Principles of Plant Genetics and Breeding

Introduction
Don L. Keim
BREEDING COTTON 549
Industry highlights
Cotton breeding
Delta and Pine Land Company, One Cotton Row, PO Box 157, Scott, MS 38772, USA
Modern upland cotton, Gossypium hirsutum, is an allotetraploid which, though primarily self-pollinated, is readily cross-pollinated
by insects. Cotton varieties are developed primarily by pure-line breeding techniques. But many varieties have traditionally been
mixtures of closely related genotypes as a result of cross-pollination, cotton morphology, and breeding procedures. Uniquely, the
primary economic value is not the seed itself, but in the fiber produced as an extension of the seed coat cells.
Some terminology unique to cotton is explained below:
1 Lint: the economic fiber that is separated from the seed during ginning. After ginning and cleaning, the lint is compacted
into a bale. In the USA, each bale is identified and sampled for quality determination and marketed in bale units of approximately
480 pounds net weight of lint.
2 Cotton boll: a single fruiting body that contains at harvest the seed and attached lint fibers, and the dried ovary wall (burs).
3 Lint percent: the proportion of the lint to the harvested seed cotton (seed with lint attached).
4 Fiber quality: the characteristics that affect the processing of lint into yarn and finished textile goods. Four primary characteristics
that affect the marketability and pricing of fiber are grade, length, micronaire, and strength. Many other characteristics
affect the usability of fiber, some of which are uniformity, elongation, and short fiber content. Many fiber characteristics
have a genetic component, but a major portion of the variation is due to environmental, harvest, and ginning effects. Fiber
characteristics are determined in the commercial channels, and in most breeding programs, through the use of an HVI (high
volume instrumentation) machine.
5 Grade: the combination of the amount of leaf trash and color of a lint sample.
6 Length of staple: the length of the fibers in a sample (2.5% span length).
7 Micronaire: the combination of relative fineness and fiber maturity of a sample.
8 Strength: the amount of force required to break a specified bundle of fibers.
Historically, a major factor contributing to the high cost of cotton production has been in the area of insect management. The
insects most affecting US production have been the bollworm/budworm complex, Heliothis virescens and Helicoverpa zea, and
the boll weevil, Anthomonas grandis. Recent adoption of new practices has dramatically reduced control costs associated with
these two pests. The government-sponsored boll weevil eradication program has practically eliminated boll weevil as a pest in
major areas of the US Cotton Belt. Additionally, in 1996, farmers began adopting the use of transgenic cotton which contains a
gene that confers resistance to the bollworm/budworm complex. This gene was developed by Monsanto and was given the tradename
Bollgard® in cotton.
Cotton breeding as compared to breeding in other field crops is affected by two key factors. First, the fiber must be removed
from the seed (ginning). Second, the remaining short fibers (seed fuzz) must also be removed with acid (delinting) so that the seed
is “flowable” and can be planted with modern equipment. These processes, ginning and delinting, are costly in resources (time,
effort, money). Additionally, cotton breeding is expensive due to much hand harvesting (hand picking), the limited number of
seed per boll, and the need for hand selfing.
These factors, along with the need for expensive fiber quality evaluations, make the resource cost per unit of genetic gain much
higher with cotton when compared to several other crops (Figure 1). Given fixed resources, the genetic gain will generally be less
with cotton.
Until recently, cotton could have been considered to be relatively unbred when compared to the “state of the art” in crops like
corn, soybean, and wheat. In the past, resource limits had forced cotton breeders to have fewer populations with small population
sizes and to advance limited numbers of relatively unselected strains.
The recent widespread adoption and use of transgenic varieties has greatly increased the value of seed. Seed companies reaping
this value have been able to place more resources in the hands of breeders. The recent gains in lint yield and fiber quality have
been a direct result of these increased resources and the efficiencies gained from modernization of breeding procedures.
Breeding program
A major effort in this breeding program has been to maximize the gain enabled by the added resources made available. Several
technique and procedural changes have been adopted to increase the efficiency of the utilized resources. This has resulted in an
increased number of populations, larger population sizes, heavy selection pressure, and the early identification of superior strains.