Technical Abstract: Plant biodiversity theory suggests that increased plant species diversity contributes to the stability of ecosystems. In managed grasslands, such as pastures, greater stability of herbage production would be beneficial. In this retrospective study, I used data from three reports from the 1930s, 1940s, and 1960s to determine whether increasing mixture complexity contributed to yield stability with time. Yield data from a total of 181 mixtures of grasses and legumes (two to seven species per mixture) in experiments of 3 to 6 yr duration in Connecticut, Pennsylvania, and Utah, USA were used in the analysis. Regression was used to examine relationships among herbage yield, stability of yield [measured as the interannual coefficient of variation (CV)], and diversity measures [Shannon diversity index (H), species richness (S), and evenness, (J)]. In several instances there was no relationship between herbage yield or yield stability and the complexity (number of species) of the mixture. In one experiment, the proportion of legume in the sward seemed to be the controlling factor regarding herbage yield and yield stability. Monocultures and binary mixtures frequently had the highest interannual CV (e.g., less yield stability) than more complex mixtures. Within forage mixtures, however, there was no consistent relationship between herbage yield or yield stability and measures of species diversity. Species identity and composition of forage mixtures may be more important determinants of herbage yield than simply the number of species.