Somatotropina bovina recombinante (BST): efeitos de dose, início de aplicação e intervalo de aplicação no desempenho produtivo de vacas mestiças Holandês-Gir

Abstract

The bST effect was evaluated in two experimental units at doses of 0, 250 or 500 mg starting at 40 or 63 days in milk (DIM), injected every 14 days in F1 Holstein-Gir cows, in experimental unit 1, and injected every 12 or 14 days in ¾ and 7/8 Holstein-Gir cow, in experimental unit 2, until 250 days in milk (DIM), on milk yield and composition, body weight and body condition score (BCS), reproductive parameters, SCC and mastitis. The trial was conducted in a randomized block design with 20 animals by group in a 2 x 2 factorial arrangement (two doses, two starting DIM) in experimental unit 1 and 2 x 2 x 2 factorial arrangement (two doses, two starting DIM and two injected intervals) in experimental unit 2, both with an additional treatment control group. Two hundred eighty cows, 100 (15 primíparas and 85 multíparas) F1 Holstein-Gir and 180 (63 primiparous and 117 multiparous) ¾ and 7/8 Holstein-Gir were blocked by calving date, during the period of June to December 2008, totaling 20 blocks and assigned to treatments according to experimental unit. Experimental unit 1: (bST dose mg/DIM to treatment starting) 250/40; 250/63; 500/40; 500/63; Control (no bST). Experimental unit 2: (bST dose mg/DIM to treatment starting/ injected interval): 250/40/14; 250/40/12; 250/63/14; 250/63/12; 500/40/14; 500/40/12; 500/63/14; 500/63/12; Control (no bST). Cows were kept in rotational grazing system during the rainy season and in confinement during the drought, and in both periods were supplemented with a concentrate mixture according to milk yield. Milk yield was recorded twice a week and milk composition recorded biweekly. Body weight and BCS were recorded at the beginning of the trial and every 30 days until the end. Mastitis cases and reproductive performance were measured. In F1 Holstein-Gir cows the bST 250 and 500 mg increased milk yield, FCM 3,5% and ECM only until 130 DIM (P0.05). There is no difference in milk production, FCM 3,5% and ECM between bST 250 or 500 mg treated cows (P>0,05). An average increased of 2.0 and 1.8 kg of milk per cow/day, corresponding to 14.5 and 12.9% was observed, respectively, for the 250 and 500 mg dose, considering 250 DIM. However, in ¾ and 7/8 Holstein-Gir cows the bST 250 mg did not increased milk yield, FCM 3.5% and ECM (P>0.05).The bST 500 mg increased milk yield, FCM 3.5% and ECM until 220 DIM (P0.05). An average increased of 1.4 kg of milk per cow/day, corresponding to 7.3% increase in milk yield was observed for the 500 mg dose. In both experimental units, treatment with bST did not affect milk composition, SCC, body weight and BCS and no difference was observed between the bST-treated groups (P>0.05). The production of fat, protein, lactose and total solids (kg/d) increased in the bST-treated animals compared to the control group (P0.05). In F1 Holstein-Gir cows there was an interaction between dose x starting DIM on milk yield, FCM 3,5%, ECM, fat, protein, lactose and total solids (P0.05). The use of 250 mg dose starting at 40 DIM or 500 mg dose starting at 63 DIM resulted in better lactation performance, respectively, 16.5 e 16.2 for milk yield. However, in ¾ and 7/8 Holstein-Gir cows there was a interaction between dose x injected interval on milk yield, FCM 3.5%, ECM, fat, protein, lactose, and total solids (P0.05), with better productive performance for bST 500 mg dose injected either 12 or 14 days interval (20.8 and 21.7 kg of milk yield, respectively), and bST 250 mg only injected with 12 days interval (20.4 kg of milk). There was no difference on mastitis incidence, between cows treated and not treated with bST (P>0.05) in both experimental units, however, the interaction between dose x starting DIM was detected for bST treated cows (P0.05) in experimental unit 2, with higher mastitis incidence for cows starting at 40 DIM treated with 500 mg of bST (43,6%) than for cows receiving 250 mg bST (20,5%) starting at same period. Relative to reproductive parameters, there was a difference on bST response behavior between the experimental units. Related to experimental unit 1, there were no differences on days open, services per conception, days from calving to first service and conception rate to first service, between cows treated and not treated with bST and between treated cows (P>0.05). However, in experimental unit 2, days open and services per conception was higher in cows treated with 500 mg of bST compared to control group and cows receiving 250 mg of bST (P0.05), but there was no difference on conception rate to first service between cows treated and not treated with bST (P>0.05) and between cows treated with bST 250 or 500 mg (P>0,05).