Seasonal occurrence of infective nematode larvae in California high Sierra pastures grazed by cattle.

Worm-free Holstein calves were used to assess the seasonal variation in numbers of nematode infective larvae on herbage at 2 commercial cow-calf ranches in the Sierra Nevada mountains. Both ranches used Herefords as their basic breed. One ranch near Sattley, California, is at an elevation of approximately 1,500 m above sea level, whereas the study area on the 2nd ranch near Susanville is at an elevation of approximately 1,250 m above sea level. Worm-free tracer calves were allowed to graze for approximately 30 days and were then kept free of further nematode infection until euthanasia and necropsy approximately 30 days after removal from the pastures. At necropsy, the number, species, and stages of development of nematodes in the gastrointestinal tract were determined. The most common species of nematode in the abomasum was Ostertagia ostertagi and in the small intestine Cooperia oncophora. Infection levels decreased during early summer and were low in midsummer to early fall. In late fall and winter, infection levels began to increase, and highest infection levels were obtained during the spring. A low level of inhibited development (up to 18%) of 4th-stage O ostertagi occurred in November and December. Considerably higher levels of inhibition (up to 61%) were observed in March, April, and May. Inhibition of development in Cooperia spp was not noticed. Other nematodes seen were O lyrata, C mcmasteri, C punctata, Trichostrongylus axei, Haemonchus sp, and Nematodirus helvetianus.