The effect of diverse pasture and Italian ryegrass on urine patch size and plant nitrogen uptake

The intensification of the dairy industry in New Zealand has prompted the use of fertiliser and legumes to produce productive pastures with high nitrogen content. High quality pastures create an imbalance between nitrogen supply and animal demand, the N taken up by pasture species and utilised for animal products eg. meat and milk, the excess N (75-95%) returns to the soil in excrement, especial urine. Urine N is a major source of N loss in pastoral farming systems. The N load onto pastures is a function of urine volume, urinary N concentration, urination frequency, cow density and the urine patch area, the plant uptake following this indicates the overall loss. Diverse pastures and pastures containing Italian ryegrass are increasingly being used in dairy pastures to improve dry matter production and potentially reduce N loss, however there is little information on the effect of these pastures on the urine patch area (a function on N loading) and plant uptake over winter. The trial investigated the effect of diverse pasture mixes and those containing Italian ryegrass on the area of urine patches using thermal imaging technology to record the ‘wetted’ area of urination events. There was no significant effect of species diversity and the presence of Italian ryegrass on urine patch size. There urine patch size was significantly higher in the morning compared to the afternoon (0.35 v’s 0.16m2 ±0.03 respectively). This effect occurred because of higher urine volume and pasture cover. Calibration curves were generated to determine urine volume from a certain patch area. A clear increase in urine area in short pasture was shown, however the range of urine volume generated was higher than results obtained from urine sensor harnesses. The plant N uptake was significantly higher inside urine patches than outside (143 v’s 62 kgN/ha respectively), a result of a two-fold increase in pasture yield and increased N content in pasture. The overall N loading per ha inside urine patches ranged between 16.5 and 23.1kgN/ha, and an excess N after N uptake between 12.2 and 17.9kgN/ha. The increase in urine patch size in the morning compared to the afternoon is thought to be a function of increased urine volume that occurs through diurnal variation and the pasture height which had been grazed for much longer during the morning measurements. The higher N uptake inside the urine patches occurs because of the higher N input and supply to plants, resulting in higher growth rates and DM accumulation, and also higher N% in the plant tissue. Combined there was not a large difference in the N loss between treatments, a major factor influencing this was the stock density on the pastures based on DM cover. There needs to be further research into the diurnal variation ion urine patches, along with variation in urine volume and N concentration the N loading onto a soil can be better understood.