Uncertainty of Manure Output Estimated with Intrinsic Markers for a Lactating Dairy Herd

<b><sc>Highlights</sc></b> <list list-type=bullet><list-item> We simulated the use of intrinsic markers to measure manure output rates on commercial dairies. </list-item><list-item> Weekly (vs. monthly) sampling frequency, sampling replication, and analytical replication reduced uncertainty. </list-item><list-item> The uncertainty of manure output was lowest in scenarios with low concentrations of marker in bedding material. </list-item><list-item> Intrinsic markers can be used to measure manure output at pen or barn scale, inclusive of bedding and other materials. </list-item></list> <b><sc>ABSTRACT.</sc></b> Accurate and precise manure output estimates are critical to modeling environmental impacts from livestock farms and designing manure handling and treatment systems. Through simulations and uncertainty analysis, we examined the potential to use the intrinsic markers undigested neutral detergent fiber (uNDF), acid-insoluble ash (AIA), and ash to estimate monthly manure output at farm scale. We compiled values reported in the literature to design scenarios representing uNDF, AIA, and ash markers with wood shavings bedding (uNDF-Shavings, AIA-Shavings, and Ash-Shavings, respectively), and sand bedding with a uNDF marker (uNDF-Sand), with manure representing combined excreta and discharged bedding. We assumed daily feed and refusal weights were recorded, bedding usage rates were measured at sampling occasions, and agitated manure was sampled from a transfer pit. For Ash-Shavings, we considered a typical commercial farm milk pick-up, sampling, and analysis scheme. Within each scenario, we varied the sampling frequency (weekly, monthly) and sampling and analytical replication (none, triplicate) for feed, bedding, and manure material. Then, we propagated elemental uncertainties to compute the uncertainty of monthly manure output (Δ) at a 95% confidence level relative to the mean. For all scenarios, Δ exceeded ±30% when sampling inputs monthly without sampling or analytical replication. Weekly sampling without sampling or analytical replication reduced Δ to ±17%–36%. With weekly sampling of inputs and sampling or analytical triplicates, Δ reached ±12%, ±13%, ±14%, and ±35% for uNDF-Sand, AIA-Shavings, Ash-Shavings, and uNDF-Shavings, respectively. In general, uncertainties in feed and manure marker concentrations were the largest contributors to uncertainty in manure output except with uNDF-Shavings, where uncertainty in the mass of bedding also contributed substantially. Considering that few methods currently exist to estimate manure output at pen or barn scale, our results suggest that marker-based estimates may be reasonably accurate and precise when marker concentrations in bedding are low. Still, more research is needed to examine temporal and sampling variability in outflowing manure in association with management events (e.g., stall cleaning and grooming) and to improve analytical precision for marker concentration.