Nutritional risk factors in Setaria anceps for milk fever incidence in dairy herds of the humid tropics1

Setaria anceps is a pasture grass commonly utilized in dairy farms in the humid tropics. It is noted for its good dry matter yield and nutritional quality. However, a high incidence of milk fever among dairy cows grazing this pasture has been claimed by the Costa Rican farmers. We therefore collected and analyzed 48 samples of S. anceps in the Quesada district of San Carlos county in Costa Rica to determine the mineral composition and potential milk fever risk factors associated with consumption of this pasture. An equal number of samples of the pasture Cynodon nlemfuensis were taken as a control. This is the most common pasture of the area and a low incidence of milk fever has been related to its consumption. Forage samples were measured for Ca, P, Mg, K, Na, S and Cl and the Cation–Anion Balance (CAB, mEq kg−1 of DM) was calculated as [(Na++K+)−(S2−+Cl)]. Values for Ca, P, Mg, K, Na, S and Cl in S. anceps were, 0.25, 0.19, 0.24, 3.5, 0.55, 0.17 and 0.84% of dry matter and for C. nlemfuensis 0.31, 0.22, 0.24, 2.67, 0.04, 0.27 and 0.59. CAB was 861 mEq kg−1 of DM in S. anceps and 500 in C. nlemfuensis. The significantly (**P<0.01) higher CAB, K and Na values, and lower (**P<0.01) S values in S. anceps strongly suggest that its electrolyte composition and balance could be key nutritional risk factors increasing the susceptibility of dairy cows to milk fever incidence. Since average calcium consumption in the area is in the order of 45 g cow−1 day−1, cows grazing S. anceps should be offered low calcium diets (less 20 g cow−1 day−1) to reduce their susceptibility to milk fever.