Guideline Index

Chapter 12: Nitrogen and Nitrogen Fertilisers

12.5 Expected pasture growth response

Pasture growth responses to N are greatest when conditions for pasture growth are optimal – in other words, when soils are not dry or waterlogged, when there is adequate sunlight, and temperatures are not too hot or cold. Optimal temperature for growth is between about 18-25oC for temperate grasses such as ryegrass, cocksfoot, phalaris and prairie grass, with a slightly higher upper limit (26-28oC) for tall fescue, and between about 25-35oC for subtropical grasses such as kikuyu. Growth usually ceases above about 30oC for ryegrass, cocksfoot, prairie grass and phalaris, about 35oC for tall fescue and about 40oC for kikuyu, and below about 4oC for the temperate grasses and 10oC for the subtropical grasses, as explained in Section 12.4.3.3.

In southern Australia, the highest responses of pasture growth to N are in mid- to late spring, and can range from 12 to 25 kg DM/kg N, depending on climatic conditions, species present (grass-dominant pastures respond best) and soil fertility. These high response rates can continue into summer under adequate irrigation, as long as temperatures don’t exceed about 30oC. In late autumn through to early spring when pasture growth rates are slow (between 5-15 kg DM/ha/day), then responses to N fertiliser are likely to be between 0-5 kg DM/kg N. Late autumn and early winter are not very effective times at which to apply N.

In northern Australia, the highest responses of pasture growth to N are in late winter through to mid-spring (pastures based on ryegrass), and mid-spring through to early autumn (pastures based on kikuyu), and can range from 15 to 30 kg DM/kg N, depending on climatic conditions, species present (grass-dominant pastures respond best) and soil fertility. In the cooler months from mid-autumn to mid-winter, pasture growth rates are slow (between 5-20 kg DM/ha/day), and responses to N fertiliser are likely to be between 0-5 kg DM/kg N. Late autumn and early winter are not very effective times at which to apply N.