Impacts of Feeding Monensin to High Producing Dairy Cattle fed a Contemporary California Diet.

This study determined effects of adding sodium monensin, the active ingredient in Rumensin 90TM, to a contemporary California diet fed to high producing early lactation multiparity dairy cattle on dry matter (DM) intake, milk and milk constituent yield, and potential environmental impacts. Two five-week experimental periods were used in a replicated 2 x 2 Latin square on a dairy farm near Hanford (CA, USA). Four ‘high group’ pens (i.e. cows moved from fresh pens but not confirmed pregnant) were used. Rations were formulated identically, except for the mineral premix which contained, or did not contain, 11 mg monensin/kg DM. Milk composition and yield was collected one day prior to each period end. Blood samples were collected from the tail vein of 18 cows 2 days prior to each period end during morning lock-up and analyzed for plasma volatile fatty acids, glucose and amino acids. Milk production was 4.27 kg/cow/day higher (P<0.01) with monensin feeding, with no change in composition. The digestible energy (DE; MJ/kg DM) and net energy (NEL; MJ/kg DM) differed numerically (P≤0.15) suggesting higher DE availability and post absorptive utilization efficiency (i.e. NEL). Production responses appear to have been driven by up regulation of lactose production supported by lower blood plasma concentrations of acetic, butyric, propionic and glutamic acids (P≤0.05), and higher plasma glucose (P=0.10) in cows fed monensin. In contrast, plasma threonine and tryptophan were higher in cows fed monensin (P≤0.05). Blood plasma metabolite levels were likely driven by increased ruminal degradation of organic matter, especially neutral detergent fiber (P<0.05). However, body condition score change, DM intake and microbial crude protein output were not monensin impacted. Incorporation of monensin to diets of lactating dairy cattle in California could maintain milk yield but allow reduction of cow numbers by ~160,000 with an associated estimated reduction of 19.6 M kg methane annually, compared to cows not feed monensin