TheImprovement ofTropical and Subtropical Rangelands

ORITERIA FOR PLANT SELECTION 205
An example ofa succeuful plant improvement program is the extensive
study, selection, and breeding ofplants in the genus AgropJ/ron
by the U.S. Department of Agriculture's Agricultural Research Service
group at Logan, Utah. Considerable progress has been achieved
in the development ofnew varieties of wheatgrasses and crosses with
cloeely related species (Dewey, 1983). Aaay and Knowles (1985) documented
the release of seven improved cultivars of AgropJ/ron and
Elrmu for rangeland improvement. Because of their importance for
forage and conservation 1118, many other taxonomic groups of grasses
are the subject of efforts in genetic improvement. They would include
BouteloutJ (Heizer and Hassell, 1985), PtJnicum, Sorghutrum,
and Trip6tJcum (Vogel et al., 1985), as well as Cenchru, ErtJgrostis,
DigittJritJ, LeptochlotJ, Muhlen6ergitJ, SettJritJ, and Sporo6olu (Voigt
and Oake, 1985).
The improved palatability of range plants is often mentioned
as a desirable plant improvement goal, but according to a review
by Voigt (1975), the factors that influence palatability must first
be understood. Plant chemical composition is a major palatability
factor, but morphology, succulence, disease resistence, and stage
of growth are also involved. The selection of superior plants with
these characteristics is necessary to produce plants with improved
palatability, and to relate palatability to management strategies.
One of the most critical needs is for grasses with a superior
ability to germinate and establish thelDlelves in stressful environments.
Wright (1975) has reviewed the work of plant breeders in
developing species with special traits useful in rangeland situations.
He cited results in the improvement of PtJnicum tJntidottJie for increased
drought tolerance, BouteloutJ eriopodtJ for improved seed set,
B. curtipendultJ for greater seed dormancy to avoid exce88ively early
germination, and P. o6tuum for improved seed viability.
Heinrichs (1975) pointed out that in many rangeland areas, the
ecarcity of adapted legumes appears to be a problem for maintaining
the nitrogen level in the soil adequate to support good plant growth.
Most of the research work in the United States has been done on
legume forbs such as Trifolium, MedictJgo, and VicitJ. According to
Rumbaugh and Townsend (1985), more than 25 improved cultivars
of legume species adapted to humid and semiarid rangelands are
now available, but the greatest challenge is for an improved legume
cultivar adapted to range lites receiving lea than 250 mm of precipitation
annually. There is also a need to develop improved varieties
ofleguminous shrubs. The International Livestock Centre for Africa,