The spatial distribution and area coverage of urine depositions in grazed pastures in NZ

Abstract

In grazed pasture systems, grazing animals deposit urine and dung causing high nutrient loading to a relatively small proportion of the total grazed area. The majority of ingested nitrogen (N) is excreted in urine, and high N loading in the urine patch is of particular environmental concern because of the potential for leaching of soil nitrate N from the patch and the subsequent potential degradation of ground and surface water quality. Animal stocking rate and stock type are key factors driving the quantity of nutrient, especially N, which may be deposited as urine and dung to pasture soils. Therefore the total paddock area receiving urine deposits in any time period is critical to the understanding of nutrient cycling and nutrient loss in grazed pasture systems. Some theoretical estimates of this critical area coverage value have been made by researchers, but quantitative field spatial data is scarce and seasonal components have often been ignored in earlier estimates. Consequently, much variability and uncertainty still surrounds the current estimate of annual urine patch area coverage in grazed pasture systems, and how area coverage varies with different stocking rates and stock type.

A new method using global positioning system (GPS) and geographic information system (GIS) technology was developed and successfully used to quantify the spatial distribution and area coverage of urine patches deposited by grazing animals. Accurate measurements of urine patches over a period of twelve months were made for two farm types, which are typical of New Zealand grazed pasture systems: (i) an irrigated dairy farm and (ii) a sheep and beef hill country farm. At two field sites urine patch areas were visually identified from the enhanced pasture growth in those areas and their area and spatial location recorded with GPS at regular time intervals. The GPS data was analysed in a GIS system. These data were then used to calculate seasonal and annual urine patch coverage in the paddocks on an area basis.

On the dairy farm at an effective stocking rate of 3.5 cows ha-1 (c. 32 standard stock units [ssu] ha-1) the observed annual urine patch area coverage was 22%. The mean urine patch radius was 30.5 cm. Mean urine patch radius ranged from 27 to 35 cm between seasons. The observed average urine patch area was 0.28 m2. Urine patch coverage was shown to increase with increased stocking rate (cows ha-1 or cow grazing days).

For the sheep and beef farm at an effective stocking rate of 15.1 ssu ha-1 the observed annual urine patch area coverage was 16.8% for the study paddock. Average urine patch coverage was 19.3% and 14.3 % on flat areas (0 – 3°) and hill slopes (7 – 15°) respectively. This result confirms that a higher grazing pressure occurred on flat areas than on hill slopes. For the observed one-year period, the mean urine patch radius was 25 cm. Mean urine patch radius ranged from 19 to 29 cm between seasons. The observed average urine patch area was 0.19 m2.

This study has successfully used GPS and GIS technology to make initial measurements of the spatial distribution and area coverage of urine depositions in grazed dairy and sheep and beef pastures in New Zealand. Field measurements for this study are ongoing.