Effects of dairy cow breed and dietary forage on greenhouse gas emissions from manure during storage and after field application

On-farm greenhouse gas (GHG) emissions from cows, manure, and fields (to produce feed) comprise more than 72% of the United States milk carbon footprint. Recent studies examined the impact of dietary strategies on enteric methane emissions, however, tradeoffs between enteric methane and manure related GHG emissions have not been determined. Thus, the objective of this study was to determine the carry-over effects of dairy cow breed and diet on manure composition and manure GHG emissions during storage and after field application. The GHG emissions were measured using static chamber method for 50 days (d) during storage and 50-d after field application (30-d in the fall and 20-d in the following spring) of manure collected from four Holstein and four Jersey cows fed either alfalfa silage or corn silage based diets containing low forage neutral detergent fiber (FNDF) or high FNDF. None of the interactions among treatment factors were significant (P > 0.10). Manure composition was affected by both FNDF level and FNDF source but not cow breed. For example, manure pH was lower for low FNDF-fed than high FNDF-fed cows and concentrations of organic matter, total carbon, and neutral detergent fiber were greater in manure of corn silage-fed than alfalfa silage-fed cows. Except for starch (which was in low concentration), all the measured manure characteristics changed during the storage period. Treatments did not affect either hourly CO2, methane and nitrous oxide emissions, nor cumulative emissions (over 50-d of storage or over 50-d after land application), except for a tendency (P < 0.10) to emit 22% lower manure CO2 by high FNDF-fed cows than low FNDF-fed cows. Cumulative methane and nitrous oxide emissions were respectively 25 times greater and 19 times lower during the 50-d manure storage period than the subsequent 50-d after field application. Cumulative field nitrous oxide emission was 17 times greater during spring than fall. Depending on mode of expression (emissions per kg manure or per kg milk or per cow basis), manure of low FNDF-fed cows tended to emit 51 to 72% greater 100d (combined storage and field) non-CO2 GHG emissions than high FNDF-fed cows. However, in this study, neither cow breed nor FNDF source affected the 100-d combined non-CO2 GHG emissions. (C) 2020 Elsevier Ltd. All rights reserved.