ISSUED: 8-74
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.

Barn Management
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. 3).
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 vertically.
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.
Feel of Cured Leaf Relative Humidity (Percent)
High case 90 to 100
Medium to high case 85 to 90
Medium case 80 to 85
Low to medium case 75 to 80
Low case 70 to 75
Dry to low case 65 to 70
Dry 60 to 65
Dry to brittle 55 to 60
Brittle 50 to 55
Fragile 0 to 50
a From Kentucky Agricultural Experiment Station Bulletin 501 "Principles of Burley Tobacco Barn Operation".

Air Curing
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 escape.

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.

Gas Equipment:
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.