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Forage Yield and Yield Component Response to Divergent Selection for in Vitro Dry Matter Digestibility in Smooth Bromegrass¹

Forage yield and quality tend to be negatively correlated with each other. Therefore, in selecting for improved forage quality, it is of interest to evaluate correlated selection responses for forage yield. The objective of this study was to evaluate forage yield, tiller density, and tiller dry weight of first generation smooth bromegrass (Bromus Inermis Leyss.) synthetics selected for high (B8HD) and low (B8LD) in vitro dry matter digestibility (IVDMD) and their parent population (B8). Each population was seeded in seven randomized complete blocks at Arlington, WI in 1981. Samples were taken at weekly intervals, 14 times throughout each of the 1982 and 1983 growing seasons for determination of forage yield, tiller density and tiller dry weight. Large-plot yield trials were established in 1982 at Arlington, Marshfleld, and Spooner, and harvested in 1983 and 1984. Tiller dry weight increased during each growth period, while tiller density remained rather constant after June 1982. Forage yield of small-plot samples was reduced Jl 1.5% in B8HD and 5.5% in B8LD for a normal two-cut management. These reductions were almost entirely due to reductions in tiller dry weight, suggesting possible inbreeding depression for tiller dry weight. Conversely, in large-plot trials there was no evidence of selection response or inbreeding depression. The lack of agreement between large-plot and small-plot yield estimates could be due to population x environment interactions and/or inherent differences in sampling techniques. It was concluded that forage yield was not significantly altered by selection for IVDMD. Future evaluation of forage yield should be based on the more accurate large plot determinations solely or as a supplement to smallplot yield component determinations.

Key Words: Bromus inermis Leyss. • Correlated selection response • Inbreeding depression • Tiller density • Tiller dry weight

² Associate professor, Univ. of Wisconsin-Madison, and former graduate research assistant (currently, graduate research assistant, Dep. of Agronomy, the Pennsylvania State Univ., University Park, PA 16802).

Received for publication June 14, 1985.