Partitioning variation in nutrient composition data of common feeds and mixed diets on commercial dairy farms

A large project involving commercial dairy farms was undertaken to identify important sources of variation in composition data of common feeds and mixed diets. This information is needed to develop appropriate sampling schedules for feeds and should reduce the uncertainty associated with the nutrient composition of delivered diets. The first subproject quantified sources of variation in the composition of corn and haycrop silages over a 2-wk period. Silages from 11 commercial dairy farms in Ohio and Vermont were sampled daily over a 14-d period. Most silages were sampled in duplicate each day, and all samples were assayed in duplicate. Total variance was partitioned into analytical, sampling, farm, and true day-to-day components. Farm was the largest source of variation, but within-farm variance was our primary interest. Sampling variance comprised 30 to 81% of within-farm variance depending on nutrient and type of silage. For dry matter, true day-to-day variation was the greatest source of variance, but for most other nutrients, sampling was the largest source of within-farm variation. The second subproject consisted of sampling feeds and total mixed rations (TMR) from 47 commercial dairy farms across the United States. Feeds and TMR were sampled monthly. Because samples were not assayed in duplicate, source of variation included farm, month, and residual (sampling plus analytical). For corn and alfalfa silages, month-to-month variation over a 12-mo period comprised about twice as much of the total within-farm variation as did day-to-day variation over a 14-d period in the first subproject. Although month-to-month variation was greater than sampling variation, sampling still accounted for 9 to 37% of the total within-farm variance for those 2 feeds. For TMR, sampling plus analytical variance accounted for approximately 40 to 70% of the total within-farm variance (depending on the nutrient). Variance components were estimated for several nutrients and for several common feeds. The contributions to total variance differed depending on feed and nutrient, but the information provided will help in determining whether on-farm samples should be taken and if so, how often. A major implication of this project is that sampling is a substantial source of variation in silages, concentrates, and TMR, and data from a single sample are likely not highly reliable.