The impact of using Overseer 6.2.2 in reducing nitrogen losses to water on dairy support farms using kale and fodder beet forage systems

Abstract

Recent intensification of farm systems (especially within Canterbury) has resulted in increased nitrogen losses to water. This has created health risks and the eutrophication of waterways. Winter grazing of dairy cows on forage crops (such as kale and fodder beet) is a particularly “nutrient leaky” system. High stocking rates result in a build-up of soil nitrogen from the nitrogen loading in urine patches. The nitrogen is not taken up by plants (as winter is a low growth season) and is readily leached from the soil. Government and regional councils have responded by creating regulatory control around nitrogen losses to water for farming systems. The Canterbury regulations are set out in the Land and water Regional Plan (LWRP). The Selwyn Te Waihora zone (a particularly high risk zone) has specific policies and rules outlined in Variation 1 in the LWRP, including nitrogen loss reductions by 2022 (22% for a dairy support farm). Overseer is the computer modelling tool used by regional councils to measure and regulate nitrogen leaching losses. This research project focuses on management options to reduce nitrogen losses to water, as modelled by Overseer 6.2.2. Due to the irregular updates and new versions of Overseer released this area of study is going to need constant attention, as results will change. Four case study dairy support farms were used to determine common industry practise for wintering systems. This information was used (along with industry recommendations) to create an Overseer base model for both a kale and fodder beet system. This model was used to determine the success of four management strategies in reducing nitrogen leaching losses. Nitrogen use, feed supplement nitrogen content, soil type and the use of a catch crop were all modelled. Fodder beet and Kale were not directly compared using Overseer as inconsistencies in the model called into question the reliability of results. Instead a thought experiment was used for a direct comparison. A number of inconsistencies within the Overseer 6.2.2 model impacted on results. Crop feeding levels were variable between crops and yields, feed supplement had no effect on nitrogen leaching losses and nitrogen losses from kale crops seemed low when compared with published research.