Estimating Excreted Nutrients to Improve Nutrient Management for Grazing System Dairy Farms

SIMPLE SUMMARY: Improving nutrient use on dairy farms is closely linked to manure deposition, collection and use. Nutrients excreted daily on each one of five visits over a year were calculated based on the energy requirements of grazing lactating cows and information about diets fed to herds on 43 diverse Australian farms. Cows on larger farms, from bigger herds and producing greater milk volumes excreted the most nutrients. Over a year, dairy herds excreted 24, 4, 20, 3, 5 and 3 t of N, P, K, S, Ca, and Mg, respectively, which was up to 66% of most nutrients imported onto farms. Excreted nutrients were allocated to paddocks, dairy shed, yards, feed pads, holding areas and laneways depending on the time the herds spent in these places, with the most deposited in paddocks and the least in dairy sheds. Better collection of nutrients excreted in feed pads and holding areas would reduce nutrient losses by almost a third. The nutrients excreted in paddocks were similar to fertiliser P, and often greater than the fertiliser N, K and Mg applied. However, excretion in paddocks was not uniform, with most nutrients deposited in paddocks closest to the dairy shed. Australian nutrient management tools need to account for excreted nutrients to support identification of on-farm nutrient management opportunities. ABSTRACT: Improving nutrient management in grazing system dairy farms requires determining nutrient flows through animals, the placement of cows within farms and potential for collection, and the re-use and loss of nutrients. We applied a model incorporating data collected at a range of temporal and spatial scales to quantify nutrient excretion in all locations that lactating herds visited on five days over a year on 43 conventional and organic grazing system dairy farms. The calculated nutrient loads excreted by cows in different places were highly skewed; while N, P and K deposited loads were consistent across the year, S, Ca and Mg loads varied between sampling times and seasons. The greatest mean and range in nutrient loads were deposited in paddocks, with the smallest amounts deposited in dairy sheds. All excreted nutrient loads increased with farm and herd sizes and milk production. Mean daily loads of 112, 15, 85, 11, 22 and 13 kg of N, P, K, S, Ca and Mg were deposited by the herds which, when standardised to a 305-day lactation, amounted to 24, 4, 20, 3, 5 and 3 t excreted annually, respectively. In addition to routine manure collection in dairy sheds, ensuring collection and recycling of nutrients excreted on feed pads and holding areas would decrease potential nutrient losses by 29% on average. Non-collected, recycled nutrients were disproportionately returned to paddocks in which cows spent time overnight, and except for S and Ca, nutrient loading rates were greater than rates applied as fertilisers. These data demonstrate the extent of excreted nutrients in grazing dairy systems and indicate the need to account for these nutrients in nutrient management plans for Australian dairy farms. We propose incorporating excretion data in current budgeting tools using data currently collected on most Australian grazing system dairy farms.