Simple SummaryGastrointestinal (GI) nematode infections are a significant health and welfare threat to calves in pasture-based rearing systems. Sustainable control requires an efficient and cost-effective diagnostic tool. The serum pepsinogen assay is a long-established tool to monitor the infection level. However, the relatively high cost and lack of standardisation hinder the broad implementation of this method. Here, the more cost-efficient and robustOstertagia ostertagi-Ab ELISA is evaluated as a potential alternative diagnostic method in first-season grazing (FSG) calves. We performed both a literature review of studies where this method has been applied in FSG calves and conducted field studies in Belgium and Sweden to compare results from theO. ostertagi-Ab ELISA with the serum pepsinogen assay. We conclude that theO. ostertagi-Ab ELISA test is a valuable monitoring tool in FSG calves and could complement or replace the serum pepsinogen assay.AbstractTheO. ostertagi-Ab ELISA assay is widely used as a diagnostic tool for monitoring gastrointestinal (GI) nematodes using milk samples from adult dairy cows. This assay is potentially also useful to analyse serum samples from first-season grazing (FSG) calves, providing a more cost-effective and robust diagnostic technique than the current serum pepsinogen assay. However, a comprehensive evaluation of its use in serum samples from FSG calves has not yet been conducted. In this study, we first reviewed the available scientific literature in which theO. ostertagi-Ab ELISA was applied to serum samples from FSG calves. Then, a field study was conducted to compare results from theO. ostertagi-Ab ELISA assay with a serum pepsinogen assay on a set of 230 serum samples from 11 commercial dairy herds (seven in Belgium and four in Sweden). The literature review showed an increase in mean antibody levels, expressed as optical density ratio (ODR) values, from <0.4 (early grazing season) to values of 0.7–1.1 (late grazing season). Three out of five studies found a negative correlation betweenO. ostertagiantibody levels measured during the late grazing season and weight gain, while the other two studies found no correlation between the two variables. Our field studies showed a good correlation betweenO. ostertagiantibody levels and the results from the pepsinogen assay. Both indicators were negatively related to average daily weight gain in the Belgian herds, but not in the Swedish herds. Overall, the results suggest that theO. ostertagi-Ab ELISA test can be a useful tool in FSG calves and could replace the use of the serum pepsinogen assay at the end of the grazing season for general monitoring purposes.