EFFECTS OF DIETARY MOLYBDENUM ON PRODUCTIVITY AND METABOLIC PARAMETERS OF LACTATING BEEF COWS AND THEIR OFFSPRING

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Abstract
A study was undertaken to determine cow lactation and suckling calf growth responses to varying concentrations of molybdenum (Mo) in the diet of lactating beef cows. Twelve cow-calf pairs, at approximately 31 d postcalving, were assigned to one of three dietary treatments for 9 wk. The experimental design was a split-plot with dietary treatment at the whole plot level and time (wk) at the subplot level. Cows were group fed 11.6 kg dry matter (DM) d−1 of their respective diets which contained corn silage and barley in a ratio of 77: 23, DM basis. Mo, as ammonium molybdate [(NH4)6 Mo7O24∙4H2O] was added to the barley such that Mo concentration in the diets was 0.60 (0Mo),19.3(20Mo) and 34.8 (40Mo) mg kg−1 DM. Nutrient composition of the diets averaged 14.3% crude protein, 0.52% calcium, 0.38% phosphorus, 0.13% sulfur and 5.9 mg kg−1 copper (Cu), DM basis. No feed was offered to calves. Cow milk yield and composition, cow and calf body weight and cow and calf blood samples were obtained weekly. Liver biopsies were performed in calves during weeks 1, 4 and 9. Milk yield, determined by an oxytocin/hand milking method, declined more rapidly (P < 0.05) for cows consuming 40Mo than for 0Mo or 20Mo. Diet did not influence fat, protein, lactose and Cu concentrations in milk (P > 0.05) but milk Mo concentration increased (P < 0.05) with each increment in dietary Mo. Cows given the 40Mo diet had a more rapid decline (P < 0.05) in plasma Cu concentration and ceruplasmin (Cp), a principal cupro-enzyme in plasma, activity than cows given the 0Mo and 20Mo diets. Cow plasma Mo concentrations were increased with each increment in dietary Mo (P < 0.05). Reduced milk yield and increased milk Mo levels caused lower (P < 0.05) calf average daily gains, but did not influence calf plasma Cu, Cp or liver Cu concentrations. Key words: Cow, calf, lactation, molybdenum, copper