Sustainable intensive farming systems

Abstract

Dairy systems contribute 20 to 30% of livestock emissions and excretions in Europe. Cows convert food unsuitable for human consumption (grass, forages, byproducts) into high quality food (milk), but nutrient conversion is 20 to 30%, so 70 to 80% of nutrients are excreted. With indoor systems, wastes can be contained and spread on the land when plant uptake is optimal. The main driver of environmental impact is production efficiency, i.e. milk or wastes per unit input. Efficiency is a function of animal numbers (milking cows and replacements), milk yield and culling rate. Higher- yielding fertile cows produce more milk per lactation and lifetime, so 'unproductive' emissions and excretions for maintenance and rearing are spread over more litres of milk. Poor fertility in modern dairy cows increases culling, which offsets efficiency gains from high milk yield. Our recent work has studied nutrition, insulin and fertility. Insulin stimulates oestrous cycles, but reduces oocyte quality. Dietary manipulation of insulin at strategic phases of the reproductive cycle doubled the proportion of cows pregnant at 120 days in milk. Diets for high-yielding cows contain more concentrates, starch and fat, and less fibre than grass-based systems, which reduce methane emissions. Breeding for low methane emissions may be possible in future; we find variation among cows with the same milk yield and diet. Nitrogen and phosphorus excretions are reduced by increasing production efficiency and by reducing excess inputs. Scope for reducing excesses is greater in higher-yielding cows, but trade-off in nitrous oxide emissions must be avoided. A whole-system approach is needed which considers environmental cost of diet formulation as well as economics.