STUDIES ON FERTILIZER NITROGEN AND CUTTING MANAGEMENT OF SOWN GRASS AND LEGUME PASTURES IN PURE AND MIXED SWARDS

Three field experiments were conducted at the Teaching and Research Farm of the Department of Crop Science, Faculty of Agriculture, University of Nigeria, Nsukka. In the first experiment, the effects of fertilizer-N application and cutting management on establishment, growth and yield of guinea grass (Panicum maximum) were investigated. The experiment was a 4 x 4 factorial laid out in a randomized complete block design with three replications. Treatments comprised four levels of Nitrogen of 0, 150, 300, and 450 kg N ha-1 and four harvesting frequencies of 3-, 6-, 9- and 12-weekly intervals. A second experiment was conducted using guinea grass/verano stylo (Stylosanthes hamata) mixed swards. The experiment was a 3 x 3 x 2 factorial laid out in a randomized complete block design with three replications. Treatments comprised three swards types (pure grass, pure legume and grass-legume mixed swards), three harvesting frequencies of 4-, 8-, and 12-weekly intervals and two levels of nitrogen at 0, and 300 kg N ha-1. The third experiment was conducted to evaluate the effects of two cutting frequencies (4- and 8-weekly intervals) on the growth and yield of four legumes (Lablab purpureus, Stylosanthes hamata, Centrosema pascuorum and Stylosanthes guyanensis) and four grasses (Sorghum almum, Panicum maximum, Chloris gayana and Andropogon gayanus). Basal applications of muriate of potash (75 kg K ha-1) and single superphosphate (44 kg P ha-1) were also made by broadcasting in all the experiments. In the first experiment, tiller number per square metre, plant height and grass dry matter yield were increased by 44%, 6% and 53%, respectively, during the establishment year when fertilizer-N rate was increased from 0 to 450 kg N ha-1. Grass dry matter yield was increased by 41% and 149% when the interval between cuts was increased from 3- to 12-weekly intervals at the first and fourth harvest years, respectively. Plots cut at 3- or 6- weeks intervals exhibited much greater weed proportion than those cut 9-, or 12-weekly intervals. Increasing the interval between harvests from 3 to 12 weeks increased the proportion of stem relative to leaf blade in the grass swards. The proportion of inflorescence, plant height and dry matter percentages, were generally increased by increasing the interval between cuts. Tiller number decreased with increasing interval of cut. The total annual herbage dry matter yield was increased by 32% and 48% in 2001 and 2004, respectively, when fertilizer-N was increased from 0 to 450 kg N ha-1. Increasing the N rate from 0 to 450 kg N ha-1 increased the proportion of stem relative to the leaf blade in the grass swards. Plant height and tiller number were increased with increase in N application while the dry matter percentage was generally decreased with incremental application of nitrogen. A combination of 12-weekly interval of cut with 300 kg N ha-1 gave a significantly higher forage grass herbage dry matter yield compared with the values got from the 3- or 6-weekly interval combined with any N rate.  Fertilizing at 300 or 450 kg N ha-1 when combined with cutting at 9 or 12-weekly interval between cuts was more effective in suppressing weed growth and weed dry matter as a proportion of the total herbage dry matter. The N % and crude protein contents in the leaf, stem and in total herbage dry matter decreased significantly with increasing interval between cuts. Yield of crude protein in the total herbage was significantly higher when cutting was done every 6 weeks compared with when done every 9 weeks. Nitrogen fertilizer x cutting frequency interaction effects on N %, crude protein contents and crude protein yields in the crop fractions and in total herbage dry matter were not statistically significant. On the average, the crude protein and mineral contents were higher in the leaf fraction than in the stem and inflorescence fractions. In the second experiment, herbage production was 16% and 69% higher on guinea grass/stylo swards in 2006 and 2007, respectively, compared with the yields for the pure guinea grass swards. Grass dry matter yield as a proportion of the total herbage dry matter was significantly increased from 45% to 61% in 2007 with 12-weekly cutting interval compared with 4-weekly interval. The weed proportion was significantly reduced from 28% to 19% with the infrequent cutting interval of 8-weeks compared with the 4-weeks in 2007. Cutting frequency did not influence the proportion of verano stylo in 2006 and 2007. However, the legume dry matter yield was generally increased with longer intervals between cuts. The effects of fertilizer-N rates on the proportions of grass, legume and weed in 2006 and 2007, were not statistically significant. The total herbage dry matter yield was significantly higher in guinea grass/stylo swards when cutting was done at 8-weekly interval compared with sole guinea grass or verano stylo at any interval between cuts. Crude protein yield of the guinea grass/stylo sward was higher than that of pure guinea grass sward by 8 %. In the third experiment, Stylosanthes guyanensis had the highest dry matter yield among the four legumes in 2007 when 8 weeks interval between cuts was employed. Among the four grasses, Chloris gayana produced significantly greater dry matter yield than Sorghum almum but had a similar yield value with Panicum maximum or with Andropogon gayanus in 2006. Andropogon gayanus yielded more than Chloris gayana but had a similar yield value with Sorghum almum or with Panicum maximum in the first harvest year. Cutting interval did not influence the extent of legume cover. However, the extent of grass cover was increased by 30% when the interval between cuts was increased from 4 to 8 weeks. Weed cover in the legume plots was not influenced by cutting management. However, weed cover in the grass plots was significantly depressed by 21% with increased interval of cut from 4 to 8 weeks. The dry matter percentages among the grasses and the legumes were generally increased by increasing the interval between cuts. In 2006, Stylosanthes hamata had significantly higher dry matter content among the legumes than Lablab purpureus and Stylosanthes guyanensis but did not differ with Centrosema pascuorum. In 2007, Lablab purpureum had significantly the least percentage dry matter compared with the other legumes. Stylosathes hamata had higher percentage dry matter than Centrosema pascuorum but did not differ with Stylosanthes guyanensis. Grass dry matter content did not vary among the grass species in 2006. In 2007, Sorghum almum had higher (P<0.05) dry matter content than Chloris gayana but had similar value with Panicum maximum or with Andropogon gayanus.