By James P. Gilreath, Bielinski Santos and Phyllis R. Gilreath

About  235 pounds of actual methyl bromide per treated acre is the percentage of methyl bromide in 350 pounds of a 67/33 formulation. This is thought to be the lowest rate for maintaining good nutsedge control under typical application conditions based on use of methyl bromide with standard low or high density polyethylene (ldpe or hdpe, respectively) film mulch which has low retention capacity for methyl bromide. 

In the past six years, considerable research has been conducted with new films, referred to as VIF or virtually impermeable film which has much higher fumigant retention capacity compared to ldpe and hdpe. 

Research and grower trials in tomato and pepper have established that methyl bromide rates can be reduced by one-half with VIF products and still maintain nutsedge control and crop yields comparable to a full rate with standard films. Unfortunately, there are two drawbacks to most VIF products: cost and handling characteristics.

Today, all VIF is made in Europe and must be imported, thus resulting in much higher cost than standard film. Also, most VIF products are more difficult to lay than standard films in that they are prone to linear sheer under too much tension. Handling characteristics among VIF materials differ significantly, but all are based on polyamides, such as nylon, for their barrier properties and these polyamides do not stretch well. Also, none are embossed at the present time. 

In the past two years, we have examined the barrier properties of metalized films under field conditions, first with Inline and more recently with methyl bromide. In each case, application of Inline or methyl bromide in conjunction with metalized film greatly increased the retention of the fumigant.

In the case of methyl bromide, nutsedge control was obtained with 175 lb./acre of 67/33 under Canslit metalized film that was equal or superior to that obtained with the full 350 lb./acre rate under standard ldpe or hdpe film in each of  four experiments. Grower trials confirmed these results. The retention of methyl bromide and resultant nutsedge control with Canslit metalized film was similar to what we obtained with VIF at every rate of methyl bromide, ranging from 88 to 350 lb./acre of 67/33. 

While it is possible to use VIF or Canslit metalized film to reduce methyl bromide usage rates by one-half, successful use involves more than just reducing gas flow and laying mulch film. Methyl bromide has a high vapor pressure, which means that at typical application temperatures it rapidly becomes a gas and can do so even within the tubing and gas knives. This is an advantage for reduced rate application, but does not solve one inherent problem — uniformity of application.

Typical gas rigs employ three knives per bed. A good fumigation job requires that all three knives deliver the same amount of product per minute so the application rate is uniform in the area being fumigated. When the rate is reduced, there is less fumigant in the system and more opportunity for the formation of bubbles as the methyl bromide “boils.”  This “boiling” easily can be visualized by inserting small sight glasses in the application equipment at the flow divider just ahead of the tubes which carry the fumigant to the knives. (Fig. 1) 

Under normal conditions, a certain amount of back pressure exists in the application system and can be measured at the flow divider by installing a pressure gauge. Application of a full 350 lb./acre rate will generate in excess of 30 psi of back pressure at this point. Reducing the methyl bromide flow rate to deliver lower rates per acre will reduce the back pressure measured at the flow divider. 

Our experience indicates that back pressure below 15 psi results in nonuniform distribution to the three knives which means inequalities in rate across the bed.  Usually the edges suffer the most and this can be observed later in the season as poor nutsedge control on bed shoulders. 

To increase back pressure when using low rates of methyl bromide or any other fumigant, you must decrease the flow capacity of the delivery system between the flow divider and the gas knives. This can be accomplished in two ways. First, you can use a smaller diameter tubing to deliver fumigant to the gas knives. Standard gas rigs use 1/4 inch inside diameter tubing. We have found the use of poly tubing ranging from one-sixteenth to one-eighth inch inside diameter is necessary to achieve balanced or uniform delivery of greatly reduced rates of methyl bromide. Tubing of this size is not readily available, but can be obtained and is an important modification for using reduced rates of methyl bromide with a highly retentive film. 

Fine tuning of flow capacity or rate of any tube can be accomplished by increasing or decreasing the length of the tube connecting the flow divider to the gas knife. There is a certain amount of friction loss of flow within any size tube which increases with increased length and decreased tubing inside diameter. Typical length for one-sixteenth and one-eighth inch tubing is 5 ft; although  longer tubing has been used when trying to achieve really low rates.

Color-coded tubing is available which can be a big help when adjusting flow rates. Yellow tubing has the thickest walls and smallest inside diameter of one-sixteenth inch. Black tubing is available in one-eighth inch inside diameter (Fig. 2 below). These tubes all fit the same size connector, making it easy to switch from one flow capacity to another. Select the tube needed for the desired flow capacity, then once installed, adjust the flow regulator valve for the required flow rate on the flow meter. 

A second way to decrease flow and increase back pressure is to use orifice plates (Teejet flow regulators) in the tubing at the top of the gas knife fitting. To use these plates, you have to know what flow rate you need in each tube. Since the flow rates of orifice plates are based on water, you have to do some mathematical conversions to methyl bromide or choose one on the high side and try it. You do not want a plate which gives you the exact same flow rate as what you need; you want one with a slightly higher flow rate so that clogging potential is lowered.

The plates have numbers stamped on them which tell you the size of the hole in the plate (Fig. 3 below). Keep a supply of various sizes on hand. Orifice plates work over a more narrow range of rates than tubing because the restriction in flow occurs at one point rather than over a length of tubing. 

The system we use is commercially available (manufactured by Mirruso Enterprises Inc., available through Chemical Containers Inc.) and constitutes an easily installed, simple modification. It consists of a flow divider with a small sight glass for each knife, a 0 to 30 psi pressure gauge and small diameter poly tubing. The sight glasses are equipped with standard quick connect (insert friction connectors) couplings on top so the poly tube can be connected and disconnected easily. Similar couplings are located on the top of the gas knives.

Sight glasses allow you to monitor flow and detect plugging of chisels or lines. Plugging can be a significant issue with low rates of fumigant; thus, fumigant filtration is important and filters must be checked periodically and maintained free of trash to assure consistent flow.

When using reduced rates of fumigant: the flow rate has been greatly diminished so accuracy and uniformity of delivery are critical. A common observation on commercial farms is tractor movement as soon as the fumigant flow valve is opened. There is a much longer delay in supplying all the knives uniformly when the rate is reduced, so tractor movement should not begin until all lines are fully charged. This condition easily can be monitored by observing the sight gauges and back pressure gauge. 

Once the back pressure stabilizes, fumigation can begin. Addition of an inline check valve at the top of each gas knife can be beneficial because it diminishes loss of fumigant out of the line to the knife. By keeping the line full all the way to the gas knife, there are fewer delays in fumigant delivery and less time wasted purging air from lines. This would be especially important for those growers who use radar controlled fumigant delivery systems.

Rate reduction with methyl bromide works when combined with a highly retentive mulch film like VIF or metalized film. In addition to the use of the right film, success requires close monitoring, assuring not only the correct rate, but also uniform delivery to all three knives in the bed. Nonuniformity guarantees poor fumigant performance at any rate, but with reduced rates of methyl bromide, the results can be even more dramatic.

The simple modifications described can greatly improve uniformity of delivery and performance. These modifications are relatively inexpensive and are readily available as a package. Before trying rate reductions growers should modify their fumigation equipment to allow better control over uniformity of flow. This can mean the difference between success and failure.

James P. Gilreath is a professor and Bielinksi Santos is a post doctoral associate at the UF/IFAS Gulf Coast Research and Education Center, (813) 633-4127. Phyllis R. Gilreath is an Extension agent for Manatee County, (941) 722-4524 ext. 229.