There are three aspects to grassland and pasture quality: sward quality, nutritional quality and animal health.
Sward quality is influenced more by other agronomic practices as well as grazing management, however maintaining a good level of soil fertility and incorporating a balanced crop nutrition programme should be an important part of grassland management.
There is also indirect evidence that several nutrients including potassium, molybdenum, boron, zinc and manganese reduce disease infection in cereals, so by extension this is also likely to be the same for forage grass species.
Nutritional quality of grass is typically measured by digestibility, protein and dry matter content which can all be influenced by crop nutrition.
Digestibility depends on growth stage and nutrition – young, leafy swards have higher D and ME levels than those that have started to head or contain a lot of dead material. Grasslands with a balanced nutrition strategy will be more palatable than those that are under fertilised.
Protein levels depend on grass growth stage and are affected by crop nutrition, especially nitrogen fertilization. Levels increase in early spring. Protein formation depends on the plants ability to take up nitrogen from the soil – so can be influenced by nitrogen applications but also by potash and sulphur levels and soil pH.
Dry Matter Content is the most variable and the least controllable. Low dry matter is often associated with wet weather and poor growing conditions. As well as having a direct effect on yield, this also tends to reduce the quality and lead to lower daily intake. Also, in silage a high dry matter content (>40 %) can leads to lower intake.
Animal health and trace element nutrition should also be considered as an aspect of grass quality. Grazing animals have different requirements for trace elements to those that are required for grass growth so it is important to ensure these elements are present in the grass in sufficient levels to meet the animals’ nutritional requirements. Sodium and selenium are both essential for animal health but are not required by plants and guidelines for other trace elements such as magnesium, zinc and copper are higher in grazing animals than present in grass.
High rates of nitrogen can also cause problems with silage fermentation due to excess nitrate having a negative impact on the fermentation process and will produce silage that is less palatable and the animals will be less keen to eat it.
Nitrate is generally taken up by grasses quicker than it is incorporated into proteins and until used this excess known as luxury uptake is stored in the leaves. Excess nitrate will be present if insufficient time is allowed between application and mowing and can also occur under conditions of poor growth eg low light levels, cool temperatures. It is also a problem if there is a dry spell after application, when nitrate cannot be taken up by the roots, followed by a period of wet weather that results in luxury uptake. The plant cannot convert it to protein quickly enough so it accumulates in the plant.
High rates of nitrogen may cause a reduction in crop sugars as they are used to provide energy for the increased rate of plant growth and for the manufacture of plant proteins and this increased growth rate may in turn lead to lower crop dry matter content although in practice this is often not significant.
Excess nitrate can be avoided by following the Grassland Rule to apply nitrogen at the rate of no more than 2.5 kg N/ha/day.
Phosphorus plays a role in many plant metabolic processes and enzyme activities, so even though phosphorus demand is low compared to that of nitrogen its availability is essential. Where there is a deficiency or low availability of phosphorus the nutritional quality and digestibilty of the forage will be reduced.
Phosphorus is very immobile in soil and its availability is limited by both pH and by distance from plant roots.
Potassium is the nutrient taken up in the greatest quantity by grassland swards.
Potassium has a wide ranging role in the plant affecting nutrient uptake, photosynthesis, rate of growth and feed value.
If adequate amounts of potassium are not available the rate of growth and yield will be restricted, however there can be dangers to animal health if excess potassium is applied with an increased risk of hypomagnesaemia.
Sulphur is essential in the formation of proteins amino acids and enzymes and so is crucial for growth and development.
Magnesium also needs to be considered. Magnesium is an essential nutrient, and in situations where magnesium is deficient, there will be a significant reduction in grassland quality and may have serious consequences on animal health.
Sodium and selenium are both essential for animal health but are not required by plants and guidelines for other trace elements such as magnesium, zinc and copper are higher in grazing animals than present in grass.
Sodium is important for animal nutrition and often the grass contains an insufficient amount to meet the requirements of grazing animals.
Symptoms of sodium deficiency in cattle may include low appetite, reduced fertility and reduced milk production. Sodium also increases the palatability of forage, therefore increasing dry matter intake.
Fertilisers containing sodium increase the sodium levels in grass and have also been shown to increase productivity.
Selenium is an essential nutrient for animals, necessary for the formation of glutathione peroxidase, an anti-oxidant, but is not widely recognised as a plant nutrient.
Growers can also influence grassland quality with the following agronomic practices:
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