The ratio of glucogenic to lipogenic acids in ruminal VFAs can have a direct impact on nutrient utilization. The objective of this study was to compare the effects of an extended-release injectable dewormer [LongRange® (eprinomectin)] to a traditional shorter-acting injectable dewormer [Ivomec® (doramectin)] on rumen VFA profiles and blood metabolites. Ruminally cannulated lactating cows (2yr of age) were randomly allotted to one of the two dewormers (n = 5/treatment) at the time of being moved to spring grazing (day of AI). Blood samples, rumen pH, and rumen fluid were collected prior to treatment on d 0, d 15, and approximately every 30d thereafter until d 137 and analyzed by analysis of repeated measures. Fecal samples were collected at the start, middle, and end of the study to determine parasite fecal egg count. At the initiation of the study only 3 animals had any detectable parasites by fecal egg count and at the middle of the study all animals treated with the traditional dewormer had detectable parasites. Animals treated with the extended-release dewormer had no detectable parasite load throughout the study. There was no effect of treatment or treatment by time on rumen pH (P = 0.64, 0.55) or circulating NEFA concentrations (P = 0.60, 0.65) but both changed over time (P ≤ 0.01). Circulating glucose concentrations were not affected by treatment (P = 0.71) or time (P = 0.15), but there was a treatment by time interaction (P = 0.03) with glucose concentrations increased on day 32 in the extended-release dewormer. Concentrations of rumen ammonia, total VFAs, acetate, propionate, isobutyrate, butyrate, isovalerate, and valerate were not impacted by treatment (P > 0.20), but there was an effect of time (P = 0.10). In summary, controlling parasite load during a 5mo grazing season can improve nutrient utilization in cattle.