Characteristics of tall versus short-type varieties of forage sorghum grown under two irrigation levels, for summer and subsequent fall harvests, and digestibility by sheep of their silages

Changes in the response of short (Tal) and tall (FS-5) types of forage sorghum varieties to irrigation level during summer and subsequent fall growth were measured with respect to crop yield, assimilate partitioning, chemical composition and digestibility. The digestibility by sheep of silages made from Tal or FS-5 plants grown for summer and fall harvests was also investigated. During summer cycle, FS-5 plants attained maximal height of 3.2 m and stem diameter of 2–2.1 cm, whereas the upper height of Tal plants was lower (1.7–1.8 m, P<0.05), and their stems were thinner (diameter of 1.7 cm, P<0.05). Within each genotype, plants morphology and chemical composition were not affected by irrigation from 180 to 250 mm. Dry matter (DM) yields of the summer plants ranged from 25.3 to 30.1 t DM/ha and was not affected by genotype or irrigation level. Yield of the fall harvests attained to lower levels of 12.5–14.6 t DM/ha. Crude protein (CP) content was higher in Tal than in FS-5 biomass. During the ensilage process, differences in composition that were found in the green forages, disappeared in the two genotypes, resulting in similar in vitro DM digestibility (0.59–0.61) for both growth cycles. During ensilage most of the soluble carbohydrates and in the case of Tal also part of the solubilized hemicelluloses were converted mainly into lactate, acetate and ethanol, resulting in similar and low pH values (3.7–4.0). High DM recovery during ensilage was found in all the genotypes. Voluntary intake by sheep of DM and NDF from diets based on the fall silages was higher than that of the summer silages however, there was no difference in forage intake between the two genotypes. In vivo DM digestibility by sheep was similar (0.61–0.64) in the two genotypes irrespective of the growth season, and reflected their similarity in NDF (0.51–0.55) and cellulose (0.61–0.64) digestibility. The main difference between genotypes was observed in CP intake and digestibility that were higher (P<0.05) in Tal than in FS-5 silages grown in both cycles.