A brief overview and simulation of the effects of some feeding strategies on nitrogen excretion and enteric methane emission from grazing dairy cows

Abstract

Dairy farming is under increasing pressure to reduce its environmental impact; foremost of these being nitrogen (N) discharges in urine and enteric methane (CH₄) emissions. This work reviews the literature supporting specific feeding strategies on reducing urinary nitrogen (UN) and scales each strategy up to a farm level using a mechanistic cow model (Molly) within a whole farm model (WFM). It then explores the capacity of each strategy to reduce UN and collateral enteric-methane (CH₄) emissions from those on a baseline allpasture dairy farm (BL) from Waikato, New Zealand (3.45 cow/ha; 1300 kg milksolids/ha/yr). The feeding strategies simulated were: 1) N fertilisation level; 2) Use of high sugar ryegrass cultivars; 3) Timing of pasture allocation; 4) Timing of defoliation at specific leaf stage; and 5) Maize silage supplementation at the beginning and end of lactation. The simulations showed different effectiveness on UN reductions according to feeding strategies. Except for the lowest and highest levels of N fertilisation (100 and 300 kg N/ha/yr) all feeding strategies increased enteric-CH₄ emissions/kg dry matter intake (DMI). Such increments were small, but had a large impact on CO₂ equivalent (CO₂-e) emissions/DMI and per unit of milksolids (MS). The biggest reduction of CO₂-e/herbage DMI, (6.3%) from BL, was obtained with the lowest level of N fertilisation (100 kg N/ha/yr). This work highlights an important trade-off: feeding strategies aimed to reduce UN may result in concurrent increases in CH₄ emissions, with an overall increase in environmental impact measured in CO₂-e