Effect of source and level of forage in the diet on in vitro ammonia emission from manure of Holstein and Jersey dairy cows

Discipline: environment; Keywords: manure ammonia, emissions, high and low roughage diet, breed, NDF.

The nitrogen (N) use efficiency (conversion of feed N into milk N) of dairy cattle is poor, typically ranging from 25 to 35%. The unused feed N is excreted about equally via the faeces and urine, although the proportion depends on the crude protein (CP) level and the ratio of rumen degradable protein (RDP) to rumen undegradable protein (RUP) in the diet. The excreted N is lost at each stage of manure management (e.g., during collection, storage, and after land application) in several forms: ammonia (NH3), nitrous oxide (N2O) and nitrate. Ammonia volatilization is the major form of N losses to the environment and accounts for 15 to 50% of the excreted manure N. Ammonia released into the atmosphere has mainly negative consequences. It forms particles of less than 2 μm in size, which might affect human health. Upon re-deposition, NH3 can cause acid rain and soil acidification, eutrophication of aquatic ecosystems, and biodiversity loss. Additionally, NH3 is an indirect source of N2O, the most potent greenhouse gas from agricultural sources. Furthermore, NH3 emission represents a loss of manure N that would otherwise be available for crop fertilization. Thus it is important to reduce the atmospheric emissions of manure NH3. To do so, one needs to study the various factors influencing emission of manure NH3. Therefore, the objective of a study by Drs M. E. Uddin and M. A. Wattiaux was to determine the effect of iso-nitrogenous and iso-starch diets with varying levels and sources of forage neutral detergent fibre (NDF) on in vitro NH3-N emissions from manure of Holstein and Jersey cows. Their study was published in JDS Communications, Volume 2 of 2021, page16 to20, the title being: Effect of source and level of forage in the diet on in vitro ammonia emission from manure of Holstein and Jersey dairy cows

In the study the potential NH3-N emission of reconstituted manure was determined, using an in vitro protocol. Faeces and urine were collected from a companion study designed as a Latin square in which four Holstein and four Jersey cows were fed diets containing two levels of forage NDF [low-forage NDF (19%) vs. high-forage NDF (24%; dry matter basis)] from either lucerne or maize silage (70:30 vs. 30:70 ratio of lucerne silage NDF: maize silage NDF) arranged as a 2 × 2 factorial. All diets contained similar levels of CP (17%) and starch (23%), and had forage-to-concentrate ratios of 55:45 and 68:32 for low- and high-forage NDF diets, respectively. Measurements of NH3-N emission were conducted in a laboratory-scale chamber with 16 gram of reconstituted manure (urine plus faeces) incubated for 48 hours at 15°C with sampling at 1, 3, 6, 12, 24, 36, and 48 hours. Hourly NH3-N emissions data were analyzed using a repeated-measures mixed model. The fixed effects were breed, forage NDF level, forage NDF source, time of sampling, and all possible interactions; cow was included as a random term. The cumulative 48-hour NH3-N emissions and the scaled-up emissions accounting for daily output of manure from each cow were analyzed using the same model but without time of sampling.

Level and source of forage in the diet tended to influence the pattern in hourly rate and 48-hour cumulative emission, respectively. Accounting for daily manure volume differences, low-forage NDF diets led to lower estimates of daily NH3-N emissions than high-forage NDF diets (20% on a cow basis, 15% on a raw manure basis, and 18% on a manure-N basis). Compared with Holsteins, Jerseys emitted 17% lower estimated NH3-N on a cow basis, mainly due to lower manure excretion but tended to emit 15% more NH3-N expressed on a manure-N basis. 

The results of the study suggested that cow breed and dietary forage NDF level should be considered in the prediction of NH3-N emission from the dairy industry.