The percentage of plantain in pasture that can be entered in Overseer can be anywhere between 5 and 100%. However, the effect of plantain on urine N load is capped at 60% of diet. Why is this?
Currently OverseerFM reflects the effect of plantain in the animals’ diet on urine nitrogen (N) concentration and partitioning of excreted N to urine or faeces. N in urine deposited on soil is the largest source of nitrate leaching in grazed farm systems.
Other effects are not reflected due to insufficient data on their size and potential interaction with soil type and climate. Examples are increased water uptake and nitrification inhibition (both potentially reducing N leaching), reduced winter growth and increased area covered by urine with increasing risk of urine patch overlap (both increasing risk of N leaching).
The effect of plantain on urine N load is capped at 60% of plantain in the diet to avoid significantly over-estimating the effect of plantain on N leaching. Data from animal trials (Figure 1) show that the urine N load continues to drop with further increasing plantain content in the diet. However, OverseerFM does not reflect the effects of greater urine volume when plantain content increases, which theoretically results in an increasing area covered by urine and more chance of urine patches overlapping (i.e. areas with double the amount of N deposited, or even more). Therefore, the chance of exaggerating the reduction in N leaching is greater at high proportions of plantain in the diet. Until we have more evidence of the impact of increased overlap on any reduction in effectiveness of high plantain diets, we have taken a precautionary approach in the model.
Figure 1. Reduction in urine N concentration with increasing plantain content in the diet, as measured in a range of experiments, and reduction in urine N load as estimated in Overseer.
Overseer calculates the urine patch N load as 750-(PP * 500), where PP is the proportion of plantain in the diet and 750 is the default N load of urine patches. The proportion of plantain in the diet is calculated from the plantain content of pasture and the proportion of pasture in the total diet.
 Bryant, R. H., Snow, V., Shorten, P. R. and Welten, B. G. (2019) Can alternative forages substantially reduce N leaching? Findings from a review and associated modelling. New Zealand Journal of Agricultural Research 63: 3-28. https://doi.org/10.1080/00288233.2019.1680395
 Shepherd, M. (2020) Recommendations for the implementation of plantain in Overseer. AgResearch report RE450/2019/069. Report for Forages for Nitrate Leaching (FRNL) and Overseer Ltd. https://www.dairynz.co.nz/media/5793403/frnl-overseer-2-recommendations-for-implementation-of-plantain-based-pasture-in-overseer-after-frnl-feedback-final-20-feb-2020.pdf
The Forages for Reduced Nitrate Leaching programme combined the expertise and resources of 10 commercial monitor farms that included Māori agribusinesses, three Crown Research Institutes (AgResearch, Plant & Food Research, Manaaki Whenua – Landcare Research), one university (Lincoln University), and two industry-good bodies (DairyNZ and the Foundation for Arable Research). The main funder of the programme was the Ministry of Business, Innovation and Employment (MBIE) with the six programme partners providing co-funding. For more information, go to dairynz.co.nz/FRNL