Mitigating GHG Emissions in Dairy Production.

Comprehensive inventories of greenhouse gas (GHG) mitigation options for animal agriculture have been published recently. For dairy production systems, management option include (1) manipulation of dietary components (e.g., forages, concentrates) and use of feed additives (e.g., oils, tannins) to reduce enteric methane (CH₄), and (2) methods to collect, process, store and land spread manure to abate CH₄, and nitrous oxide (N₂O) emissions from dairy barns, manure storage and soils. After a brief review of recent practice inventories, we will focus on some dairy nutrition factors that impact GHG emission.  For example, altering the supply and source of non-cell wall and cell wall carbohydrates in dairy cow diets lead to substantial changes in CH₄ production: decreasing forage level from 68 to 47% of dry matter intake (DMI) has little impact on performance but reduces both daily CH₄ emission and CH₄ emission per unit of milk.  A large substitution of alfalfa silage with corn silage does not have a profound impact on milk production or CH₄ emission, but fermentation of fiber in corn silage yielded substantially more CH₄ than the fermentation of fiber in alfalfa silage. Genetic selection for feed efficiency is a potential tool to reduce enteric CH₄: cows with higher gross feed efficiency have lower CH₄ emission per unit of milk than low gross feed efficiency cows. The urea contained in dairy cattle urine is the principal source of ammonia (NH₃) and N₂O emission from dairy farms. Our recent analyses found that milk urea nitrogen (MUN) may be used to not only evaluate feed N use efficiency (the proportion of feed N transformed into milk N), but also urinary urea N excretion, and NH₃ and N₂O emissions from dairy farms. Expansion of economic optimum fertilizer N rate (EONR) guidelines to include environmental outcomes can greatly reduce N₂O emissions from soils.