Leaf nutritive value related to tiller development in warm-season grasses

Assessing nutritive value of key grass species in relation to plant development is essential for producers to efficiently manage livestock enterprises. Changes in nutritive value for tiller populations of 2 common Nebraska Sandhills grasses, prairie sandreed [Calamovilfa longifolia (Hook.) Scribn.] and sand bluestem [Andropogon gerardii var. paucipilus (Nash) Fern.], in response to morphological development was evaluated at the Gudmundsen Sandhills Laboratory (GSL) during the 1990 and 1991 growing seasons. Morphological development was determined on a 40 to 60-tiller sample from each block (12 blocks in 1990 and 8 blocks in 1991) at ten-day intervals using a comprehensive staging system. In vitro dry matter digestibility (IVDMD), crude protein (CP), neutral detergent fiber (NDF) and lignin were determined for leaves and correlated with the morphological index (Mean Stage by Count), growing degree days and day of the year. Leaf NDF values of both species remained constant while leaf IVDMD declined throughout the summer indicating that decline in leaf IVDMD was caused by declining cell wall digestibility. Leaf IVDMD was influenced more by tissue aging than advancing morphological stage. Leaf CP was significantly different between years but not between species indicating leaf CP was largely influenced by environmental factors. In both species and for both years, leaf CP initially declined rapidly to low levels and then stabilized during the vegetative phase. Nutritive value of a single vegetative morphological stage over the growing season was similar to the leaf tissue of the tiller populations. Management decisions by producers depend on accurate assessment of changes in nutritive value during the growing season in tiller populations of these 2 important grasses