A urine patch framework to simulate nitrogen leaching on New Zealand dairy farms

Abstract

On New Zealand dairy farms, it is the nitrogen excreted directly onto pasture, particularly urine, that drives nitrogen (N) leaching from the farm. A new framework (UPF: Urine Patch Framework) is presented that post-processes the results of a whole farm model and runs a mechanistic soil model to simulate the urine patches. Two alternative methods to simulate the spatial distribution of urine patches were implemented and compared (Grid: spatially explicit, and Probabilistic: based on the probability of different temporal urination patterns). This paper describes the implementation of these two methods in connection with a Whole Farm Model; and compares the N leaching predictions with observed data. Two examples are provided, one analyzing the impact of urine patch overlap and another, the relative risk of N leaching at different times of urinary N deposition. The model showed good correlation and predictive ability between simulated annual N leaching results and observed data [R 2 = 94 %, mean relative prediction error (MRPE) = 10 % for Grid and R 2 = 72 %, MRPE = 20 % for Probabilistic]. The two methods produced similar results across an 8-year period for monthly and annual N leaching (R 2 = 96 %, MRPE = 10 % and R 2 = 86 %, MRPE = 8 %; respectively). Only 8 % of the paddock area was covered with multiple urinations during 1 year, but as much as 39 % of the total urine volume was deposited on overlapped patches. Systematically removing all urinary N for 1 month in either May or June reduced N leaching by approximately 20 %. Avoiding urinary N deposition during autumn or early winter could be highly effective in mitigating N leached during the following winter.

The Challenge of the Urine Patch for Managing Nitrogen in Grazed Pasture Systems

“…These approaches allow an estimate of urinary N load deposition at a range of spatial or temporal scales when linked to data about animal movements and management. Romera et al (2012) linked a mechanistic urinary N excretion model with a model predicting urinary volume to then estimate the number of, and N concentration in, urine patches deposited on a paddock as a prior step to calculating NO 3 -N leaching risk. More simply, daily urinary N excretion has been related to daily N intake either as curvilinear (Castillo et al, 2000) or linear (Vellinga et al, 2001) functions and also to milk urea-N and dietary crude protein (CP; Spek et al, 2013).…”

Trade-offs between profit, production, and environmental footprint on pasture-based dairy farms in the Waikato region of New Zealand

“…Stand-off pads are conceptually one of the most-effective means available to reduce N outflows from commercial New Zealand dairyfarming systems Romera et al, 2012). This practise has been used throughout New Zealand (Christensen et al, 2011) and other countries (Augustenborg et al, 2008).…”

“…Urine deposited on the stand-off pad is captured in a storage pond and applied to pasture at low rates and at a suitable time during which leaching risk is reduced, usually after the autumn-winter period when soil-water drainage is lower and N uptake by plants is higher. The impact of a stand-off pad on leaching levels is significant, though it is heavily dependent on the duration of time the animals are removed from the pasture and when this removal takes place (Romera et al, 2012). In one study, Ledgard et al (2006) highlighted that leaching levels were 25% lower with the use of a stand-off pad, compared to where they were not utilised.…”

Process-based modelling of NH<sub>3</sub> exchange with grazed grasslands

“…A way to estimate the temporal evolution of the urinecovered proportion of the field is to use a negative binomial distribution function for the time-space distribution of the urine patches as suggested by Petersen et al (1956), or the Poisson distribution tested by Romera et al (2012). Based on the distribution suggested by Petersen et al (1956), Pakrou and Dillon (2004) determined the proportion of the paddock covered by urine patches (P t ) after a t time period as…”

“…Figure 3. Proportion of the field covered by urine patches (P t ) calculated for sheep (a) and cattle (b) as suggested by Pakrou and Dillon (2004;P t_Pakrou ), Romera et al (2012;P t_Romera ) and when there is no overlap between the patches (D t ).…”