Principles of Plant Genetics and Breeding

covered with the bag. Pollination success varies with
the method of pollination, operator’s skill, environment,
age of the stigma, and amount of pollen, among
other factors.
Natural pollination
CMS is used for large-scale hybridization in an isolation
block where random mating populations are grown. A
fertility-restorer pollinator is planted with several malesterile
females.
Seed development and
havesting
Successful pollination is evidenced by the seventh day
following pollination. The grains are physiologically
mature when the black layer forms at the base of the
seed. This stage occurs between 35 and 45 days after
pollination.
Grain sorghum is a perennial plant. Consequently,
the plant remains green and alive, even after the grain is
matured, until killed by tillage or freezing temperatures.
The grain dries slowly. At the hard-dough stage, the
grain contains about 18–20% moisture. For effective
combining, the grain moisture content should be about
13% or less. Waiting for the grain to dry in the field
delays harvesting and increases the risk of damage by
weather factors and birds. Furthermore, delayed harvesting
also delays the rotation of sorghum with a winter
crop (e.g., wheat), Desiccants (e.g., diquat, 28% nitrogen
urea-ammonium nitrate) are applied as a preharvest
treatment on grain sorghum. Roundup Ultra® may be
used as a defoliate when the grain is for feed.
Common breeding objectives
1 Grain yield. Grain yield is one of the principal objectives
of sorghum breeding. Grain yield improvement
in sorghum has made significant strides over the years.
The success mainly stems from access to additional
germplasm from tropical accessions through the
sorghum conversion program, which has augmented
existing genetic variability and the development of
hybrid cultivars. Also, disease and insect resistance
protect grains in the field, reducing harvest losses.
2 Yield stability. Sorghum is more adapted to marginal
production environments than cereal crops such as
corn and rice. Breeders are interested in stable per-
BREEDING SORGHUM 517
formance over variable environmental conditions in
the production region.
3 Agromorphological traits:
(a) Lodging resistance. Tall cultivars are prone to
lodging. Breeders use dwarfing genes to attain
short stature. Sources of stiff stalk include the
kafir and hegari genotypes. Another way lodging
resistance is enhanced is through the development
of resistance to stalk and root diseases.
(b) Short stature. As previously discussed, breeders
use the four recessive dwarfing genes dw 1 , dw 2 ,
dw 3 , and dw 4 to develop short-statured cultivars.
Most US cultivars are 3-dwarfs (have three recessive
dwarfing genes). It should be mentioned that
taller cultivars are preferred for the production of
forage and silage, and also in dry production systems.
Short stature is desirable for mechanized
harvesting. In addition to short stature, crosses
between milo and kafir produce recombinants
with an erect head and stout stalk.
4 Adaptation:
(a) Early maturity. Early maturing cultivars are
advantageous in low rainfall regions by allowing
the crop to escape damage from drought. These
cultivars also allow an expansion of sorghum production
to regions of high altitudes and short
growing season. Genes for early maturity tends
to reduce plant stature.
(b) Photoperiod insensitivity. Photoperiod insensitivity
adapts the crop to regions of shorter
growing season.
(c) Drought resistance. Sorghum is more resistant
to heat and drought than corn. In breeding for
drought and heat resistance, breeders select for
extensive root systems that promote more extensive
exploration of the soil for moisture, as well
as plant features that reduce moisture loss from
the leaves (e.g., fewer stomata).
(d) Tolerance of aluminum. High levels of soil
aluminum reduce root development and predispose
plants to drought injury. Genotypes with
aluminum tolerances have been identified for
breeding.
5 Disease resistance. Sorghum diseases of economic
importance include the following:
(a) Rots. Sorghum is known to be attacked by both
root and stalk rots, the common ones including
Fusarium root rot (caused by Fusarium moniliforme)
and stalk rot, charcoal rot (caused by
Macrophomina phoseolina), and Periconia root
and crown rot.
(b) Blights. An important blight is the northern
leaf blight (caused by Exserohilum turcicum), a