Aflatoxin Contamination of Bt and Non-Bt Cottonseed
Tim C. Knowles, La Paz County Cooperative Extension
Vic Wakimoto, Farmer, Mohave Valley, AZ
Del Wakimoto, Farmer, Mohave Valley, AZ
Mike Keavy, Farmer, King Ranch, Parker Valley, AZ
Transgenic Bt cotton varieties that are resistant to pink bollworm should
sustain less feeding damage to bolls and cottonseed, compared to non-Bt
varieties that are more susceptible to feeding damage by pink bollworm larvae.
Prior to boll opening, the aflatoxin producing fungus Aspergillus flavus cannot
penetrate undamaged cotton bolls. Thus resistance to pink bollworm could result
in reduced aflatoxin contamination under high pink bollworm pressure.
Cottonseed aflatoxin levels of Bt and non-Bt varieties were compared at various
planting and harvest dates. Bt and non-Bt cotton varieties had similar cottonseed
aflatoxin levels. Long season production systems favored high cottonseed
aflatoxin levels, compared to short season production systems, regardles of the
cotton variety grown.
Aflatoxin contamination of cottonseed from the toxin producing fungus
Aspergillus flavus occurs primarily at elevations below 1800 feet, or
in counties along the Colorado River and in south central Arizona.
Infection results in damage to cotton lint by staining and weakening the
fibers, and when the fungus penetrates the seed, viability is lost since
seed germination is inhibited. Furthermore, the feeding of cottonseed exceeding
20 ppb aflatoxin to dairy cows is prohibited since aflatoxins can be transmitted
from the feed to the milk of dairy cows.
Only open bolls and bolls or seeds damaged by insects such as the pink
bollworm are subject to invasion by the aflatoxin producing fungus. The
critical period for boll invasion and contamination from aflatoxin is
from early August to mid-September when bolls start to open. Approximately 95%
of the seed infection and aflatoxin formation can take place during this time.
These susceptible bolls are normally located on the lower 1/3 to ½ of the plant
where dense leaf canopies maintain high humidity and prevent air movement
around the bolls. Rank cotton is usually more susceptible to aflatoxin problems.
Late season irrigation management and cotton harvesting methods can affect
aflatoxin levels in cotton seed. During high risk growing seasons, when above
normal August and September nighttime air temperatures occur, reduction of the
growing season and limiting the number of irrigations in August shows promise for
reducing aflatoxin levels. Careful management of irrigation and nitrogen to avoid
rank cotton growth can lessen fungus development. Rood cotton should not be mixed
with picked cotton since rood cotton can have aflatoxin levels up to 50 times
higher than picked cotton.
Transgenic Bt cotton may have reduced susceptibility to aflatoxin
contamination due to its pink bollworm resistance. However, under heavy pink
bollworm pressure, first and second instar pink bollworm larvae can damage
Bt cotton bolls and seed. Furthermore, even Bt cotton varieties are not resistant
to Aspergillus flavus infection following boll opening, and large quantities of
aflatoxin can form during this period. When Bt cotton growers extend the growing
season due to low cost pink bollworm control, most advantages of Bt cotton in
aflatoxin management may be lost (Cotty et. al., 1997).
Materials and Methods
Field experiments were conducted during 1995 and 1996 in Parker Valley (located
in southwestern La Paz County) and Mohave Valley (located in southwestern
Mohave County) to determine aflatoxin levels in cottonseed produced there.
In 1995, approximately 50 pounds of seed cotton was hand picked on November
7 from the bottom, middle, and top 1/3 of the cotton plants from an early
(March 15) and late (April 20) planted fields. The experiment was conducted at
V&K Wakimoto farms in Mohave Valley. In 1996, approximately 5 pounds of
cottonseed was saved from 4 replicates each of Deltapine 5415 and 33B cotton
grown in a variety test at Avi Kwa ‘Ame Farms in Mohave Valley. Also in 1996,
approximately 25 pounds of cottonseed was saved from full field (40 acre)
evaluations of Deltapine 33B and 35B harvested at several different dates
at the King Ranch in Parker Valley. Cottonseed samples were tested for
aflatoxin levels by Chandler Analytical Laboratories through Anderson Clayton
Results and Discussion
In the 1995 study, highest aflatoxin levels were observed in cottonseed from
hand harvested from the upper one third of the cotton plant compared to the
middle one third
and bottom one third portions
(Table 1). Most likely, this was caused by
damage to bolls growing at the tops of plants. It is pretty common to see the
top crop of
non-Bt cotton varieties infested with pink bollworm larvae late in the growing
Additionally, aflatoxin levels in cottonseed from the early planted (March 15) cotton field
were higher than those observed in the late planted (April 20) field. Since both fields were
harvested on November 7, the early planted field was subjected to a relatively long growing
season and probably had more susceptible (open) bolls during the critical period for infection
from August through mid-September.
In 1996, a non-transgenic variety Deltapine 5415 was grown side by side with its transgenic
Bt daughter line Deltapine 33B in a replicated upland cotton variety test located in Mohave
Valley (Table 2). At the November 15 harvest, aflatoxin
levels of Deltapine 5415 cottonseed ranged from 10 to 31 ppb, and Deltapine 33B cottonseed
had aflatoxin levels ranging from 35 to 83 ppb. Statistically, the aflatoxin levels of
cottonseed from the two varieties were not significantly different. This indicates that
although Bt cotton is somewhat resistant to pink bollworm, it may not be resistant to
infection by Aspergillus flavis. First or second instar pink bollworm larvae can survive in
Bt cotton and impart some damage to bolls and cottonseed which can be penetrated by the A.
flavus fungus. Boll damage from insects other than pink bollworm and heat stress induced
suture cracking may also predispose bolls to infection. Furthermore, Bt cotton varieties
are not resistant to aflatoxin increases occurring after boll opening.
Also in 1996, full field (40 acre) evaluations of transgenic Bt
harvested at various dates were examined in Parker Valley
(Table 3). Deltapine 35B harvested from
October 21 through
November 13 had cottonseed aflatoxin levels ranging from 2 to 16 ppb. However, Deltapine 33B
and 35B harvested on December 4 had cottonseed aflatoxin levels ranging from 58 to 411 ppb.
Therefore, Bt cotton varieties harvested during October and November had cottonseed aflatoxin l
evels acceptable for dairy cow feed, while the same Bt cotton varieties harvested in December
did not. Therefore, long or full season cotton production systems seem to favor high cottonseed
aflatoxin levels, compared to short season production systems, regardless of what cotton
variety is grown.
- Cotty, P.J., D.R. Howell, C. Block, and A. Tellez. 1997. Aflatoxin contamination of Bt
cottonseed. 1997 Cotton Report. University of AZ College of Agriculture
Series P-108. P. 435-439.
The valuable cooperation, land, and resources provided by Vic Wakimoto of V&K Wakimoto
Farms, Del Wakimoto of Ft. Mohave Avi Kwa ‘Ame Farms, Mike Keavy of Parker Valley King Ranch,
and John Villalobos of CRIT/ACCO Joint Venture Gin is highly appreciated.
This is a part of publication AZ1006:
"Cotton: A College of Agriculture Report," 1998, College of Agriculture, The University of
Arizona, Tucson, Arizona, 85721.
Any products, services, or organizations that are mentioned, shown, or indirectly
implied in this publication do not imply endorsement by The University of Arizona.
The University is an Equal Opportunity/Affirmative Action Employer.
This document located at http://ag.arizona.edu/pubs/crops/az1006/az100610e.html
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