HARVESTING AND CURING BURLEY TOBACCO
Ira E. Massie and Jones H. Smiley
Department of Agronomy
A well-cured burley crop depends on
cutting your tobacco at the right time, housing it correctly, practicing
good barn management, and bulking it properly. Curing burley is more than
just drying the leaves. You must control temperature, humidity, and air
circulation if you hope to market good yields of high quality tobacco.
Many fine crops are injured by improper handling, inadequate housing, and
lack of control over curing conditions. Furthermore, proper harvesting
and curing can often improve some of the poorer crops. Remember, the care
and good management you used from plant bed to cutting time must be continued
in the barn if burley is to return maximum profits.
Ripening does much to improve the quality
of burley tobacco. Cut tobacco when nearly all the upper leaves show a
distinct yellow tinge (Fig. 1). The added growth and improved quality of
ripe tobacco more than make up for the loss of lower leaves. In very humid
harvest seasons, cutting when the middle leaves show a distinct yellow
tinge may be desirable.
Leave the tobacco in the field in the
standing stick long enough for it to wilt (your housing facilities will
partly govern the length of time) but never longer than 3 to 5 days. Loss
in fresh weight will be about 20 per cent by the end of five days with
little or no loss in value. Fully matured tobacco is not likely to sunburn;
however, green tobacco often sunburns. Sunburned tobacco should be left
standing in the field for about three days to reduce damage.
Good housing practices are essential
to control the curing of burley tobacco. Many crops that come from the
field in fine condition are seriously damaged by poor housing facilities.
Each bent should be filled completely
from top to bottom as the crop is put in the barn. Leave enough space under
the lowest rails so you can use heat if necessary. Starting the fill on
the southwest side of the barn takes advantage of air movements in the
early stages of curing. Space sticks about 6 to 8 inches apart on rails
and spread the stalks on sticks. (Fig. 2.) Make sure that the leaves are
hanging down and not doubled up. Also, tip leaves should fall between lower
sticks. Never hang fresh-cut tobacco (tobacco harvested earlier) under
partially cured tobacco. Water evaporating from the fresh tobacco may cause
partly cured tobacco to darken.
If you do not have enough tobacco to
fill your barn, space it throughout the barn so air can circulate evenly
through the tobacco.
Locate the barn on an open, well-drained
area with sides facing the direction of the prevailing winds for best ventilation.
The best location is on a ridge, hill, or a high point in the field (Fig.
Provided that one side faces the prevailing
winds, the structure's length and height will not noticeably affect air
circulation within the barn. Width is the most important dimension affecting
ventilation. Width determines (1) the distance the air must move as it
passes through the barn and 2) the quantity of tobacco through which the
air must pass.
A standard barn is 40 feet wide and
60 feet or more long with a sidewall 20 feet high and a gable roof of 1/3
pitch. The only ventilators are full-length sidewall vertical doors equivalent
in area to at least 1/3 of the sides. Since larger tobacco now is being
grown, some farmers are building barns with rails up to six feet apart
When repairing old barns or building
new ones, provide ample ventilators on the sides (Fig. 4, Fig. 5). In barns
36 to 40 feet wide, hinge at least 1/3 of the boxing or siding to permit
opening and closing. Wider barns need even more ventilation.
Ventilators are not needed at the ends
of burley barns unless prevailing winds strike the end of the barn and
tier rails run parallel with the width. If prevailing winds strike the
side of the barn and tier rails are parallel to width, the barn should
be remodeled and tier rails changed to run parallel to the length of the
barn (Fig. 5) and ventilators should be provided on the sides. Research
at the Kentucky Agricultural Experiment Station has shown that ventilators
in the roof are almost useless.
Because of reduced acreage, the smaller
32-foot wide pole type barns are becoming popular and practical. Plans
for these lower cost barns are available from your county Extension agent.
The final quality of cured tobacco
is determined very largely by moisture conditions which prevail inside
the tobacco barn during the curing period. High moisture causes tobacco
to cure too slowly, producing red or house-burned leaf and heavy losses
in weight. When tobacco stays in "brittle case", it cures too fast, causing
a greenish-tinged, mottled, or pie-bald leaf.
Moisture can be controlled in burley
barns fairly well through proper use of ventilators, plus careful use of
heat in humid weather. Burley cures favorably when the temperature inside
the barn ranges between 60 degrees and 90 degrees F, provided the relative
humidity averages 65 to 70 per cent in the barn over a 24-hour period.
In normal weather during the tobacco curing season in Kentucky, the outdoor
temperature seldom goes above 90 degrees or below 60 degrees F for any
great length of time. Therefore, favorable curing conditions depend largely
on whether relative humidity can be kept around 65 or 70 per cent.
Cured tobacco leaves are very sensitive
to changes in the moisture content of surrounding air and can be used to
roughly determine relative humidity in the barn (Table 1). When samples
feel "dry to low case", the humidity is about right for best curing.
Table 1: Feel of Cured Tobacco Flyings in Relation
to Relative Humiditya.
a From Kentucky Agricultural Experiment Station Bulletin
501 "Principles of Burley Tobacco Barn Operation".
|Feel of Cured Leaf
||Relative Humidity (Percent)
||90 to 100
|Medium to high case
||85 to 90
||80 to 85
|Low to medium case
||75 to 80
||70 to 75
|Dry to low case
||65 to 70
||60 to 65
|Dry to brittle
||55 to 60
||50 to 55
||0 to 50
Curing conditions in the barn can be
varied by management practices. How well you cure your crop depends largely
on how well you regulate humidity, how close you space sticks, the width
of your barn, the size of the tobacco, and the amount of ventilation.
During August and September the air
is usually dry during the day and moist at night. Generally, therefore,
open the ventilators as soon as the dew dries in the morning and close
them in late afternoon.
