The effects of urea and ammonium sulphate nitrate (ASN) on the production and quality of irrigated dairy pastures in Canterbury, New Zealand

Abstract

This paper examines the effects of two different forms of nitrogen (N) fertiliser, urea and ammonium sulphate nitrate (ASN) on the production and quality of spray irrigated dairy pastures in Canterbury. Pasture production and a wide range of pasture quality parameters were measured in a mowing trial of randomised design on the Lincoln University dairy farm. Treatments consisted of three fertiliser forms (urea, urea plus sulphur (S) and ASN) applied at two annual rates ofN (150 and 250 kg N ha⁻¹). Measurements included pasture dry matter (DM) yield, botanical composition, pasture N and S uptakes, 'standard' pasture quality measurements, such as carbohydrate and protein levels, and a detailed assessment of levels of individual proteins in the pasture. Under the conditions of this trial representing typical centre-pivot irrigation and high soil fertility, the total annual dry matter yield ranged from 15.9 (Control) to 19.4 (ASN 250) t DM ha⁻¹ and was strongly influenced by rate of N fertiliser application but not fertiliser type. ASN showed a small significant yield advantage (P < 0.05) over urea at the 150 N rate but not at the 250 N rate. Clover levels did not vary significantly with type or rate of fertiliser applied. Pasture quality, as measured by 'key' quality indicators was generally unaffected by fertiliser treatments throughout the season, although some differences in specific amino acid levels were detected. The implications of the impacts of N fertiliser use on pasture quality are discussed.