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Applying the right nutrients to planted pastures that are tailored to the specific plant species and local environmental conditions, can significantly boost both yield and quality. However, given the high cost of fertilisers, they must be used efficiently to ensure pastures are optimally utilised by livestock.
Renier Bothma, a soil scientist at Kynoch Fertilizers, explains that rainfall is a key factor in pasture production under dryland conditions. The more it rains, the greater the pasture growth and the higher the nutrient demand for maintaining optimal yield and quality.
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Plan before you plant
Before planting a pasture crop, several important decisions must be made. Consider the type of pasture required and when it will be needed, assess the soil’s chemical condition, determine whether adjustments are necessary, and select the most suitable cultivars for your environment. You’ll also need to decide whether to plant annual or perennial species.
When establishing perennial forage crops such as lucerne, it is essential to correct soil pH, and calcium and phosphate levels before planting. Once these crops are established, making such adjustments becomes extremely difficult or even impossible. Legumes such as lucerne, medics, and clovers fix their own nitrogen from the atmosphere, so fertilisation efforts will focus more on phosphate, calcium, potassium, sulphur, and micronutrients.
Annual pastures such as forage sorghum and winter options such as oats are generally more tolerant of soil acidity and are managed much like grain crops. Phosphate does not move easily through the soil, so later applications are typically ineffective. Calcium is corrected through liming, while potassium and sulphur can be supplemented later.
Grass crops such as ryegrass require applications of nitrogen, potassium, and sulphur, with phosphate needed only in smaller amounts. Phosphate is applied only upon establishment while nitrogen, sulphur, and sometimes potassium must be supplemented once the grass has been mowed or grazed. Grass crops also respond well to foliar nutrition.
Finally, it is essential to conduct a soil analysis before establishing pastures to guide appropriate fertilisation and ensure optimal growth and productivity.
Soil acidity
Most pasture crops grow well in soils with a pH above 4,5 (measured in KCl), but different grass species vary in their tolerance to soil acidity. For instance, Eragrostis can thrive in very acidic soils with acid saturation levels of up to 50%, whereas Smutsfinger grass performs poorly when acid saturation exceeds 20%.
Regular soil pH testing is crucial, as nitrogen fertilisation can gradually increase soil acidity. Acidic soils can reduce the yield of established pastures by limiting the effectiveness of fertilisers. Lime can be applied to correct soil pH but it moves slowly through the soil, and pH adjustment may take some time.
The importance of nitrogen
Nitrogen plays a vital role in increasing both the dry matter (DM) yield and the protein content of cultivated pastures. Higher protein levels in the leaves improve forage quality, which supports better animal performance. As a general guideline, between 200 and 250g of nitrogen should be applied for every millimetre of rainfall received. In a region with an average seasonal rainfall of 500mm, this translates to approximately 100kg of nitrogen/ha.
Nitrogen is typically broadcast over pastures, but the effectiveness of this practice depends on selecting the appropriate nitrogen source. For instance, urea is not immediately available to plants; it must first be converted by the enzyme urease into plant-available nitrogen – a process that can take one to five weeks, depending on weather conditions.
In contrast, nitrogen sources such as calcium ammonium nitrate (CAN) and ANO 21 (a liquid fertiliser) are already in the ammonium and nitrate forms, making them readily available for plant uptake. CAN and urea treated with a urease inhibitor are safe for surface application and help minimise nitrogen losses due to volatilisation.
Higher nitrogen availability in the soil results in more protein/ha, improved feed digestibility, and increased leaf production. These leafy pastures are more palatable for livestock, ultimately supporting improved meat production/ha.
Phosphate deficiencies
Most agricultural soils in South Africa are phosphate deficient. Ideally, soil phosphate levels should be around 25mg/kg. Deficiencies can be addressed either through a single corrective application or by applying liquid phosphate throughout the season alongside nitrogen.
Phosphate removal from the soil varies by crop. Grass pastures extract roughly 1kg of phosphate/tonne of DM produced, while lucerne can extract up to 3kg/tonne. When these crops are moved, phosphate should be replenished at equivalent rates.
The role of potassium
Potassium promotes the uptake of other nutrients by the plant and plays a key role in photosynthesis, growth, and the nutritional quality of pasture crops. It strengthens stems, improves resistance to drought and cold, and generally contributes to higher yields.
For most crops, the optimal soil potassium level is above 120mg/kg. Potassium can be applied before planting, at planting, or during the growing season, as it readily diffuses through the soil. However, pre-plant or planting-time applications are often the most effective. During the growing season, approximately 20kg of potassium is removed from the soil per tonne DM. If all plant material is removed, it is important to replenish the entire amount of potassium extracted.
The role of sulphur
Sulphur is often referred to as the fourth macro-element. It plays a key role in protein synthesis within the leaves of pasture crops, making it essential for optimal animal growth and development.
Many nitrogen fertiliser blends available on the market already contain adequate levels of sulphur. Ideally, soil should contain around 20mg/kg of sulphur.
While fertilisation is a costly input, it is necessary when establishing productive pastures. To ensure cost-effective feed production, fertilisers must be applied strategically. High-quality pastures should be reserved for animals with greater nutritional needs, such as dairy cows, young growing stock, and stud animals. Livestock currently not in production can be maintained on more economical feed sources. – Andries Gouws, Stockfarm Magazine
Enquiries can be directed to Renier Bothma at renier.bothma@kynoch.co.za
