Development of an on-farm model to predict flow of fecal volatile solids to the liquid and solid handling systems of commercial California dairy farms

•A model was created to predict fecal volatile solids in dairy farm manure.•A key limiting factor to model creation was time cattle spent on concrete surfaces.•Collected data showed amounts of time that cattle spend on concrete surfaces.•Model evaluation showed that most (~77%) fecal VS was from lactating cows.•The model showed liquid fecal VS of 65% for freestall and 37% for drylot dairies. A source of methane (CH4) emissions from dairy farms arefecal volatile solids (VS) produced by cattle, which is impacted by herd size, cattle type, feed intake/composition and farm management practices. Where cattle deposit fecal VS in their pen is important in this regard since that deposited on concrete, which will be handled and stored in a liquid form, is a likely source of CH4 emissions, whereas fecal VS deposited on drylot surfaces will be handled and stored in a dry form and is a minor source of CH4 emissions. Our objective was to create a model to assist dairy farmers and regulators make informed evaluations of impacts of dairy farm management practices on potential CH4 emissions from fecal VS. Evaluation of initial model outputs led to on-farm data collection of the model inputs which influenced predicted fecal VS entering the liquid and solid manure handling systems. A key input limitation was lack of information on cattle fecal deposition locations within pens. Data collection used four dairy farms to measure time that cattle spent on concrete surfaces within day among season, as well as other model inputs. The final model, populated with collected data, showed that lactating cattle contribute the overwhelming proportion of fecal VS, 77% in the composite dairy farms, and that a composite freestall dairy farm had 65% of total fecal VS deposited on concrete annually with the remainder on drylot surfaces. In contrast, a composite drylot dairy farm had 37% of fecal VS deposited on concrete annually.