Biomass and nutritive value of forage soybean lines [Glycine max L. (Merr.)] in northwestern Puerto Rico

Abstract

Ten lines of forage soybean [Glycine max L. (Merr.)] were evaluated using two planting dates (PD) in September and January; and two crop harvest physiological stages (CHPS), R2 (full bloom) and R5.4 (most pods 51 to 75% full), a total area of 496 m2 with an Oxisol soil, under conditions of temperature and precipitation not limiting crop growth. The experimental design was one of split plots in randomized complete blocks, PD constituting the complete plots, CHPS the subplots, and soybean lines the sub-subplots. Dry matter yields (DMY) were higher (P menor que 0.01) for R5.4 than R2 (6,452 vs. 5,436 kg/ha), fluctuated (P menor que 0.058) from 5,300 to 6,573 kg/ha among the soybean lines and did not differ between the two PD. Plant populations were about 260,000 kg/ha and were not affected by the three principal factors. Plant height varied (P menor que 0.01) from 84.2 to 93.2 cm among the soybean lines; differed (P menor que 0.05) between the two PD (89.2 cm, September vs. 87.5 cm, January) and (P menor que 0.01) between the two CHPS (72.4 cm, R2 vs. 104.3 cm, R5.4); and was also affected by all of the double and the triple interactions of the three factors. The proportion of leaf in the total DM favored (P menor que 0.01) R2 over R5.4 (44.1 vs. 41.1%) and involved an interaction (P menor que 0.05) of CHPS x soybean line. Regarding the chemical composition of the forage, crude protein (CP) varied among the soybean lines (P menor que 0.05) and between the CHPS (P menor que 0.01) and was affected (P menor que 0.01) by an interaction of these two factors (29.6 to 33.6, R2 and 23.1 to 26.7, R5.4). Acid detergent fiber (ADF) behaved like a CP with respect to significance of the independent variables (21.0 to 25.6, R2 and 26.2 to 33.3, R5.4); whereas neutral detergent fiber (NDF) did not differ among soybean lines, but was higher (P menor que 0.01) for R5.4 than for R2 (41.0 vs. 33.5), and involved an interaction (P menor que 0.01) of CHPS x soybean line. It is concluded that, according to both agronomic characteristics and chemical composition, all the soybean lines evaluated performed adequately, but SF-6, SF-24, SF-50, SF-88 and SF-110 outperformed SF-1, SF-2, SF-22, SF-57 and SF-72; harvest at the R5.4 stage was advantageous in achieving greater DMY without unduly sacrificing the excellent chemical composition of R2 forage; and the magnitude of PD effects was not large.