RATIONING DAIRY COWS TO MINIMISE ENVIRONMENTAL IMPACT

摘要

The environmental impact of livestock production includes emissions of greenhouse gases (GHG), land use e.g. competition for food from arable land between humans and livestock, land use change e.g. rainforest destruction and land management e.g. pollution of air and water from livestock units. The UK population of dairy cows decreased by 36% between 1990 and 2010 - a major factor in the 20% decrease in agricultural GHG in the period. The GHG of concern are methane from enteric fermentation and nitrous oxide from manure. It is difficult to measure GHG directly on farms and indirect indicators e.g. feed conversion efficiency (FCE) and nitrogen use efficiency (NUE, milk N as a proportion of total N intake) and the proportion of the herd calving for thefirst time annually can be used to assess probable GHG/kg milk and rates of change. The proportion of potentially human-edible feed e.g. cereal grain and soyabean meal in dairy cow rations is relatively low compared with monogastric livestock, consequently more milk energy and protein is produced than edible energy and protein consumed. Concern has been shown over Imports of soyabean meal in terms of land use change, despite much soyabean production being from land which has been in arable cropping more than 20 years. The average CP of grazed pasture, at 228 g CP/kg DM, is too high in relation to the requirement of the cow. Consequently NUE is less than 0.20 for some pasture-based systems. Pasture should be supplemented strategically with feeds low inCP to reduce total N intake and increase NUE. Additional expected benefits to pasture supplementation include increased intake of ME, increased milk yield and/or reduced weight loss in early lactation, improved fertility and increased stocking rate perhectare of pasture. Methane per kg milk is lower for maize silage than for grass silage-based diets. Selecting concentrate raw materials for reduced carbon footprint (CFP) in a maize silage-based ration for a 40-litre cow also results in increased NUE, reduced human-edible proportion of total diet DM, but increased cost. Soyabean meal, despite a relatively high CFP, is chosen for the low CFP concentrate because it has a more favourable ratio of DUP to CFP than the other raw materials. Herd replacementsconstitute 27% of total herd methane emissions. Increasing conception rate from <40% to that achieved by the best producers (60%) would reduce herd methane emissions by 20%. Increasing cow longevity from 3 to 4 lactations reduces lifetime methane per litre of milk by 13% with little change in lifetime NUE. There is scope for increasing FCE by improved accuracy of ration formulation and diet presentation. Realistic practical targets are a FCE of 1.5 kg milk per kg total diet DM intake and a NUE of 0.33.