Grass Tetany
Additions of potassium fertiliser to pastures have been associated with incidences of grass tetany (or hypomagnesaemia) in dairy cattle. This condition develops when insufficient magnesium is absorbed from the diet.
Application of potassium fertiliser can predispose animals in two ways. Firstly, improved plant availability of potassium can reduce the magnesium concentration of pasture through dilution in additional dry matter produced, and in direct competition for root uptake. Secondly, high levels of potassium relative to magnesium in the diet can reduce the ability of the gut to absorb magnesium.
Problems with grass tetany are more likely to occur in the spring when pasture magnesium levels are naturally at their lowest (<0.20%), and demand for magnesium by animals is greatest. Lactating cows are more susceptible to the development of grass tetany, and early symptoms include increased excitement and nervousness.
Fertiliser potassium is only one of several factors which can contribute to the risk of grass tetany. Temperature can influence the incidence of grass tetany, with a prolonged period of cold (<14oC), wet days raising the likelihood of deaths. Pasture species, composition, soil type and availability of other nutrients also influence whether a paddock is tetany-prone or not and some of these things can be managed to minimise the risk of animal health problems associated with fertiliser potassium applications to pasture.
Strategies for Managing Grass Tetany Risk
Risk Managing the following aspects of the dairy system and potassium fertiliser program can help minimise the risk of grass tetany while maximising pasture production. The following information should assist in the development of a tetany risk management strategy to suit individual dairy operations.
1. Time of Potassium Fertiliser Application
Application of potassium fertiliser usually, but not always, results in a reduction in pasture magnesium concentration very soon after application which can increase the risk of grass tetany. Trials in the North Canterbury region of New Zealand have shown that this effect lasts for a relatively short period of time with pasture magnesium concentrations returning to near original levels within two months of potassium application. Therefore, on paddocks which have a high risk of grass tetany it is best to avoid applying potassium to paddocks just prior to or following calving.
2. Application Rate
Trials on New Zealand North Island dairy ryegrass and grass/clover pastures found that the effect of potassium application on pasture magnesium content was apparent even at low application rates (25kg potassium/ha). Application rates up to 250 kg potassium/ha had only slightly greater effects on pasture magnesium concentration. This is supported by Australian data. (See table.)
|
This table shows the changes in the relative potassium and magnesium content of a pasture following the application of potassium fertiliser. |
|
However in all cases, increasing rates of potassium will continue to increase pasture potassium concentrations and therefore the ratio of potassium to magnesium in the diet. Therefore it is important to only apply as much potassium as necessary to sustain maximum pasture production, and to match this with other nutrients which may be limiting. This is best achieved by basing fertiliser application rate decisions on the results of a soil and plant tissue testing program.
3. Soil Application of Magnesium
Application of magnesium fertilisers can improve the magnesium content of pastures but may not be sufficient to raise dietary magnesium to an acceptable level. Long term maintenance of soil magnesium levels using magnesium fertilisers may be effective in low to moderate tetany risk areas.
4. Dietary Supplementation
Direct dietary supplementation of magnesium has proven a more effective method of improving magnesium intake in high risk situations. Dusting of pastures or supplementary feeding with magnesium compounds is common. In intensive industries it is also possible to give daily magnesium drenchings year round or during risk periods. A veterinarian or animal nutritionist will be able to advise on a drenching program. Dietary supplementation must start 2-3 weeks prior to calving and continue for 4-8 weeks after calving.
5. Maintain the Legume Component of Pastures
Because of their superior ability to accumulate potassium, grasses often have a higher tissue potassium concentration than accompanying legumes in a mixed pasture. For this reason grass tetany incidence is more likely on pastures which are heavily dominated by the grass component. Ironically, where soil potassium levels are moderately low, ensuring adequate pasture potassium nutrition can help to improve the legume component of a pasture and therefore reduce the risk of grass tetany. The effect of nitrogen application on the legume content of pastures must also be monitored carefully.
6. Ensure Adequate Phosphorous Nutrition
In some cases inadequate phosphorous nutrition can contribute to incidences of grass tetany. Phosphorus fertility is important to ensure optimal plant uptake of magnesium and calcium. Furthermore, pasture plants which are phosphorous deficient will be stunted and tissue potassium will be more concentrated.
7. Application of Nitrogen Fertilisers
The effect of nitrogen application on pasture potassium concentration is difficult to predict. Where soil and forage potassium levels are already high, application of nitrogen fertiliser tends to increase pasture potassium concentrations. Where pasture nitrogen concentration is less than 2%, application of nitrogen is likely to have a dilution effect and result in lower pasture potassium concentration. Recent research in New Zealand has shown no effect of nitrogen application on the potassium concentration of pasture.
8. Grazing Management
If it is possible to identify areas which have a greater risk of inducing grass tetany (for example paddocks which receive large amounts of dairy effluent which is high in potassium), avoid grazing these areas for one month prior to and two or three months after calving.
9. Precision Application
Spatial variation in application rate of potassium whether by paddock, landscape unit or using grid sampled data can minimise the over-application of potassium to areas with high soil potassium and under-application to areas with low soil potassium. As a result production is maximised and risk of luxury uptake and metabolic health issues is minimised. This approach requires a thorough soil testing and monitoring program for all paddocks on a farm.