Midsummer Dectomax® Application for Spring Calving Cow-Calf Herds in the Northern Great Plains

G.P. Lardy¹, C.S. Stoltenow¹, J.T. Seeger², and K. Odde²

¹Department of Animal and Range Sciences,

North Dakota State University, Fargo

²Pfizer Animal Health

ABSTRACT

A multi-year, multi-faceted study was conducted at North Dakota State University’s Central Grasslands Research Extension Center near Streeter, North Dakota, United States, to evaluate under field use conditions the efficacy of doramectin at 500 mg/kg topically in beef cows and their calves when compared to non-medicated controls. One-hundred-twenty (120) spring calving cow/calf pairs harboring natural internal and external parasite infections and grazing native pastures from June through September were utilized. The pairs were randomly allocated to one of two treatment groups with four replicates and placed on eight separate breeding pastures throughout the grazing phase of the study. On Day –40 (40 days prior to Dectomax application), the 120 cows were weighed, their body condition scores (BCS) evaluated and fecal samples were collected. On Day 0, the day of treatment, the 60 cow/calf pairs allotted to treatment group two (T2) were treated with 500 mg/kg of doramectin applied topically down the midline of the back. The 60 cow/calf pairs in treatment group one (T1) received no anthelminthic treatment and served as a non-medicated control group. Individual fecal samples were collected from all pairs (cow and calf) from each treatment group of each replicate on Day 0, 21, 42 and 84. All fecal samples were quantitatively examined for parasite eggs by a parasitologist at South Dakota State University. Individual weights were assessed for all cows and calves on Days 0, 21, 42 and 84 and BCS were assessed for all cows on Days 0, 21, 42 and 84. On Day 84, the calves were weaned and the cows were palpated to determine pregnancy status. Also, on Days -3 (3 days prior to treatment), 4, 11, 18, 25 and 32 (weekly) five randomly selected cows in each replicate were examined for the presence of flies. The fly numbers were counted and recorded by an entomologist from North Dakota State University. At the time of weaning/pregnancy exam, Day 84, all of the cows from the four treatment (T2) replicates minus the cows determined to not be pregnant were individually examined for lice then commingled as a unit and held separate, as were the cows from the four control (T1) replicates, for the duration of the study. These cows were all individually examined by the study investigator for the presence of lice once monthly for the months of November through March. All of the steer calves from the study were commingled and committed to a backgrounding / finishing program to evaluate the effects of the prior application of doramectin on slaughter parameters including carcass quality. Three cow/calf pairs were removed from the study for the purpose of data evaluation as they failed to meet all of the study parameters. There were no significant differences between the doramectin-treated animals and the non-medicated controls for any variable measured except the fecal egg counts on the doramectin treated cows on Days 21 and 42 were significantly less than the non-medicated control cows; the BCS of the doramectin-treated cows was significantly lower on Day 84 than the non-medicated control cows; and the lice counts on the doramectin treated cows were significantly lower on Days 161-162, 184-185, 210-211 and 238-239 than on the non-medicated control cows. No adverse reactions to treatment were observed at any time during the study.