Long-term effects of multiple applications of nitrogen fertiliser on grazed dryland perennial ryegrass/white clover dairy pastures in south-west Victoria. 2. Growth rates, dry matter consumed, and nitrogen response efficiencies

Abstract

A 3-year grazing experiment undertaken on a commercial dairy farm in south-west Victoria ascertained the effects of multiple applications of different rates of nitrogen (N) fertiliser over autumn and winter on pasture growth rates, dry matter (DM) consumed, and N fertiliser response efficiencies. Four treatments, replicated 3 times in a randomised block design, comprised: zero N (A); 3 applications of 25 kg N/ha (B); 3 applications of 50 kg N/ha (C); and 3 applications of 75 kg N/ha (D), with N applied as urea (46% N).

Treatments B, C, and D increased pasture growth rates relative to Treatment A, with Treatments C and D having the greatest affect. Treatments B, C, and D also increased pasture DM yield, with Treatment D having the greatest effect. Total pasture DM consumed in 1997 was 7.1, 7.9, 8.2, and 8.9 t DM/ha for A, B, C, and D, respectively. For 1998, the values were 5.4, 6.5, 7.2, and 8.7; and in 1999 5.7, 6.6, 6.9, and 7.7 t DM/ha.

During 1997, N response efficiencies were 11, 7, and 8 kg DM/kg N for treatments B, C, and D, respectively. Based on a cost of AU$400/t urea (including freight, handling, and spreading), Treatment B was the most economical, costing 8 cents/kg additional pasture DM produced. Nitrogen application resulted in response efficiencies of 14, 12, and 15 kg DM/kgN in 1998 and 13, 8, and 9 kg DM/kg N in 1999 for Treatments B, C, and D, respectively. In 1998 the most economical application was Treatment D, costing 6 cents/kg additional pasture DM produced, whilst in 1999 Treatment B was most economical costing 7 cents/kg DM for additional pasture.

The apparent recovery of applied N fertiliser by the pasture was calculated and found to fall within the range reported in the literature. Apparent recoveries estimated during 1997, 1998, and 1999, respectively, were Treatment B: 62, 78, and 70%; Treatment C: 50, 69, and 45%; and for Treatment D: 45, 82, and 51%.

It is concluded that, for the range of N fertiliser rates tested, N fertiliser is a cost-effective option for the production of extra pasture DM during autumn and winter on grazed perennial ryegrass dairy pastures in south-west Victoria.