The development of systems of milk production and grazing management based on low stocking rates and very low artificial nitrogen inputs.

Abstract

There is increasing pressure on to reduce nitrogen (N) inputs to agricultural production systems within the European Union. This three-year experiment examined the impact of lowering N-input/ha on milk output, carrying capacity and N losses. In Ireland, a dairy cow is classified as excreting 85 kg organic N per year. There were four treatments involving annual stocking rates and fertilizer N inputs as follows: (1) 2.5 cows/ha & 350 kg/ha (Intensive), (2) 2.5 cows/ha & 250 kg/ha (Moderate), (3) 2.1 cows/ha & 175 kg/ha (Extensive) and (4) 1.75 cows/ha & 80 kg/ha (Minimal). Swards were initially composed predominantly of perennial ryegrass and contained white clover. The primary aim was to supply sufficient pasture to meet the feed requirements of the lactating cows during the main grazing season. Subject to meeting this requirement the objective was to produce enough grass to meet winter-feed requirements as grass-silage. Production of grasssilage was indicative of carrying capacity. There were 18 cows per treatment each year. Concentrates fed were 595 kg/cow/year. There were no significant differences in yields (mean ± SEM kg/cow/year) of solids-corrected milk (6210 ± 97), fat (263 ± 4.4), protein (225 ± 3.3) and lactose (301 ± 5.2) between treatments combined over years. Silage production was sufficient to meet winter-feed requirements (i.e. 1.40 t DM/cow) on all treatments except Moderate, which was 0.87 of requirement. Measurement of soil mineral N concentrations indicated largest losses from Intensive during the winter. However, measurement of nitrate N in drainage water during the winter indicated low concentrations (mg/litre) from all treatments; 2.4 from Intensive, 2.0 from Mininal, 0.9 from Moderate and 0.9 from Extensive. The comparably high mean concentrations associated with Minimal were attributed to the high proportion of white clover in these swards and the breakdown of clover stolon releasing mineral N into the soil during the winter months. The main findings were: (1) No difference in milk output per cow even under low fertilizer N inputs (2) A relationship between requirement for fertiliser N and stocking rate along the line: Fertilizer N req. = (SR x 300) – (300 + background-N) Where SR is stocking rate in cows per ha and background N is the release of N from net mineralization of soil organic matter N. The average value for background-N is around 130 kg/ha. (3) Very high levels of productivity from grass + white clover swards receiving 80 kg N/ha/year with around 80% of the carrying capacity of the Intensive treatment. (4) Very low losses of nitrate-N in drainage water under organic N loads of up to 300 kg/ha. Losses of nitrate-N in drainage water accounted for less than 5% of N losses in the experiment except on the clover-system. It is likely that denitrification and losses of di-nitrogen (N2) and nitrous oxide (N2O) gasses were the main pathways for loss. This is consistent with the heavy wet imperfectly drained soils, high rainfall, intermittent soil saturation and the mild conditions experienced at Solohead.