Keterinary Parasitology, 40 ( 1991 ) 87-98 87 Elsevier Science Publishers B.V., Amsterdam Effects of time of day, season and stratum on Haemonchus contortus and Haemonchus placei third-stage larvae on irrigated pasture R.C. Krecek a, H.T. Groeneveld b and J.A. van Wyk c aDepartment of Parasitology, Faculty of Veterinary Science, University of Pretoria, Private Bag X04, Onderstepoort O110, South Africa bDepartment of Statistics, University of Pretoria, Pretoria, South Africa CHelminthology Section, Veterinary Research Institute, Onderstepoort O110, South Africa (Accepted 8 April 1991 ) ABSTRACT Krecek, R.C., Groeneveld, H.T. and van Wyk, J.A., 1991. Effects of time of day, season and stratum on Itaemonchus contortus and Haemonchus placei third-stage larvae on irrigated pasture. Vet. Par- asitol., 40: 87-98. The effect of time of day, season and stratum of herbage and soil on the availability of Haemonchus contortus and Haemonchusplacei third-stage larvae ( L3 ) on pasture was assessed. Feces from infected calves and lambs were placed on pasture plots and samples of upper herbage, lower herbage, mat and soil were collected at five intervals per day throughout the daylight hours on 18 sample days over 12 months. Using recovery rate factors derived from a preliminary investigation on the efficacy of larval recovery from each stratum, the data on larval recoveries were analyzed for the effect of season, time and stratum, and their interactions. Significant (P< 0.05) differences were found for season, stratum and the season-with-stratum interaction for both parasites. No significant differences were detected for larval counts at different times of the day. Larval recoveries of//. contortus were larger throughout the study than those ofH. placei. Most H. contortus L3 were recovered in the summer and autumn, and H. placei in the spring and summer. For both parasites, the recoveries of larvae from the upper and lower herbage were larger than those from the mat and soil. The implications of these findings are discussed in terms of control strategies. INTRODUCTION Information is needed on the effect of the environment on the ecology of free-living stages of ruminant parasitic nematodes. Present control strategies are complicated by the problem of anthelmintic resistance, particularly with Haemonchus contortus in the Southern Hemisphere (Waller, 1987; Van Wyk and Malan, 1988 ). A better understanding of the characteristics of the pas- ture stages of these parasites may lead to the development of alternative strat- 0304-4017/91/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.