Effect of Sustained-Release Somatotropin on Performance and Grazing Behavior of Ewes Housed at Different Stocking Rates

This study evaluated the effects of recombinantbovine somatotropin (bST; one injection of 320 mgper ewe) on milk production and composition and onthe grazing behavior of multiparous ewes in the thirdto fourth lactation. Forty Comisana lactating eweswere divided into four groups: 1) untreated, grazingon natural pasture (botanical composition: 35% ofGraminaceae, 49% of Fabaceae, 6% of Cruciferae,10% of other families) at a low stocking rate (16 m2/d); 2) untreated, grazing at a high stocking rate (8m2/d); 3) treated with bST, grazing at a low stockingrate; and 4) treated with bST, grazing at a highstocking rate. The diets of the ewes were supplementedwith vetch and oat hay (500 g/d) and withconcentrate (500 g/d). Treatment increased milkproduction (923.8 vs. 669.5 g/d) but had little effecton fat and protein contents. Administration of bSTsignificantly increased herbage intake; the effect onintake was more marked at the high stocking rate.Under these grazing conditions, the treated ewesreduced selective intake behavior and, thus, achievedgood feed intake despite the low biomass availability.