Sorghum and black oat forage production and its nutritive value under phosphate levels

Phosphorus (P) is one of the most limiting mineral elements for biomass and grain production in tropical soils. This study was undertaken to assess the influence of P on herbage accumulation (DM) and the nutritive value of forage sorghum (Sorghum bicolor) and black oat (Avena strigosa) in succession. Evaluated treatments were P fertilization levels of 0, 50, 100, 150 and 200 kg of P2O5 ha-1 distributed in a randomized complete block design with three replicates. The treatments were applied at sorghum seeding in the summer 2010/2011 and 2011/2012. Black oat was seeded following sorghum in 2011 with no additional P fertilization. Herbage production and its nutritive value were assessed by successive cuts. The greatest sorghum DM yields were obtained at the highest phosphate level tested (200 kg P2O5 ha-1), with residual response in subsequent black oat. There was no effect of P fertilization levels on the nutritive values of both crops, considering crude protein (CP) levels, in vitro dry matter digestibility (IVDMD), neutral detergent fiber (NDF) and acid detergent fiber (ADF), what demonstrates that P addition has no effect in forage nutritive value, especially when the soil P levels are classified as medium or high. The plant P recovery efficiency decreased when increasing P fertilization levels for both sorghum and black oat. The level of 50 kg P2O5 ha-1 year-1 presented the greatest P recovery by plants, which supports the idea of less fertilizer use with more efficiency.