Estimating nitrogen excretion and deposition by lactating cows in grazed dairy systems

Abstract

Large N surpluses are a feature of most dairy farms worldwide. Despite the predominance of grazing systems in Australia, increasing animal densities and greater reliance on purchased feed mean that feed nutrient inputs and the role of grazing animals in N flows and deposited loads have grown. However, quantifying nutrient intakes and N excretion is difficult on commercial farms due to challenges in estimating pasture dry matter (DM) intake by grazing cattle. The aim of the present study was to quantify for commercial dairy farms, N excreted by lactating herds, the relative amounts of excreta N versus farm N inputs, and N loads deposited to different within-farm locations. Data were collected on at least five occasions from 43 diverse grazing system farms located in different climatic zones. An animal performance method for estimating annual DM intake was modified to calculate daily N intake and excretion and animal feed N use efficiency (NUE; milk N secreted/feed N intake). On average, these herds received 52% of their energy requirements from supplementary feeds despite the grazing base. Mean N intake (545 g cow–1 day–1), which greatly exceeded recommended dietary intakes, resulted in mean excretion of 433 g N cow–1 day–1 and low mean NUE (21%; range 11–39%). Calculated annual N flows through the lactating herds were equivalent to 69% of total N inputs onto these farms. When excreted N was apportioned to the locations visited by the lactating herds, deposition to pasture paddocks was not uniform. Almost 50% more excreted N was deposited to paddocks that were closer to the dairy shed, and approximately twice as much N was returned to feedpads and holding areas as deposited in dairy sheds and yards, with the potential for N accumulation and loss in these places. On average, 20% more N was deposited as excreta on pasture paddocks by the lactating herd than was applied as commercial fertiliser (168 vs 141 kg N ha–1 respectively). These results demonstrate that quantifying excreta N loads and spatial nutrient distribution by lactating cows can assist in improving N management in grazing system dairy farms.