Principles of Plant Genetics and Breeding

514 CHAPTER 30
Breeding nursery
Hybrid evaluations
Off-season nursery
Lubbock
College Station
Figure 3 Map of Texas showing the locations used by the TAES sorghum
breeding program for breeding, selection, and evaluation.
growing environments. In our high
plains nurseries, the environment
is typical of temperate production
regions and different traits are of
importance. The evaluation of material
at these locations allows us to
select widely adapted germplasm.
Many traits are important in our
breeding program (both inbred and
hybrids). Since grain sorghum producers
are paid based on grain
yield, it remains the most important
trait. Most every breeding program
measures the yield potential in
hybrids, as the correlation between
inbred yield and hybrid yield is
rather poor (Rooney 2004). In addition
to breeding for yield, any factor
that reduces yield becomes important
as well. Therefore, breeding
for drought tolerance and disease
and insect resistance have priority as
well. Diseases of importance in US
sorghum include stalk lodging, grain
mold, anthracnose, downy mildew,
head smut, sooty stripe, and leaf
blights. Insects of economic importance
include the sorghum midge,
greenbug, and chinch bug. Finally,
grain quality in sorghum has become more important. Until recently, sorghum in the US was used exclusively as feed grain, but the
development of food quality sorghum hybrids has resulted in an increase in the sale and use of sorghum in food products. A major
goal of our program is to produce sorghum lines that will make hybrids with grain quality suitable for both feed and food use.
Prior to testcrossing, the new B-lines must be male-sterilized. B-lines are sterilized using a backcross program in which a standard
A-line with a similar pedigree to the new B-line is used as the source of the male-sterile cytoplasm (Figure 4). The new B-line
is then used as a recurrent parent to produce an A-line that is genetically identical to the B-line (except that it is male-sterile). In
each generation of backcrossing, plants and progenies that are fully male-sterile and are the most similar to the B-line are
selected. The sterilization process usually requires a minimum of five backcrosses and most sorghum breeding programs utilize
winter nurseries to reduce the amount of time required for sterilization.
Our program begins testcrossing in the F 5 for R-lines and in the BC 3 of sterilization for A/B-lines. New R-lines are testcrossed
to A-line testers and new A-lines are testcrossed to R-line testers to determine each new line’s general combining ability and the
fertility of the hybrids. Lines that produce high-yielding hybrids with appropriate agronomic parameters are advanced for additional
testing. These lines are hybridized to several potential parental lines of the opposite group to identify those hybrids with
good general and specific combining ability. These hybrids are tested in multiple locations and those lines that produce hybrids
with high yield, good stability, good agronomic characteristics (height, maturity, etc.), and acceptable abiotic and biotic stress tolerances
will be released. These releases are then tested by private industry to determine if they will be used in hybrid releases for
producers (Rooney 2003a, 2003b).
In addition to hybrid performance, private companies must consider the parental line’s performance with regard to seed
production. Seed producers must be able to consistently coordinate flowering of the A-line and the R-line. If the two lines have a
poor “nick”, the A-line will have extremely low seed set and, consequently, poor yields of hybrid seed. The pollinator line should
start shedding pollen prior to the emergence of stigmas in the A-line and the R-line must continue to shed pollen throughout
the flowering of the A-line. In addition, the pollen shed from the R-line should be consistent and relatively unaffected by normal
environmental conditions. Obviously, the R-line must consistently restore fertility to the hybrid and the A-line parent must produce
seed yields high enough to justify seed production costs.
In addition to breeding for grain sorghums, there are directed breeding programs in sorghum to improve the crop for other uses.
These include the improvement of sorghum for forages, such as silage, grazing, and hay. Other programs focus on the use of
sorghum for sweeteners, in which the crop is used much like sugarcane. All of these uses have resulted in the development of
numerous genotypes suitable for the production environments and purposes of the crop. The challenge for sorghum breeders is to
continue future improvements given the relatively small number of researchers actively engaged in this field of research.
Beeville
Corpus Christi
Weslaco