Pollination, lygus infestation, genotype, and size of plants as affecting seed setting and seed production in alfalfa

Interest in the problems of alfalfa seed production has been revived largely as a result of greatly declining yields in formerly highly productive areas. Present information appears to indicate the predominant importance of four factors, namely, (a) insect pollination and tripping, (b) lygus infestation, (c) varieties and strains of alfalfa and the genotypes of individual plants, and (d) environmental conditions and cultural practices affecting growth and development of plants. Three sites were selected for the investigation as being typical of conditions under which alfalfa seed is produced in Utah and other western states, while clonal lines from six genotypes of known fertility level gave material representative of a wide range in alfalfa types. When insect pollinators were excluded by caging, plants of the various clonal lines produced from 10 to 17 pounds of seed to the acre with self pollination, as compared with 154 and 245 pounds per acre, respectively, with open pollination in comparable adjacent plots at two widely different sites. Similarly, where lygus bugs were a factor, the yield was 66 pounds per acre compared to 175 pounds for nearby plots that were dusted frequently with a partially effective insecticide. Where conditions were favorable for pollination and seed setting, the larger plants produced more seed than did small plants of the same clone, owing in part to the greater abundance of flowers. The relationship between seed yield and size of alfalfa plants, however, is subject to the effects of factors influencing pollination and seed setting. Data show a statistically significant negative correlation between lygus populations of plots receiving different insecticidal treatment and the percentage of tripping. Since tripping affords a highly reliable index of pollinating insect activity under field conditions, low tripping frequency associated with high lygus population suggests the possibility of an unfavorable effect of lygus infestation on the activity of pollinating insects in alfalfa fields. The potential seed setting capacity of individual plants and genotypes of alfalfa seems to be closely associated with response to controlled pollination. The effects of cross pollination in particular appear to afford a good index for a preliminary estimation of seed setting potentials, although self pollination stimulated by artificial tripping seems also to have high predictive value for the reason that tripping is an essential condition for highly efficient pollination. The significance of plant genotype in relation to the production of alfalfa seed is apparent from the yields of clonal progenies ranging from 37 to 441 pounds to the acre.