Selection for reduced neutral detergent fiber (NDF) concentration has been used as a mechanism of improving intake potential of perennial grasses by ruminant livestock. However, reduced NDF concentration is typically associated with reduced forage yield, although the reasons for this genetic correlation are unknown. The objectives of this study were to determine the relative contributions of pleiotropy, linkage, and drift to the genetic correlation of NDF concentration with four agricultural fitness traits: forage yield, survival, seed yield, and lodging. These traits were measured on four smooth bromegrass (Bromus inermis Leyss.) populations that had undergone one cycle of divergent selection for NDF concentration. Selection responses for forage yield were linear and homogeneous, suggesting pleiotropic effects. Growth and expansion of the cell wall appears to be essential for accumulation of forage yield, lending a certain allometry to these two traits. However, natural selection within swards appeared to regulate this response, because forage yield responses disappeared by the third production year. Selection responses for survival and lodging were linear, but nonhomogeneous, suggesting linkage. Selection responses for seed yield appeared to be regulated by all three phenomena, with drift (asymmetry) the most important. Seed yield is highly sensitive to inbreeding depression, which occurs as a result of drift. Short-term divergent selection experiments, analyzed by a factorial ANOVA model, provide a mechanism to identify genetic phenomena responsible for observed genetic correlations.
