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Many dairy cattle are housed under drylot conditions, which generally are not considered conducive to elevated populations of gastrointestinal parasites. Thus, an examination of the potential benefits to be obtained by anthelmintic treatment of lactating cows under these circumstances was initiated. Because there were no statistically significant differences in milk production between control and treated groups, routine deworming cannot be considered economically sound under the management conditions described.

Two worm-free calves were allowed to graze on irrigated pasture with a naturally infected herd for each of 34 one-month periods from November 1979 to August 1982. After each grazing period, the calves were transferred to a cement-floored pen for 3 weeks and then were euthanatized and necropsied. Ostertagia ostertagi and Cooperia oncophora were the most prevalent species of nematodes recovered. Adults and larvae of Ostertagia spp and Cooperia spp were most numerous in winter and spring and least numerous during summer. The proportions of Ostertagia spp that were inhibited as fourth-stage larvae increased in late fall, peaked from March through April, and then decreased to low values during summer. The maximal inhibition in 1980, 1981, and 1982 was 72, 65, and 62%, respectively. The number of larval Cooperia spp was highest in winter months and, except for one grazing period when 55% of the Cooperia spp were larvae, the total numbers represented less than 15% of the nematode population during all grazing periods. Other nematodes encountered were Trichostrongylus axei, Haemonchus spp, O lyrata, and O occidentalis in the abomasum; C surnabata, C punctata, Nematodirus helvetianus, T colubriformis, and Bunostomum phlebotomum in the small intestine; and Oesophagostomum venulosum and Trichuris ovis in the large intestine.

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.

Worm-free Holstein tracer calves were used to assess the seasonal variation in number and course of development of nematode parasites of cattle on an irrigated pasture located in the California Sierra foothills. The most common genera of nematodes found in the abomasum and small intestine were Ostertagia and Cooperia, respectively. The infective Ostertagia larvae on herbage were most numerous in early spring and lowest in summer months. Arrested development of Ostertagia occurred in mid-to-late spring. The percentage of the population of Ostertagia found to be arrested larvae was constant (73% to 95%) over 4 years, even though the total number of nematodes in the genus was markedly different. Levels of infection with Ostertagia and Cooperia were found to correlate with pasture contamination occurring in the preceding fall and concurrent winter months. In the cecum and large intestine, Oesophagostomum venulosum was found to be the most common nematode. It is suggested that this species may replace Os radiatum as the cattle nodular worm in regions where climate is similar to that of the southwestern coast of North America. Evidence of seasonal hypobiotic development of Oe venulosum was found. Other genera of nematodes observed in tracer calves were Trichostrongylus, Haemonchus, Bunostomum, Trichuris, and Dictyocaulus.

Oxfendazole, methyl 5(6)-phenylsulfinyl-2-benzimidazole carbamate, was given by oral drench (5 mg/kg) to California lambs harboring natural infections of gastrointestinal nematodes. Ninety-nine percent of adult Ostertagia circumcincta, Ostertagia trifurcata, Trichostrongylus axei, Trichostrongylus vitrinus, Trichostrongylus colubriformis, Nematodirus filicollis, and Nematodirus spathiger were removed by the anthelmintic.

The relationship of anthelmintic treatment for subclinical gastrointestinal nematode parasitism with resultant milk production was studied on 3 California dairies where animals were maintained on a pasture-based nutritional program. All of the animals that started lactating (freshened) during a 12-month period participated in trials in which coumaphos was given as a feed top dressing to animals averaging 30 days into lactation. All of the animals that freshened during the subsequent 12-month period from 2 of the same 3 dairies participated in trials in which thiabendazole was administered at or within 2 weeks before freshening. Anthelmintic administration during 2 consecutive lactation cycles under the conditions of the present study resulted in no consistent reproducible significant changes in milk production.

During the period extending from Dec 1, 1975, to Dec 22, 1976, 3 distinct peaks in the concentration of infective nematode larvae on pasture forage were detected by use of helminth-free tracer calves. Two of these peaks, 1 occurring in the spring and 1 in the fall, consisted mainly of Ostertagia ostertagi and Cooperia sp. It is concluded that these 2 peaks can be anticipated annually and constitute unsafe periods of grazing. The 3rd peak was comprised largely of Haemonchus placei and Cooperia sp. It is concluded that this peak occurred as the result of unusual rainfall and reduced evaporation in August. Arrested developed (24% to 30%) of O ostertagi larvae occurred only during the spring peak. This finding indicates that on irrigated pastures in California, type I ostertagiasis can be expected to occur as the result of infection during the spring and/or fall peaks, whereas type II ostertagiasis can be expected to occur in the fall as a result of infection obtained during the spring peak. Arrestment in development of Cooperia sp occurred during spring (14%) and fall (23%) peaks. Haemonchus placei larvae increased their propensity for arrestment in development from zero to 82% during the months following the peak level of infection in summer. Other nematodes encountered were Trichostrongylus axei and Nematodirus helvetianus.