PRODUCTIVE RESPONSE OF LACTATING DUAL-PURPOSE COWS GRAZING IN AN AGROSILVOPASTORAL SYSTEM DURING THE DRY SEASON SUPPLEMENTED WITH LOW LEVELS OF CRUDE PROTEIN

Background. Cattle production in tropical regions of Mexico is in dual-purpose farms (DP) on unintended agrosilvopastoral systems (ASPS) with pastures dominated by tropical grasses with scattered shrubs and trees. During the dry season due to the forage diminished nutritional value and availability most farmers supplement their cattle to sustain milk production and weight gains of calves, without taking into consideration the wide variety of forages available in grazing lands to decide the composition of supplements offered to their cattle. Objective/hypothesis. The objective of the study was to determine the effect of low levels of crude protein (CP) content in supplements (S10 = 100, S11 = 110 and S12 = 120 g CP/kg of DM) on the performance of lactating Brown Swiss (BS) cows on an ASPS during the dry season. We hypothesize that low levels of CP in supplements of lactating grazing cows on an agrosilvopastoral system during the dry season would not affect animal performance. Methodology. The study was carried out in the dry season (March to June of 2012), in a commercial DP in the State of Mexico. Eighteen multiparous BS cows and their calves were used in the study, randomly allocated to three groups (six cows/group), to receive one of three supplements (4.5 kg DM/cow/day), consisting of 100, 110 or 120 g/kg DM of crude protein (CP) S10, S11 and S12, respectively. Data were analysed as a complete random experimental design with a mixed model. Results. There were no significant differences in most of the animal performance variables (P > 0.05), except for fat protein corrected milk (FPCM) where S12 (6.4) was significantly higher than S10 and S11 (4.7 and 4.5 kg/day, respectively). Milk protein yields (kg/day) significantly increased as the CP level increased in supplements. High levels of milk urea nitrogen (MUN) were detected (mean 14.0 mg/dL). Implications. In similar DP farms where cattle have access to other sources of forages like shrubs and trees, it is possible to reduce the CP content of supplements and to increase the efficiency of utilization of those alternative forages as well as a reduction in supplementation costs. Conclusion. There were no significant differences in the performance of BS cows grazing on an ASPS during the dry season receiving supplements with low crude protein levels. When correcting to fat-protein milk yields a significant difference was found in favour of supplement with 120 g CP/kg of DM.