Comparative environmental impacts of intensive all-grass and maize silage-supplemented dairy farm systems: a review

Abstract

Abstract New Zealand dairy farmers are lifting stocking rates and increasing available feed through nitrogen (N) fertiliser applications to pasture, growing maize for silage and other supplementary crops for silage or grazing on-farm, and/or procuring feed supplements off-farm. This has raised concerns about the possibility of increased risk of nutrient losses to waterways and the atmosphere. This paper reviews NZ and overseas data on the integration of maize silage into dairy systems. Maize silage is a low protein forage which helps optimise animal protein intake and reduces N loss. Maize silage-supplemented dairy farms leached more nitrogen per hectare but less per kg milksolids (MS) than intensive all-grass systems. Feeding maize silage on a feedpad and spreading the resulting effluent uniformly over the farm further reduces N leaching. In the Resource Efficient Dairying (RED) trial, total emissions of nitrous oxide (N2O, a potent greenhouse gas) for the maize-supplemented farmlet was 14% lower on a per hectare basis and 22% lower on a kg MS basis than the all-grass system when both received 170 kg N/ha as urea. The increases in maize dry matter production in response to incremental additions of N and water, where production is constrained by these inputs, can be 2-3 times greater than that for pasture. Using a feed and stand-off pad and managing maize growing through minimising tillage effects, determining soil N status at planting and timing N applications appropriately further reduce the environmental impact of maize silage-based dairy systems. Keywords: all-grass, environment, greenhouse gases, intensive dairy systems, maize silage, nitrates