Abstract
Background:
The selection of sheep with high genetic resistance to gastrointestinal nematodes is a sustainable alternative for parasite control.

Aim:
This study was performed to categorize three breeds of hair sheep according to their resistance to gastrointestinal nematodes during the peripartum period using hematocrit (HCT) and to compare these results with categorizations derived from the nematode egg count per gram of feces (EPG).

Methods:
Parasitological records from two studies involving 46 Katahdin × Pelibuey and 25 Blackbelly ewes were used, along with information from pregnancy (week 22) to lactation (week 13) of a flock of 31 Pelibuey ewes. All ewes of the three breeds were naturally infected by grazing. The ewes were categorized as resistant, intermediate, or susceptible in each breed and by physiological stage (gestation or lactation) using the EPG ± three standard errors. We also categorized ewes based on their HCT ± one standard deviation.

Results:
During pregnancy, resistant ewes were those with less than 257, 148, and 96 EPG for the Blackbelly, Katahdin, and Pelibuey breeds, respectively, while in lactation, resistant ewes had less than 1587, 912, and 310 EPG, respectively. In the classification by HCT, Blackbelly ewes had values lower than 31.0%; therefore, only intermediate (HCT of 24.4%–31.0%) and susceptible ewes (HCT <24.4%) were identified. Among the Katahdin, the resistant ewes had only 149 EPGs were recorded during lactation, thereby making the classification by lactation-HTC (94 EPG) comparable to the classification by EPG. In Pelibuey ewes, classification by HCT during early lactation (week one-four) allowed the selection of resistant ewes with higher EPG (379 EPG) compared with the EPG classification (80 EPG), but intermediate and resistant ewes had similar EPG.

Conclusion:
Classification by hematocrit and nematode egg per gram of feces allows the selection of ewes with resistance to gastrointestinal nematodes at the beginning of lactation.

Key words: Genetic selection, Haemonchus contortus, Pelibuey, Peripartum, Resistance