Effects of cutting management on growth and regrowth after cutting, and digestibility of a sorghum-sudangrass hybrid cultivar, sudax sx-11

There is a recognized need for high quality forage during the mid to late summer in the southeastern United States. The introduction of sorghum sudangrass hybrids has given the producer another group of summer annual grasses which have a high yield potential. There is inadequate information on the effects that management and environment have on factors influencing forage quality of these newer summer annual grasses. Experiments were begun in May of 1967 to study the growth rate, rate of regrowth after cutting and digestibility of a sorghum sudangrass hybrid cultivar, Sudax SX-11. Two field experiments were conducted on the University of Tennessee Plant Science Farm, Knoxville, Tennessee, one in 1967 and another in 1968. Two small sample nylon bag digestion experiments were used, one to determine the digestibility of harvested whole plants and another to study the digestibility of component parts of the plants. Dry matter production curves were constructed by harvesting appropriate treatments at weekly intervals. The percent leaf, stem and inflorescence in the plant was determined on a dry matter basis each v:eek, along with plant height and the number of stems per plant. It was found that the growth rate, expressed as either cumulative dry matter production or plant height, decreased with the number of uniform cuttings and as the season progressed. This decrease due to the number of uniform cuttings was greater when plants were older at the time of the uniform cutting. Total dry matter production and digestible dry matter production per hectare was greater for the more mature management, but the quality of the forage would probably have been lower due to lower animal voluntary intake. The percentages of leaves, stems, and inflorescences on a dry matter basis were related more to plant height than to time after cutting or to management practices. The percent digestible dry matter was related more to plant height and morphological composition, than to time after cutting or to management practices. The percent digestible dry matter generally decreased from approximately 90% for the first week after cutting when the plants were 59 cm tall to less than 60% six to seven weeks after cutting when the plants were 250 cm tall. Since the digestibility of the stem decreased more rapidly with maturity than the leaf or inflorescence fractions, the decrease in digestibility of Sudax SX-11 plants with maturity may be due to lower digestibility of the stem fraction of the plant.