Internal Parasite Resistance Selection Method for Sheep

1999 Annual Report for FS99-088

Project Type: Farmer/Rancher
Funds awarded in 1999: $4,844.00
Projected End Date: 12/31/2001
Region: Southern
State: Texas
Principal Investigator:

Internal Parasite Resistance Selection Method for Sheep


In the Southern United States–with long warm seasons and abundant rainfall—internal parasites are the major impediment to sheep production. The warm and moist environment is favorable for the transmission of adult parasites into the host where they reproduce and do their damage. Producers have typically approached the problem of internal parasite control with frequent and repeated use of two or three classes of anthelmintics (chemical parasiticides). While the short-term result is control by suppression, in the long-term this can result in chemical-resistant parasites and animals with weakened natural immune systems.

The hair-type breeds of sheep are known to have a higher level of natural resistance to internal parasites. However, this genetic trait will vary considerably among individual animals. Our approach to the problem of managing internal parasites in sheep is to select individuals for breeding stock that have the highest level of natural parasite resistance and then proliferate this genetic trait in our breeding program.

Hair sheep are very suitable for small farm operations or grazed in a multi-species program with cattle. They will select forbs and browse not usually selected by cattle. Also, sheep and cattle do not share internal parasites which make them very compatible as ‘parasite vacuums’ for each other in a lead/follow grazing scheme.

The goal of this project is to develop a producer-friendly and economically viable method for selecting parasite resistance in a specific flock and then to define a program for minimum chemical input that will do no harm to the natural defenses of the selected sheep.


Frank Craddock

Texas A&M University
Tom Craig

Texas A&M University