Abstract
Background: Trace metals are supplemented to cattle to prevent nutrient deficiencies. Levels supplemented to mitigate worst-case basal supply and availability scenarios can, however, result in trace metal intakes far above nutritional requirements of dairy cows with high feed intakes.
Objectives: We evaluated Zn, Mn and Cu balance in dairy cows from late lactation through the subsequent mid-lactation, a period of 24 wk characterized by large changes in dry matter intake.
Methods: Twelve Holstein dairy cows were housed in a tie-stall from 10 wk before to 16 wk after parturition, and fed one unique lactation diet when lactating, and a dry cow diet otherwise. After two weeks of adaptation to the facility and diet, Zn, Mn and Cu balances were determined at weekly intervals, by calculating the difference between total intakes and complete fecal, urinary and milk outputs, with the latter three fluxes quantified over a 48-h period. Repeated measures mixed models were used to evaluate the effects on trace mineral balances over time.
Results: The Mn and Cu balances of cows were not significantly different from 0 mg·d-1 between 8 wk prepartum and calving (P ≥ 0.54), when dietary intake was the lowest of the period evaluated. However, when dietary intake was highest, between wk 6 and 16 postpartum, positive Mn and Cu balances were observed (80 and 20 mg·d-1, respectively, P ≤ 0.05). Cows were in positive Zn balance throughout the study except during the first 3 wk after calving during which Zn balance was negative.
Conclusions: Large adaptations occur in trace metal homeostasis in transition cows in response to changes in dietary intake. High dry-matter intakes, associated with high milk production of dairy cows, combined with current Zn, Mn and Cu supplementation practices may exceed regulatory homeostatic mechanisms resulting in potential body accumulation of Zn, Mn and Cu.
Daniel, Jean-Baptiste