Forage Sampling Factors Influencing the Variability of In Vitro Fermentation Results of Grass Selections

Variation in in vitro fermentation estimates of forage digestibility is a problem. Only a few studies have quantified the extent of laboratory variation, and even less information is available on variability associated with sample collection, processing, and storage prior to analysis. A study was designed to evaluate the effects of source of samples and maturation on genotype selection in a breeding program. Variance was partitioned into genotype effects; sampling procedures; effects of handling, drying, grinding, and storing samples; and laboratory analysis. The sampling procedure consisted of collecting bulk and tiller samples of 31-day regrowth forage of five clones from each of five Panicum coloratum L. and seven Cenchrus ciliaris L. selections (genotypes). Tillers in the early inflorescence stage were selected while bulk samples consisted of the top growth from one-half of each clone. Each sample was randomly divided in the field, and bulk samples were again subdivided following drying and grinding. Laboratory analyses were run on duplicate 0.5-g samples. Bulk samples exhibited a wider range of digestibility among selections, and approximately the same genotypic rankings as did tiller samples. Bulk samples collected at three ages (approximately 4, 6, and 8 weeks of regrowth) showed genotype ✕ age interactions, indicating the need to sample at more than one stage of maturation. The largest component of variance (3.01 to 8.1) was associated with selections (genotypes). The second largest component of variance (1.81 to 2.65) was associated with clones. Clones accounted for 14 to 25% of total variance. Relatively little variance was associated with handling, drying, grinding, and storage of samples. From 9 to 22% of total variance was associated with in vitro analysis (duplicate samples). The components of variance for laboratory analysis ranged from 1.18 to 2.38, which is similar to that reported in other studies. The results indicate that for indeterminate plants, and others not varying widely in date of flowering, bulk sampling is acceptable. Improvement in precision of the in vitro procedure may be realized by sampling multiple clones of the same genotype and by multiple laboratory analysis. Grass breeders should sample plants at more than one stage of maturation to gain an improved assessment of forage digestibility potential.