Reliability of a single fecal egg per gram determination as a measure of individual and herd values for trichostrongyle nematodes of cattle.

OBJECTIVES: To determine precision of a single fecal egg count, to evaluate the necessity of repeated testing to ascertain the number of parasite eggs found per gram of fecal material (EPG) of a given animal, and to infer the number of cattle that must be sampled to estimate the mean EPG value of a given herd.
DESIGN: Calves born in 4 successive years were tested for 3 consecutive days in May, July, and September to determine their fecal EPG value. Results were analyzed to determine important sources of variance in the EPG values. ANIMALS: 427 Black Angus calves. PROCEDURES: Nematode EPG values were determined by zinc sulfate flotation of fecal samples taken from the rectum. Variance components were estimated by restricted maximal likelihood procedures.
RESULTS: The largest source of variation in fecal egg counts arose from differences among calves. Variation associated with sampling within calf was larger than that associated with year of sampling or day when samples were taken. Repeatability of EPG determinations was between 0.4 and 0.7.
CONCLUSIONS: Collection of 3 replicate samples reduced variance associated with a single calf mean by 25 to 30%. Additional replication would further reduce the variance, but at a diminishing rate. To accurately estimate mean EPG for a herd, a randomly drawn sample should be similar in composition to the herd and include at least 1 animal from the high EPG group, which often constitutes 15 to 20% of the calves. To ensure that, on average, 95% of the samples drawn contain 1 or more animals with high EPG, the sample must include 15 to 20 animals within the grazing group under study.