Recommended number of sheep sampled in programs for internal parasite control can lead to incorrect decisions Worm egg count per gram (WEC) is used as a management tool to decide when to drench sheep to reduce the worm burden. It is also used in sheep genetic evaluations. In order to determine the appropriate time of drenching, or of recording a whole group in genetic evaluation, WEC is monitored in a random sample of animals from the group. The question that emerges is: what is the adequate sample size to determine if the average WEC has reached 500 o 1000? In this paper we examine two data sets with average WEC of 629 and 1499. They correspond to WECs frequently encountered in Uruguay. Thresholds beyond which drenching is recommended, or are considered appropriate for genetic evaluation, vary depending upon pasture availability and quality, and sheep condition. The former value is relevant for poor pastures and poor sheep condition, whereas the latter value is relevant for good pastures and sheep in good condition. When thresholds were set a 500 and 1200 in each data set, we found that with the recommeded sample size of 10 to 20 individuals, incorrect decisions would be frequently made. The minimum sample size should be 20 and preferably it should be greater. Given the insufficient sample size currently used, we recommend a revision of the topic and an experimentally and statistically based re-formulation of sampling guidelines. Keywords: worm egg count, faeces, sheep, sample size
Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume Electronic Poster Session - Biology - Disease Resistance 3, , 232, 2018
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