Dairy farm system solutions that reduce nitrate leaching and their consequences for profitability: Using plantain, fodder beet and oats on a Canterbury case study farm

Abstract

Dairy products provide nutrition, energy and income for much of the world. It is currently necessary to continue their supply albeit in a more environmentally sustainable manner. Excess nitrate (NO₃⁻) from dairy cow urine patches can leach from soils with significant consequences for receiving waters. Some potential on-farm management solutions exist, but can be costly to adopt. The research aim of this thesis was to test prescribed management solutions for reducing NO₃⁻ leaching by 20%, in comparison to an existing farm management baseline for the 2017/2018 dairy season, whilst maintaining profitability. Nitrate leaching and profitability were estimated for a south Canterbury case study dairy farm using the models FARMAX Dairy and OVERSEER® Nutrient Budgets. Prescribed management practices from the Forages for Reduced Nitrate Leaching (FRNL) programme were modelled to achieve this target. The principles were: (i) reducing nitrogen (N) in cows’ diets through low-N feed (fodder beet), (ii) recapturing N from soils through catch crops (oats) and (iii) diluting urinary N (through ingested plantain). Two crop treatments were applied to the Baseline to address (i) and (ii). Plantain was included in pastures to address (iii). A number of key assumptions were made about plantain’s efficacy for reducing NO₃⁻ leaching. Plantain was not expected to persist in pasture swards without active management and so a persistence curve and maintenance treatments were incorporated. A sensitivity analysis investigated the influence of soil type and poorer persistence of plantain on treatment success. Most treatments reduced NO₃⁻ leaching, but significant management inputs were required to achieve a 20% reduction from the Baseline. Plantain was identified as the key forage for reducing NO₃⁻ leaching. When plantain was included in pasture swards and undersown every second year to increase its presence, NO₃⁻ leaching could be reduced by 21-24%, however, profitability was reduced by 5-10%. Fodder beet and oats had little impact on NO₃⁻ leaching because the crop area was small in comparison to the rest of the farm (4%). There were no treatments that achieved a 20% reduction in NO₃⁻ leaching and maintained profitability. The implications of this modelling study for real-life application are that if plantain can be maintained in the pasture sward at high enough levels NO₃⁻ leaching can be substantially reduced, though this would likely result in a loss of profit.