidity levels plus required for enzyme reactions. Nitrogen – Used to form microbial protein.
Magnesium Absorption in cattle
Magnesium is an essential mineral in cattle required to participate in a number of bodily functions such as bone formation, cell replication, hormone regulation, nerve impulses, muscle contraction as well as fertility. The majority of magnesium absorption (Roughly 35%) takes place in the rumen(Elliott 2009) . Magnesium in generally not stored in large amounts within the body, so therefore a change in the pasture coupled with the increase of production through lactation can make cows prone to magnesium deficiency otherwise known as grass tetany. Below is a diagram (Figure 1) showing the uptake, flow and excretion of magnesium in cows(Elliott 2009).
Other factors increasing susceptibility to magnesium deficiency There are a number of further factors in which can impact on the likelihood of cows being affected by magnesium deficiency. Firstly the age of a cow is a large determining factor resulting in tetany. As the cow ages her milk production increases until the age of about 4 years until it stabilises(Grunes 1979). In older cows the absorption of magnesium decreases and this coupled with a high level of milk production results in magnesium deficiencies. Another cause of magnesium deficiency in older cows is their higher level of bodily fat which lowers the magnesium availability within their bodily fluids(Elliott 2009). A US research paper in 1995 found that certain breeds are more susceptible to magnesium defies as their bodies were able to absorb magnesium differently. In the study it was found that of the 5 breeds tested being Angus, Hereford, Holstein and Brahman.
The most susceptible by far to magnesium disorders were the angus cattle(Littledike et. Al 1995). In today’s breeding environment where a large emphasis is placed on production ability, animals with high growth rates as well as metabolic activity place increased production demands on their body which in turn results in an increased use of minerals and in some cases induces a magnesium deficiency. Sudden starvation such as yarding cows for long periods of time when they are prone to magnesium deficiency can also impact upon magnesium within the body.
The fat levels in cattle should be monitored so they do not push too high or low and in the case that they may be too high rather than starving the animal(s) in question they should instead be put on a careful diet. Also the elements within the soil and plants can impede with the available magnesium in pastures. Elements such as sodium (Na), potassium (K), chlorine (Cl) and magnesium (Mg) can affect this balance(Elliott 2009). Timing of calving is an imperative aspect of managing magnesium deficiency within cattle and therefore must be managed appropriately. Cows calving in autumn and winter are most susceptible to magnesium deficiencies primarily due to the change of composition in pastures at that time, of these months, the most severely affected months
Treatment and Prevention
In order to prevent the condition of magnesium deficiency there are a number of different treatment options available which include treating feed rations with magnesium oxide, use of magnesium loose lick blocks, magnesium bullets as well as medicating water. These options vary greatly in price and some can prove to be more effective than others(Elliott 2009). Magnesium blocks are available to producers however they are generally quite expensive. The use of blocks makes it hard for cows or calves to get the correct amount of magnesium oxide. Generally blocks contain roughly 16% magnesium. Variable consumption of these blocks can lead to some of the cattle not receiving the desired amount of magnesium and therefore failing to solve magnesium deficiency(Elliott 2009). The use of magnesium in feed rations is one such recommended method of preventing magnesium deficiency.
Generally this involves using the active ingredient of Causmag which generally costs around $500 per ton and is mixed into rations such as molasses, oats or hay. It is recommended to include salt and lime to these rations as salt ensures sodium is supplied thus preventing the impacts of potassium, Lime is high in calcium which is known to act with magnesium to increase concentration in the body (Aydin et. al 2008). Salt can also be used as a diet regulator as adding more salt will reduce the amount consumed and likewise reducing salt will increase the ration consumed(Elliott 2009). Magnesium bullets are another form of treatment that can be used to treat the condition, they can be relatively expensive costing roughly $9, however they are very effective for just under 3 months. Usually this form of treatment is used leading into or during early winter to combat the period of time in which cattle are most affected by magnesium deficiency (Aydin et. al 2008).
Magnesium bullets slow release magnesium at a rate of about 2 grams per day, this is generally enough to prevent the onset of grass tetany however can prove to be insufficient in cows producing very high levels of milk(Elliott 2009). Lastly magnesium can be added to water troughs through use of magnesium sulphate otherwise known as Epsom salts or magnesium chloride can be added. Generally it is recommended that these solutions be mixed at a rate of 3 grams per litre. This method however is only effective whereas troughs are the only source of water available to stock(Elliott 2009)..
Prevention of the condition
In order to prevent magnesium deficiency it is best to implement management strategies to minimise the impact of low magnesium pastures. Firstly producers must investigate the levels of magnesium in their soils as well as determinant factors such as the pH and the potassium level. Management strategies which can be implemented into production systems include the following (Grunes 1979): •Altering the time of calving to a period of time where magnesium levels in the pastures are less affected by seasonal conditions. •Limiting stress on stock in months leading up to calving. This involves refraining from mustering and transporting cattle. •Maintaining a level plain of feed, or introducing stock to greener pastures gradually, this can be done by leaving a gate open between new and old pastures so that the cattle are able to return to their old feed for periods until they are used to new pasture. •Prevention of starving cattle.
Predicting the Likelihood
Grass & crop tetany
In any pasture and crop there are 2 elements that regulate the availability of magnesium within the soil, these are calcium and potassium. When cold temperatures occur with temperatures not exceeding 7 degrees Celsius during the day plant roots go into a temporary hibernation where they cease taking nutrients from the soil. This results in a potassium build up making magnesium less available to the plants once they start producing again (Elliott 2009).
Post drought grass Tetany
When there is in an imbalance of potassium, calcium and magnesium whether caused naturally or via application of fertilisers the onset of a drought can cause feeds to be toxic to cattle. In drought conditions plants die resulting in no uptake of nutrient from the soil. As a result potassium levels can rise suppressing magnesium uptake from the soil. When a drought is broken, the first shoots of pastures can be very high in potassium and thus toxic, to avoid this it is recommended that pastures should have more than 12 centimetres of growth before attempting to graze them after a drought. In the meantime hay feeding can avoid this feed gap (Elliott 2009).
Penned feeding grass tetany
In feedlot situations there is a rare chance of cattle being affected by tetany through the high levels of manure they produce. The manure they produce is high in some nutrients such as potassium and a change event such as high rain can cause this to leech into the soil, given the right conditions grass can germinate giving similar results of post drought tetany where early shooting grass contains very high levels of potassium(Elliott 2009).
As it can be seen grass tetany is a severe condition which can be a large economic burden, however with the correct management strategies producers can better manage grass tetany by predicting its likelihood, managing their pastures and in the case of tetany occurring implementing a treatment program to get the best production out of their cattle.
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the status of several macro and trace minerals of adult beef cattle. Journal of animal science, 73(7), 2113-2119. Oxyfertil. (2013). Roles of Ca and Mg in animals Oxyfertil. Source: http://www.oxyfertil.com/royaume-uni/role-ca-mg-animal.html