Fecal egg counts and individual milk production in temperate pastoral dairy systems of Australia

Anthelmintic use in lactating dairy cattle has been shown to result in a milk production response in some previous studies. If individual animals within a herd could be identified that would most benefit from anthelmintic treatment, this may reduce anthelmintic resistance. Australian dairy systems are predominantly pasture based, allowing sustained exposure and immune stimulation of cattle to gastrointestinal nematodes. We assessed the relationship between milk production and early-lactation fecal egg counts (FEC) to determine whether cows with higher FEC produced less milk. Ten pasture-based dairy farms in south-west Victoria, Australia, entered an observational study. Individual FEC at a minimum detectable count of 2.5 eggs per gram of feces (epg) were recorded for recently calved primiparous and multiparous cohorts on each farm. All animals were calved ≤30 d at sampling. Body condition scores were assessed at sampling and milk production data were collected from daily milk meters and herd tests to give first 100-d milk production. When separated by primiparous or multiparous status, no difference in the milk production between cows with FEC = 0 and FEC ≥2.5 epg was identified. Between-farm variation was large for FEC and milk production. Fecal egg count at a minimum detectable count of 2.5 epg detected parasitism in primiparous cows postcalving, but the presence and magnitude of parasitism measurable by FEC was not related to milk production in pasture-based Australian dairy systems. In multiparous cows, the rate detection of worm eggs at this analytical sensitivity was lower and the significance of a positive FEC at this analytical sensitivity requires further assessment to ascertain the effect on milk production. Based on our study, it seems unlikely that individual FEC results would be useful as a basis to select individual cows in south-west Victorian dairy herds for anthelmintic treatment.