Update on copper and selenium in Canadian cow-calf herds: regional differences and estimation of serum reference values.

Trace mineral supplementation of beef cattle is essential for efficient reproduction and herd health. Understanding regional differences in cow trace mineral status could inform decisions about risks of deficiencies and supplementation management. Cow-calf surveillance projects provided three opportunities to evaluate the trace mineral status of Canadian beef cow herds. Blood samples were collected at pregnancy testing in 2014 from 102 cow-calf herds and in 2016 from 86 cow-calf herds in Western Canada. In 2019, blood samples were collected at pregnancy testing from cows in 163 cow-calf herds from Eastern and Western Canada. Serum samples were analyzed for copper, selenium, and molybdenum concentrations using a plasma mass spectrometer. The prevalence of copper deficient cows sampled from the Western provinces ranged from 24% to 43% across the three periods, and was 20% from Eastern Canada in 2019. The prevalence of selenium deficient cows ranged from 0.2% to 0.4% across the three projects in Western Canada, but was higher in Eastern Canada at 4.6% in 2019. High serum molybdenum was identified in 9.4% to 14% of cows across the three periods in Western Canada and in 15% of cows sampled in Eastern Canada in 2019. Serum copper, selenium, and molybdenum concentrations varied by cow age and month of sample collection. Serum selenium and molybdenum concentrations, but not copper, varied by soil type associated with the location of the farm. A subsample of samples from cows from Western Canadian herds provided body condition score (