If you are not sure whether to open
or close the curing barn, put a few cured leaves in a sheltered place,
such as an open shed near the tobacco barn, and when these leaves are damp
and hang limp, the air is high in moisture and the barn should be closed.
In general, whenever these leaves feel drier than the tobacco inside the
barn the ventilators should be opened, but when the tobacco inside the
barn feels drier than those leaves outside, then keep the barn tightly
closed (see Table 1).
During cool periods, open the barn
to get the benefit of the warmer outside air. In cool weather the temperature
may be 10 degrees F lower in a closed barn than outside because of cooling
from evaporation. If no fire is used in curing, provide as much ventilation
as possible until curing is nearly complete.
If the weather is very dry and your
tobacco is curing too fast, close the barn in the daytime and open it at
night. This method traps the cool, moist night air and keeps the drier
daytime air out of the barn.
During some periods every year, relative
humidity cannot be adequately controlled by ventilators, and heat should
be used. Also, whenever tobacco remains in case for more than 24 hours,
houseburn will start. If you are unable to manage the barn ventilators
properly as described, it is best to put the tobacco in the barn and leave
all doors and ventilators open for 4 to 6 weeks. After the tobacco is cured,
close the barn and leave it closed.
Controlling Humidity with Heat
Heat is used primarily to control the
moisture content of the air surrounding the tobacco in the barn. If too
much heat is added, the moisture level of the air becomes too low, resulting
in too fast a cure. If the weather dries the cured leaves each day without
fire, then firing is a waste of fuel and time.
Always practice care and good management
when using supplementary heat. Control the rate of heat to keep the temperature
85 to 90 degrees F directly above the burner at the lowest level of tobacco.
You should use enough burners so that the temperature does not vary more
than 15 degrees F throughout the barn. Make all temperature measurements
at the lowest level of the tobacco.
Normally, the side ventilators of the
barn are mostly closed when heat is being used. As the warm, dry air rises
through the tobacco, it absorbs moisture released by the tobacco. The moist
air must not then be allowed to accumulate in the barn. Therefore, some
ventilators will have to be partially opened to allow the moist air to
Fuels and Stoves:
Use a number of stoves with low or
moderate fires rather than a few stoves burning with high fires (Fig. 6).
This will distribute the heat better and minimize hot spots. Coke stoves
made from 55-gallon oil drums release too much heat near the stove, cause
hot spots, and result in green or "off" colors being set in the tobacco.
Also, though coke is commonly used,
considerable labor is required to tend a coke-fired barn properly. Coke
stoves must be started outside the barn and then moved in after the fires
are started. They further require attention at least twice a day.
When properly used, one ton of coke,
on the average, will cure one acre of tobacco. Follow these suggestions:
1) Avoid extremely hot fires since they will destroy the stoves and cause
poor heat distribution; 2) After the curing season, remove the ashes and
coke from the stoves, coat the stoves thoroughly with crankcase oil inside
and out, and store in a dry place (coke stoves, when properly cared for,
will last ten years or longer).
Natural gas is an excellent fuel for
use in tobacco barns and, where available, is more economical than any
other non-solid fuel that has proved satisfactory for tobacco. All natural
gas supplied through commercial pipelines is safe for use in tobacco barns.
Gas from most private wells is also pure enough for use in tobacco barns.
The harmful impurity in natural gas is sulfur, usually in the form of hydrogen
sulfide, which is easily detected by its foul odor. Gas with as little
as one part per million of hydrogen sulfide will have a foul odor.
Propane gas or LP gas costs slightly
more than natural gas or coke. It lights instantly, is easily controlled,
and is a constant and uniform source of heat. Labor requirements for firing
with propane are comparable to those for natural gas and both of them are
much lower than coke.
When properly used, gas stoves will
last ten years or longer. However, after the curing season, store the stoves
in a dry place and store the hoses in a dark, dry place also.
Two types of gas burners, shown in
Figures 7 and 8, will operate on either natural or LP gas. They require
different orifices when using natural gas so these should be checked with
your gas dealer.
The small gas unit (Fig. 7) has a maximum
heat output of 30,000 B.T.U.. per hour. Each stove will cure an area in
the barn of about 150 square feet. The larger unit (Fig. 8) has a maximum
output of 75,000 B.T.U. per hour and will cure an area in the barn of about
500 square feet. Automatic control, available for all gas heaters, is very
helpful in adjusting relative humidity in the barn. Hygrometers or humidistats
will also help you determine relative humidity in the barn. These tools
and instructions ought to be used since they will help in curing your tobacco.
Distribution of Heat:
Hot spots will frequently occur in
localized areas directly above the stoves. These hot spots indicate poor
heat distribution which, in turn, causes green or "off" colors in the tobacco.
Hot spots are usually caused by over-firing and/or too few stoves. Moving
the stoves from place to place in the barn does not solve the problem.
You get uniform heat distribution only by using an adequate number of stoves,
each burning at a moderate rate. If you use coke, put at least two stoves
per bent, and sometimes three to a bent, in an average width barn (36-44
feet wide). All stoves should have some sort of heat spreader.
When using the larger gas burner (Fig.
8), use one stove per bent. Each arm is slotted so that heat will escape
at the proper intervals, giving good heat distribution. To check the heat
distribution in the barn, use thermometers, hygrometers, and/or humidistats.
Bulk Tobacco As Soon As Fully Cured
To prevent the darkening of cured leaf,
bulk tobacco as soon as it is fully cured. In warm weather, however, do
not bulk tobacco in high order as mold and rot may severely damage such
tobacco. Also, in warm weather, make only small bulks. If the tobacco stalks
are not fully dry, it is unsafe to leave tobacco in the bulk beyond 48
hours. Moisture from the stalk may enter the stem, causing stem rot.