Submitted to: Forage and Grazinglands
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/17/2007
Publication Date: 3/13/2008
Citation: Brink, G.E., Hall, M., Mertens, D.R., Casler, M.D. 2008. Grass Yield and Quality Affect Potential Stocking Rate and Milk Production. Forage and Grazinglands. Available: http://www.plantmanagementnetwork.org/sub/fg/research/2008/milk/. Interpretive Summary: Many cool-season grasses are available to dairy producers who utilize pastures as a primary source of feed. Quality of grasses, such as protein concentration and digestibility, is generally described as the average of the whole plant, but grazing cattle usually do not consume the entire plant. Upper layers of the grass canopy are often consumed first, and lower portions later. How the quality changes from the top to the bottom of the canopy may influence how much of the canopy is consumed and the benefit it imparts to the animal. When measured at a height representing typical grazing management (10 inches), we found that forage quality of grasses varied from the top to the bottom of the canopy depending on the growth habit (bunch vs. spreading, open vs. dense) and the season of the year (spring, summer, or fall). Protein concentration always declined from the top to the bottom for all grass types. Digestibility often followed the same pattern except in grasses having a dense, bunch-type growth habit (meadow and tall fescues), which often had similar digestibility throughout the canopy. The results suggest that to optimize utilization, pastures should consist of mixtures of grasses possessing similar growth habit.
Technical Abstract: Quality changes within grass canopies may influence pasture utilization and performance of grazing animals. Our objective was to determine changes in crude protein (CP) and neutral detergent fiber digestibility (NDFD) within vertical layers of diverse temperate grass swards. Grasses were assigned to a canopy type based on their growth habit: bunch growth with an open canopy (BO), bunch growth with a dense canopy (BD), rhizomatous growth with an open canopy (RO), and rhizomatous growth with a dense canopy (RD). The study was conducted in 2004 and 2005 on a Loyal silt loam (fine-loamy, mixed, superactive, frigid Oxyaquic Glossudalf). When mean canopy height of each grass reached approximately 10 in. during the spring, summer, and fall, the sward was harvested by vertical layers (4 to 6 in., 6 to 8 in., 8 to 10 in.) and each layer was analyzed for CP and NDFD. In the spring, summer, and fall, CP of all canopy types declined from the top to the bottom of the canopy. The NDFD of BO and RO types also declined from the canopy top to the bottom in the spring, but the reverse was true in BD and RD types. In the summer, NDFD of grasses with a bunch growth habit (BD and BO) was similar across all layers, but declined from the top to the bottom in grasses with a rhizomatous growth habit (RD and RO). In the fall, NDFD of the middle and upper layers was greater than that of the lower layer in all grass types, except the BD type, where NDFD of the upper layer was lowest. Our results suggest that at equivalent heights, forage quality differences within the canopies of temperate grasses are influenced by growth habit, and that to optimize utilization of pastures containing mixtures, grasses of similar architecture should be planted together.