A PROCESS-BASED MODEL FOR CATTLE MANURE COMPOST WINDROWS: PART 2. MODEL PERFORMANCE AND APPLICATION

A model was developed and incorporated into the Integrated Farm System Model (IFSM, v.4.3) that simulates important processes occurring during windrow composting of manure. The model, documented in an accompanying article, predicts changes in windrow properties and conditions and the resulting emissions of C and N. Our objective in this article was to evaluate the performance of the compost windrow model. Model predictions were compared to published data from an independent cattle manure composting study that characterized static (no turning) and turned windrows composted over a 188-day period. The model did very well in simulating C and N losses for the turned windrow treatment, with predicted and measured cumulative C losses of 78% and 77%, respectively, and cumulative N losses of 55% and 57%, respectively, by day 99 of composting. By day 188 of composting, predicted cumulative N loss (56%) differed from measured (58%) by 2 percentage points only, whereas predicted cumulative C loss matched the corresponding measured value (80%). For the static windrow treatment, the model also did well in simulating cumulative C loss by day 99, with a difference of 5 percentage points between predicted (50%) and measured (45%) values. On the other hand, there was a difference of 12 percentage points between predicted (31%) and measured (19%) cumulative N losses for the static windrow by day 99. This discrepancy in simulating N loss for the static windrow treatment was not surprising, as the NH3 biofiltration effect of the finished compost cover on the actual static windrow was not modeled. After simulating the turning of the static windrow treatment on day 99, predicted and measured cumulative N losses by day 188 were equal at 44%. With the compost windrow model as a component, IFSM can be used to evaluate gaseous emissions from composted manure as influenced by windrow management practices and environmental conditions, along with other aspects of performance, environmental impact, and economics of cattle feeding operations. Simulating different composting strategies showed that addition of dry material to the cattle manure at the start of composting and turning of the manure windrow during composting produced a more stable organic soil amendment but caused greater carbon and nitrogen losses.