Predicting manure volatile solid output of lactating dairy cows

Abstract

Organic matter (OM) in livestock manure consisting of biodegradable and nonbiodegradable fractions is known as volatile solids (VS). According to the Intergovernmental Panel on Climate Change (IPCC) Tier 2 guidelines, methane produced by stored manure is determined based on VS. However, only biodegradable OM generates methane production. Therefore, estimates of biodegradable VS (dVS; dVS = VS - lignin) would yield better estimates of methane emissions from manure. The objective of the study was to develop mathematical models for estimating VS and dVS outputs of lactating dairy cows. Dry matter intake, dietary nutrient contents, milk yield and composition, body weight, and days in milk were used as potential predictor variables. Multicollinearity, model simplicity, and random study effects were taken into account during model development that used 857 VS and dVS measurements made on individual cows (kg/cow per day) from 43 metabolic trials conducted at the USDA Energy and Metabolism laboratory in Beltsville, Maryland. The new models and the IPCC Tier 2 model were evaluated with an independent data set including 209 VS and dVS measurements (kg/cow per day) from 2 metabolic trials conducted at the University of California, Davis. Organic matter intake (kg/d) and dietary crude protein and neutral detergent fiber contents (% of dry matter) were significantly associated with VS. A new model including these variables fitted best to data. When evaluated with independent data, the new model had a root mean squared prediction error as a percentage of average observed value (RMSPE) of 12.5%. Mean and slope biases were negligible at <1% of total prediction bias. When energy digestibility of the diet was assumed to be 67%, the IPCC Tier 2 model had a RMSPE of 13.7% and a notable mean bias for VS to be overpredicted by 0.4 kg/cow per day. A separate model including OM intake as well as dietary crude protein and neutral detergent fiber contents as predictor variables fitted best to dVS data and performed well on independent data (RMSPE = 12.7%). The Cornell Net Carbohydrate and Protein System model relying on fat-corrected milk yield and body weight more successfully predicted dry matter intake (DMI; RMSPE = 14.1%) than the simplified (RMSPE = 16.9%) and comprehensive (RMSPE = 23.4%) models to predict DMI in IPCC Tier 2 methodology. New models and the IPCC Tier 2 model using DMI from the Cornell Net Carbohydrate and Protein System model predicted VS (RMSPE = 17.7-19.4%) and dVS (RMSPE = 20%) well with small systematic bias (<10% of total bias). The present study offers empirical models that can accurately predict VS and dVS of dairy cows using routinely available data in dairy farms and thereby assist in efficiently determining methane emissions from stored manure.

Methane emissions of stored manure from dairy cows fed conventional or brown midrib corn silage

“…An increase in DMI without change in OM digestibility would result in an increase in the amount of VS excreted. Other studies reported a positive relationship between OM intake and VS excretion (Hassanat et al, 2013;Massé et al, 2016;Appuhamy et al, 2018).…”

Modeling the Effect of Nutritional Strategies for Dairy Cows on the Composition of Excreta Nitrogen

“…Such an evaluation requires a realistic and detailed representation of the cow's complex digestive processes. Several empirical models have been developed to predict output and composition of feces and urine (e.g., Reed et al, 2014;Appuhamy et al, 2018), but such models do not allow for detailed manure characterization and do not reflect the availability of N from urinary and fecal components for subsequent losses or use by plants.…”

Increasing Dairy Sustainability with Integrated Crop–Livestock Farming

“…Average nutrients by month and cow class were estimated using a weighted average, to account for different percent contributions to each diet. Monthly manure emissions were then estimated following Appuhamy et al [55] by first calculating volatile solid excretion (VS; kg month −1 ) of all cattle on farm. Volatile solids are biodegradable and nonbiodegradable fractions of organic matter in manure [55].…”

“…Monthly manure emissions were then estimated following Appuhamy et al [55] by first calculating volatile solid excretion (VS; kg month −1 ) of all cattle on farm. Volatile solids are biodegradable and nonbiodegradable fractions of organic matter in manure [55]. Monthly VS was calculated as a function of nutritional intake (Table 2):…”