Gastrointestinal nematode effects on grazing dairy herds during development and its impact on production

Abstract

Most studies about the effects of gastrointestinal nematodes on bovines have been performed on beef herds, and relatively few trials have evaluated the effect of parasitism on dairy heifers and cows. We reviewed the results from our experiments in Holstein heifer calves grown on pastures, continuously treated with ivermectin from birth to puberty in comparison with naturally nematode infected untreated animals. Nematode eggs in feces of parasitized animals augmented rapidly during the 3rd and 4th months of life and then decreased, whereas treated animals remained egg-free throughout the same period. Treated animals grew faster than untreated ones and differences in body weight became significant in the 6th week of life. Parasite-free heifers reached puberty nearly four weeks earlier than infected ones. Besides, pelvic area at 39 weeks and at 15 months of age was greater in treated heifers when compared to parasitized animals. Age at first service was advanced by 10 weeks and age at calving by 3.5 months in treated versus untreated animals. Daily milk yield and fat content in milk during first lactation were similar in both groups but culling rate during first lactation was higher in untreated than in treated cows. Endocrine mechanisms involved in parasite action on growth, reproduction and production were investigated. Serum IGF-I levels increased from birth to 22 weeks of age and then reached a plateau in both groups, but levels were consistently higher in parasite-free heifers. Mean LH levels were diminished in untreated heifers 4 weeks before the first estrus and the amplitude of LH pulses were augmented in treated heifers when puberty was reached. Prolactin followed seasonal changes of sunlight duration and presented sporadic bursts in infected animals. These were related to high nematode infection and are probably involved in the immune response of the host. Furthermore, leptin which directly correlates with body weight during development showed a prepubertal surge two weeks before the first progesterone peak in both groups. We conclude that parasitism during growth in dairy heifers impairs full production as it delays onset of lactation and reduces productive life of the cows, and that those effects could be mainly ascribed to reduced IGF-1 levels during development.