Appendix ll-A— A Case Study of Wheat • 305 
sociatrcl with signitirant yield improv eim'iit. I'he 
shorter, stilh'r stems ot the semidwarl plants allow 
ma\imi/ation ot ri'sourees w ithout yield l eduetions. 
lm|)ro\ »'ment in thi> inluM’iMit yield components of 
st('ins p«M- unit ar(>a, kernt'Is per stem, and kernel 
weif'ht has also contrihuteil (>\t('nsi\ el\ to yield im- 
pro\ t'MU'nt. 
The use ol ap[)lied genetics in w heat im|)ro\ ement 
occurs in close harmony with total wheat manage- 
ment systems. The tarmer must integrate a huge as- 
sortiiKMit ot alternativ es in t'ach decision — e.g., an iti- 
ili\ idual producer may l)e deciding on a nitrogen 
program. It th(' tarm is irrigated, the producer 
selects nitrogen amounts and application timing 
based on soil tests, intended crop and \arietv, the 
end list' ot that crop, and watering schedules. II the 
tarm is rainit'd, the product'!' takes into account soil 
tt'sts, ci'op considei'ations, and l aint'all pi’ohahilities. 
In hotli cases pi'oduct pi'ices at the time ot sale 
must he predictetl since they go\ern |)otential gross 
returri, w hich in turn atlects the costs ol maintaining 
a (irotit margin, (it'iietic inteiaction in this svstem is 
inti'icate. The tai'mei' must first select the \arietv 
most likely to produce at the maximum economic 
level, for irrigated land, it may he a short high- 
yielding semidw ai'f either for the cookie trade or the 
e.xpoi't market. The farmer knows that part of the 
value ot his product is dependent on low' protein. 
However, ina[)[)roi)riatelv high levels of nitrogen, 
w hich greatly improv e yield, will also raise the pro- 
tein of the crop beyond acce[)tahle levels. If the ex- 
port market is strong and the total I'.S. supply re- 
duced. the higher protein may he of little economic 
conse(|uence. 
In the case of the dryland farmer, the variety 
selected mav he taller with lower yield potential but 
with much better levels of adaptation and tolerance 
to adverse env ironments. It may be designed for the 
bread industry or the export market. Part of the 
V alue is related to high-protein content. Since mois- 
ture conservation and use is critical, nitrogen ap- 
plications and amounts must be selected so that the 
plants do not waste their moisture reserve. How ev'er, 
nitrogen applied too late may not receive enough 
rain to penetrate the soil and become av ailable to the 
plants. If the plants "burn up" because of unwise 
water use early in the season, the seeds will be high 
in protein but low in yield. If inadequate nitrogen is 
av ailable, the crop will generally be low in protein. 
The abbrev iated protein story is but one of many 
examples of farm management interaction with ap- 
plied genetics in wheat production. Recent changes 
in energv' price and availability, environmental re- 
straints, marketing structures, and technology devel- 
opment are producing a new array of complex prob- 
lems. 
Genetic vulnerability in wheat 
Genetic vulnerability is defined as a high degree of 
genetic uniformity in a crop grown over a wide acre- 
age. Wheat, which is produced on about 62 million 
acres annually in the United States, has a relatively 
high level of uniformity and genetic v ulnuerability. 
In 1974,102 hai'd I'ed winter wheat v^arieties were 
grown on 36.6 million acres, with four varieties oc- 
cupying 40 percent ot the acreage. Hard red spring 
wheat varieties totaled 80 percent on 14.7 million 
aci'es, with three varieties occupying 52 percent of 
the aci'eage. Similar situations occurred with other 
classes ot wheat. Plant pests, including diseases and 
insects, have periodically caused moderate to severe 
vv heuil crop losses in years favmrable to the develop- 
ment ot strains capable of attacking current forms of 
resistance. 
Incoi'porating genetic resistance to pests has tradi- 
tionally been the responsibility of public breeders. 
Wheat is a self-fertilized plant that can be faithfully 
reproduced from generation to generation. Private 
industry has been reluctant to invest R&.D money in 
improvements since the farmer, following the initial 
seed purchase, can reproduce the crop without re- 
turning to the seed company. Thus, public breeders 
have been the main source of new varieties and have 
had the responsibility of delivering genetic im- 
provements to the producer. Wheat breeding pro- 
grams are generally designed to respond to State pro- 
duction needs. Goals and objectives are established 
by technical advisory groups that include breeders 
and scientists, growers, use industry representa- 
tives, and extension workers. 
Genetic variability is available to the breeder from 
naturally occurring sources and artificially induced 
mutations. Naturally occurring variability bas been 
collected from native plant populations throughout 
the w'orld and is maintained in the World Wheat Col- 
lection by the Science and Education Administration 
of USDA located in Beltsville, Md. Currently, about 
37,000 accessions are contained in the collection. 
Breeders use the collection as a reservoir from which 
to draw exotic genes needed to improve the value of 
their breeding programs. In addition to variability 
w'ithin w'heat varieties, the breeders can use special 
genetic techniques to draw valuable genes from re- 
lated species such as rye and various forage grasses. 
This approach, while time-consuming and costly, has 
been used in a number of variety development pro- 
grams. Mutations induced by artificial means have 
