Changes in the faecal egg counts of adult cows and in-calf heifers during winter and early lactation in Canterbury, New Zealand

Abstract

The present trial was designed to determine whether or not a breakdown of immunity to gastrointestinal nematodes occurs around parturition in both heifers and adult dairy cows in New Zealand. This was assessed by monitoring the level of nematode eggs excreted in the faeces from late autumn (May), when the animals were dried off, through to early lactation in spring (October).

On each of four farms, all the in-calf replacement heifers and an equal number of adult cows were faecal sampled every 4 weeks throughout the winter and every 2 weeks during the calving season (May - October). Faecal egg counts (FEC) and larval identification from coprocultures were conducted to determine the pattern and magnitude of any infection. Pasture samples were also collected and infective larvae contamination measured to determine the level of challenge to which the animals are exposed.

Results show that the date of sampling (August-October) had a significant (P < 0.01) effect on the FEC of the in-calf heifers, which supports the theory of a breakdown in the immunity of this class of animals during the calving season. The FEC increase observed in first calving heifers went from 20 eggs per gram (epg) in early winter to 50 epg during the calving season. However this increase was not observed in the adult cows. The difference between the animal groups was highly significant (P < 0.001) and an interaction between group and date of sampling was also highly significant (P < 0.001).

The results from the pasture showed an increase in the parasitic larvae available on pasture during the winter. The predominant species recovered from these samples were Ostertagia, Cooperia and Trichostrongylus, which are also in accordance with other reports from New Zealand.

The coprocultures performed separately on heifers and adult cows on each farm showed an apparent prevalence of Cooperia, however when inspected carefully, on most of the farms Ostertagia larvae increased the most during the calving season. This increase in Ostertagia larvae numbers is attributed to a resumption of development of arrested larvae.

On one of the farms an anthelmintic failure and/or nematode resistance to anthelmintics was observed, however more detailed analysis on this farm needs to be performed to determine the magnitude of the problem and recommend future control methods.