Field sampling strategies for studies of alfalfa forage quality

Information is scarce on sampling techniques for field studies of alfalfa forage quality. Standard formulas are available for estimating the number of samples needed for reducing error in a study, but little is known about the impact of plot sampling on forage quality. Our objectives were to compare the strategy of manual harvesting from small areas within plots with that of grab sampling mechanically harvested forage, and to determine whether the within-plot location of sampling affected forage quality in any systematic way. Alfalfa forage was sampled from swaths of mechanically clipped forage (grab samples) and from hand-clipped areas within field plots (area samples). Systematic sample location within a plot had no discernable effect on forage quality. Calculations of predicted standard errors and required sample numbers indicated that one area or one grab sample per plot with three replicates would provide an acceptable standard error for comparison of alfalfa entries for protein and fiber concentration. Within-plot variability was greater at late-summer harvests than earlier harvests, but at all harvests one sample per plot with three replicates gave adequate precision for forage quality comparisons. Higher forage quality from grab samples than from area samples at spring harvests suggested the need for caution when comparing forage quality studies done with different harvest methods; however, there were few entry × sampling strategy interactions, which suggests that relative performance of entries would be similar regardless of the method of sampling.