Urine patches indicate yield potential of cocksfoot

Dry matter (DM) production and crude protein (CP%) of sheep urine patches in a grazed cocksfoot pasture (28 day rotation with 21±1 days regrowth) were measured at Lincoln University from September 1999 to May 2001. Also, the DM response from artificial urine patches was measured over time. The rate of nitrogen (N) applied per hectare in an individual urine patch was 173 kg N/ha in autumn compared with 448 kg N/ha in spring. This stimulated a maximum difference in DM production between urine patches and controls over a 21 day period of 1970 kg DM/ha in spring (October). Smaller differences were measured in summer (380 kg DM/ha) and winter (370 kg DM/ha). The maximum difference in CP% was also in spring being 27.0% for urine patches and 18.7% for controls. DM production differences were attributed to differences in net leaf photosynthetic rate, which ranged from 23.5 in spring to 4.5 mol CO₂ m⁻² s⁻¹ in summer. Leaf photosynthetic rate was limited solely by N supply in spring, by soil moisture (mean soil volumetric water content in the top 500 mm < 20%) and leaf N content (< 3 %N) in summer and mainly by low temperatures (< 9 ºC) but also N supply in winter. The low temperature in winter also meant the duration of urine patch response was 133 days compared with 105 days in spring and 77 days in summer. In a second experiment, the DM response of cocksfoot to synthetic urine (300 kg N/ha) and irrigation was examined in ungrazed areas (four 60-day regrowth periods and one 110-day winter regrowth). From these treatments, the maximum annual DM production for cocksfoot with irrigation and N was 28.6 t DM/ha/yr compared with 9.2 t DM/ha/yr for the control. The addition of irrigation alone yielded 13.0 t DM/ha/yr compared with N alone at 23.5 t DM/ha/yr. To overcome the N stress in cocksfoot pastures a combination of applied N in spring, tap rooted perennial legumes in summer and annual clovers in winter is recommended.