A general and specific combining ability study was done on the progeny of a six clone alfalfa diallel cross. This study was done to determine the gene action controlling forage yjeld and resistance to Meloidogyne spp. during the cool season in central Chile.
The genotypes herein considered were selected from the open-pollinated variety Moapa on the basis of their resistance to nematodes and their ability to transfer this trait to their progeny.
For this study, Griffing's Method 3, Mojel 1 was used on both: the yield determination in the field and the reaction to Meloidogyne spp. under greenhouse conditions. Both experiments were organized in a Rectangular Lattice design, including three replications.
The mean squares values obtained in the analysis for GCA and SCA suggested that forage yield in the Fall and Winter was controlled by additive gene action. Also this same character was found to be controlled by both additive and non-additive gene action in early spring.
This study also yielded large differences between clones for GCA main effects. One clone had high positive values throughout this study, while three other clones had negative GCA values and signiucantly inferior performance. Still another genotype demonstrated a high GCA main effect only for the first two cuttings, and the remaining clone had negative values for the first two cuttings but the values were positive for middle winter and early spring crops.
The determination of SCA main efects indicated a superior two clone combination in three out of four euttings. One of the parental clones also had an outstanding GCA main effect for forage yield, while the other had a high GCA main effect for nematode resistance.
Regarding Meloidogyne spp. the resistan ce offered by these genotypes was controlled by additive gene action. Also no specific effects were detected, significant reciprocal differences resulted, suggesting either maternal effects or sorne selfing occurr

1 Ingeniero Agrónomo, Proyecto Leguminosas, Estación Experimental La Platina, Instituto de Investigaciones Agropecuarias.
2 Ingeniero Agrónomo M. S., Proyecto de Riego y Producción Animal, Estación Experimental La Platina, Instituto de Investigaciones Agropecuarias.