Use of monensin in lactating crossbred dairy cows (Holstein × Gyr) raised on tropical pastures with concentrate supplementation

Monensin is a feed additive produced by a strain of Streptomyces cinnamonensis that has been used to improve milk production and feed efficiency by a mechanism linked to rumen fermentation of lactating dairy cows. Evaluation on performance, digestibility and blood parameters of crossbred dairy cows supplemented with monensin is still scarce. Thus, the objective of this study was to determine the effects of monensin supplementation on intake, digestibility of nutrients, lactational performance and blood parameters in crossbred dairy cows grazing tropical pasture subjected to rotational stocking and supplemented with concentrate. Sixteen crossbred Holstein-Gyr cows (initial milk yield = 20 kg/d; body weight = 505.5 kg ± 47.64) were blocked according to stage of lactation and milk production, and assigned to two treatments in a crossover design. The treatments were (1) control diet (pasture and concentrate) or (2) control diet plus 300 mg per cow per day of sodium monensin. There were 24 paddocks used of 1750 m2 each (total of 42,000 m2) with Tanzania Guinea grass (Panicum maximum), irrigated and managed under intermittent stocking systems. Concentrate supplementation was offered after milking in individual stalls (12.5 m2). The first 18 days were considered an adaptation period, followed by 10 days of the sampling period of milk, feces, blood, pasture and concentrate. The intakes of dry matter (DM), crude protein (CP), neutral detergent fiber (NDFom) and total digestible nutrients (TDN), and concentrate as well as the digestibility of CP were similar between treatments. Monensin supplementation increased the coefficient of total tract apparent digestibility (CTTAD) of DM (P = 0.03) and NDFom of crossbred dairy cows. Blood parameters such as glucose and urea were similar among the two groups. Monensin did not affect milk yield and 3.5% fat-corrected milk, but fat and total solids contents were reduced in cows fed ionophore. There was a tendency for the beta-hydroxybutyrate (BHBA) level to be lower, while the nonesterified fatty acid (NEFA) concentration was reduced in dairy cows supplemented with feed monensin additive. These results demonstrate that monensin may increase the nutrient digestibility and fiber digestion. Furthermore, ionophore use could be beneficial to reduce fat mobilization as the ionophore tended to reduce BHBA with lower NEFA levels. Therefore, monensin has the potential to be used as an additive for crossbred dairy cows grazing on Tanzania-grass pasture (Panicum maximum) under tropical conditions.