Effects of phosphorus and potassium on growth, yield and fodder quality of IS 23585 forage sorghum cultivar (Sorghum bicolor L. Moench).

This growth analysis experiment was carried out at Khon Kaen University Experimental Farm, Khon Kaen, Northeast Thailand in the 2000, in order to justify the effects of phosphorus (P) and potassium (K) levels added to Yasothon soil series (Oxic Paleustults) on dry matter yield and fodder quality of IS 23585 forage sorghum cultivar. The experiment was laid in a 4x4 factorial arranged in a randomized complete block design (RCBD) with four replications. The P levels used were 0, 37.50, 75.00 and 150.00 kg P2O5 ha(-1) and K levels were 0, 56.25, 112.50 and 225.00 kg K2O ha(-1). The results showed that total dry weight (yield) and other growth parameters in most sampling periods significantly increased with an increase in K levels but the increase was only up to K1. Higher levels of both P and K had no significant effect on total dry weight (yield) and other growth. There were no consistent trends due to both P and K application rates. However, mean Leaf Area Index (LAI) reached a maximum value of 10.21 (P3K3) at 8 weeks after emergence (WAE) and then a decline to 5.37 (P0K3) at 10 WAE. K significantly affected LAI only at 2 and 6 WAE. Crop growth rate (CGR) and leaf area duration (D) significantly increased up to K1, whilst P did not. Both optimum total dry weight of 1,7221 kg ha(-1) (10 WAE) and seed yields of 5,169 kg ha(-1) were obtained from P0K1. Both P and K had no significant effect on 1000-seed weight. P did not affect brix value, whilst K significantly depressed this parameter. K had no significant effect on crude protein (CP), whilst P did. P had no significant effect on neutral detergent fibre (NDF), whilst K significantly did it. Both P and K had no significant effect on both acid detergent fibre (ADF) and ruminal dry matter degradability (DMD) of the sorghum plants.