EEA Manfredi, Cordoba, Argentina

Partially financed by PRODUSEM, Pergamino, Argentina

Bloat is a very serious problem for Argentine beef and dairy production. In 1991, INTA initiated a breeding program to select nondormant, bloat-tolerant alfalfa cultivars through cycles of phenotypic recurrent selection using the modified nylon bag technique. The initial results of this program, including the production of Cycle 1 seed (C-I) were published in a previous report (1).

During 1995/96, 1,195 C-I plants were evaluated for in situ initial rate of dry matter disappearance (IRDMD) after 4 h in the rumen of fistulated steers. While the average IRDMD for the spring (November) evaluation was 58.05%, the mean value for the fall (April) evaluation was 59.30%. The general average for the C-I plants was 58.67%. Ninety-eight genotypes that consistently exhibited lower IRDMD values were cloned and intercrossed in the greenhouse during July-August 1996. Harvested seed conformed the Cycle 2 (C-II) seed.

In March 1997, 1,300 C-II plants were transplanted to the field to be evaluated twice for their IRDMD during the 1997/98 season. The spring (October) and fall (March) evaluations had mean values for IRDMD of 51.39% and 48.22%, respectively. General average for the C-II plants was 49.81%. In May 1998, 148 C-II plants that consistently exhibited lower IRDMD values were selected and cloned in the greenhouse. They will be interpollinated during July-August 1998 in order to produce Cycle 3 (C-III) seed.

Figure 1. Response to selection for lower IRDMD

Based of general averages, there has been a 14.61% reduction in the IRDMD between C-0 and C-II populations (Figure 1). We conclude that selection has been effective in reducing alfalfa in situ IRDMD.

1. Basigalup, D.H., C.V. Castell & C. D. Giaveno. 1996. Breeding a bloat-tolerant alfalfa in Argentina. Report 35th NAAIC, Oklahoma City, OK, July 16-20, p. 31.