DIFFERENCES IN TOLERANCE TO INFERTILE ACID SOIL STRESS AMONG GERMPLASM ACCESSIONS AND GENETIC RECOMBINANTS OF THE TROPICAL FORAGE GRASS GENUS, BRACHIARIA

Several Brachial-la species are the most widely grown forages in tropi cal America. A field study was conducted during two seasons (1994, wet and 1995, dry) in a medium-textured Oxisol at Carimagua, Colombia. Th e main objective of the study was to evaluate genotypic variation in p lant attributes for tolerance to low fertility, acid soil stress condi tions (pH 4.9) among germplasm accessions and genetic recombinants of Brachiaria. The entries included 43 genetic recombinants selected from a breeding population, four parental accessions and an additional eig ht germplasm accessions. Small amounts of fertilizer were applied at e stablishment (kg ha(-1): 40 N, 20 P, 20 K, 14 Ca, 12 Mg and 12 S). A n umber of plant attributes including forage yield, leaf area, shoot nut rient composition and shoot nutrient uptake were measured during both seasons. Significant genetic variation was observed in several plant a ttributes such as leaf area, shoot nutrient content, nutrient partitio ning to leaves, shoot nutrient uptake, and forage yield in both season s. Forage yield ranged from 59 to 343 g plant(-1) in the wet season an d 5 to 174 g plant(-1) in the dry season. Correlation between forage y ield and shoot nutrient uptake indicated the importance of nutrient ac quisition, particularly of phosphorus (r=0.90; P<0.0001), for adaptati on to infertile acid soil stress. Two genetic recombinants (BRN093/300 9, FM9201/1873) were productive when grown under infertile acid soil s tress in both wet and dry seasons. But, neither of these two recombina nts is highly resistant to xylem-feeding insects known as spittlebugs (Homoptera: Cercopidae). One spittlebug-resistant genetic recombinant (BRN093/1371) exhibited several desirable attributes such as superior leaf area and leaf biomass, greater N content in leaves, and greater p artitioning of N and P to leaves that could contribute to adaptation a nd persistence in these soils. This recombinant is being utilized in a breeding program to develop superior Brachiaria lines. (C) 1998 Elsev ier Science B.V. All rights reserved.