 Forage evaluation and improvement research at CIAT would be decidedly
incomplete without a major effort to determine and document the tolerance that key grass
and legume species have of major abiotic stresses in humid and subhumid regions of the
tropics.Low nutrient supply is a major
constraint to forage adaptation and production in tropical acid soils. Widespread adoption
of forage cultivars depends on these efficiently acquiring nutrients from the soil and
using them for growth. Plant growth in these soils is not constrained so much by soil
acidity (i.e., hydrogen ion activity) as by aluminum (Al) toxicity and deficiencies of
nutrients such as phosphorus (P), nitrogen (N), and calcium (Ca). Plants of adapted
tropical grass and legume forages have attributes that are linked to strategies for
acquiring these nutrients in a low pH and high Al environment. Thus, understanding these
attributes is fundamental to developing more efficient screening procedures for germplasm
evaluation and/or improvement. Previous research showed that both grasses and legumes
adapt to low nutrient supply by partitioning increased dry matter to the roots at the
expense of leaf and shoot growth. Significant genetic variation apparently exists for this
ability in Brachiaria spp. and Arachis pintoi.
Research so far shows that adapted grasses and legumes must make efficient use
of both naturally occurring nutrients and those applied as fertilizer for growth, and, in
the case of legumes, for N2 fixation. Although the use of adapted forage
germplasm reduces the amount of fertilizer needed, it does not eliminate the need to apply
fertilizers.
A major constraint to small-scale livestock production in the subhumid tropics
is the shortage of forage during the dry season, which leads to overgrazing. If
smallholders are to intensify production in these environments, they must have new forage
options that tolerate long dry seasons.
Mechanisms of Aluminum Resistance in Brachiaria, and Related
Candidate Genes
We designed a low ionic-strength nutrient solution that simulated the
nutrient-deficient and Al-toxic conditions of soil solutions found in acid soils. We then
dissected the "acid-soil syndrome" into individual components that were assumed
to be most relevant to Brachiaria cultivars: Al toxicity and P and N
deficiencies. We showed that the level of Al- resistance in Brachiaria decumbens
is outstanding, compared with Al-resistant field crops. This high level of Al resistance
is not achieved by exudation of organic acids through root tips, contrary to the current
view of Al-resistance mechanisms. The challenge now is to identify specific physiological
mechanisms that confer this high level of Al resistance and to identify candidate genes
responsible for high level of Al-resistance in B. decumbens.
Screening Method to
Identify Aluminum-resistant Brachiaria Hybrids
Based on the progress made in defining mechanisms of Al-resistance, we developed
a rapid and reliable screening procedure to identify Al-resistant Brachiaria
hybrids. This method uses relative root elongation as a simple measure to identify
Al-sensitive genotypes. We adapted this method for vegetative stem cuttings and are
evaluating spittlebug-resistant Brachiaria hybrids for their Al-resistance. We
are also using this method to evaluate a hybrid population of B. decumbens
(Al-resistant) × B. ruziziensis (Al-sensitive). This will enable us to develop
molecular markers for this trait and to identify candidate genes for Al-resistance.
Field Evaluation of Brachiaria Hybrids for Tolerance of
Low-fertility, Acid Soils and Drought
Developing superior Brachiaria hybrids with improved tolerance of
low-fertility, acid soils and drought has great potential to lower input requirements and
production costs, and to reduce environmental degradation by minimizing pasture
degradation problems. We intend to achieve this objective through field evaluation of
Al-resistant Brachiaria hybrids that genetically combine efficient nutrient
acquisition and use with dry-season tolerance. Field studies are being conducted in the
Colombian savannas and Costa Rican hillsides.
Identifying Arachis
pintoi Accessions with Improved Adaptation to Low-fertility Soils and Drought
We are searching for Arachis accessions that are superior to the
commercial cultivar A. pintoi CIAT 17434 in rapid establishment and adaptation to
low soil fertility and drought conditions for multipurpose use in the tropics. Currently,
we are field-testing for genotypic variation among accessions that are highly promising as
cover or forage legumes for the Piedmont Region of the Colombian Llanos.
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