EFFECT OF TWO GRAZING PERIODS ON THE PERFORMANCE OF BULLS USING A SEMICONFINED PRODUCTION SYSTEM. LOS SANTOS, PANAMA.

Abstract

The effect of two grazing periods on the performance of bulls was evaluated using a semiconfined production system during the raining season. A complete randomized experimental design was used, with two treatments: diurnal (6:00-10:00 am) (TPD) and nocturnal grazing period (6:00 pm-6:00 am) (TPN), 4 and 12 hours in the pasture, respectively. The semiconfined production system consisted a limited grazing in Brachiaria decumbens and Digitaria decumbens/Bothriocloa pertusa mixture and confined in a cattle yard during the rest of the day with access to a supplement. Pastures were management using an 18/18 day grazing system, with an initial stocking rate of 3.5 AU/ha and N fertilization (20 kg/ha). Twelve Zebu bulls were used per treatment, with an initial body weight of 363 kg/animal. They were implanted with zeranol only at the beginning of the trial. The supplement was composed by molasses, urea and fish meal, offered to the animals in a maximum amount of 1.80, 0.015 and 0.130 kg, as fed/100 kg of body weight/day, respectively, plus a mineral supplement (0.040 kg/ animal/day). In the adaptation (20 days) an evaluation phase (92 days) the supplement intake was 40.2 and 32.0% lower with TPN than with TPD, respectively. It was related to a higher forage intake as a result of a longer time in pasture. In both treatments, pasture dry matter yield (DMY) decreased as the experimental periods advanced but the stocking rate (SR) and grazing pressure (GP) increased. They presented the following average values: 2119.0 (± 548.0) and 1525.0 (± 391.0) kg MS/ha, 4.32 (± 0.34) and 4.13 (± 0.36) UA/ha, and 3.65 (± 1.19) y 2.95 (± 1.95) kg MS/ 100 kg of body weigh/day to TPD and TPN, respectively. According to intake estimations, theses changes in DMY, SR and GP produced a decrease in the forage intake and an increase in the supplement intake, mainly with TPN. The total DM intake (DMI) of both treatments were estimated between 2.5 and 2.6 kg/100 kg of body weigh/day. The consumption of forage was estimated in 55-60 and 38-42% of DMI for TPD and TPN, respectively. The crude protein level of B. decumbens and D. decumbens/B. pertusa ranged from 6.08 to 8.32; and 10.18 to 11.59%, respectively. Average daily gain was not different between treatments (P>0.34) neither dressing percentage (P>0.36), with  average values of 1.065 kg/animal-day and 56.2%, respectively. TPN presented a lower feed cost than TPD, 31.1%. It concluded than the nocturnal grazing gave a better response han the diurnal grazing since produced a lower supplement intake and feed cost and a higher economical profit than the diurnal grazing.