Fly control is recognized as a profitable summer management practice. Depending on a number of characteristics of the farm/ranch operation, several methods of fly control may be used. More and more producers now prefer the convenience of a feed-through product.

Currently, there are a number of free-choice feed-through products available. The majority of products utilize one of two active ingredients – an organophosphate (Rabon®*) or methoprene. Producers that recognize the value of summer mineral supplementation and like the convenience of a free-choice product may question which active ingredient provides the best protection based on investment. Several aspects of each product should be investigated.

Because methoprene and the organophosphate have been cleared for use against horn flies in a feed-through product by the Food and Drug Administration (FDA), effectiveness is not an issue. Once either product is deposited in the cattle manure, it breaks the horn fly life cycle, interrupts development, and thus stops population growth of horn flies.

Analyzing the Differences
The FDA’s 2005 Feed Additive Compendium defines the differences between methoprene and the organophosphate. The FDA specifies the information that must appear on any product label and the approved additive dosage rate. The FDA statements for each active ingredient are given in Table 1.
 

Table 1 Comparison of Methoprene Versus Organophosphate Feed-Through
	Methoprene	Organophosphate Feed-Through
Caution Statement	Keep out of reach of children.  Discard empty container according to local regulations. Never reuse empty container.	Keep out of reach of children.  Harmful if swallowed or absorbed through the skin.  Causes moderate eye irritation.  Avoid contact with eyes, skin or clothing.  Prolonged or frequently repeated skin contact may cause allergic reaction in some individuals.  Wash thoroughly with soap and water after handling.  For personal protective equipment (PPE), mixers and handlers must wear: long-sleeved shirt and pants, shoes and socks and chemical resistant gloves.  User safety requirements are to follow manufacturer’s instructions for cleaning/maintaining PPE.  If no such instructions for washables, use detergent and hot water.  Keep and wash PPE separately from other laundry.  User safety recommendations are that users should wash hands before eating, drinking, chewing gum, using tobacco or using the toilet.  Users should remove clothing immediately if pesticide gets inside.  Then wash thoroughly and put on clean clothing.  Users should remove PPE immediately after handling this product.  Wash the outside of gloves before removing.  As soon as possible, wash thoroughly and change into clean clothing.  This pesticide is toxic to fish.  Do not contaminate water when disposing of equipment wash water.
Dosage	Approved dosage for beef and dairy cattle is 22.7–45.4 mg/100 lb of body weight/month.	Approved dosage for beef and dairy cattle is 2100 mg/100 lb of body weight/month (0.07 grams/cwt./day).
Indications for Use	Insect growth regulator for continuous feeding during the fly season to prevent the breeding of horn flies in the manure of treated cattle.	Control of fecal flies in manure of treated cattle.  Prevents development of face flies, horn flies, houseflies and stable flies in the manure of treated cattle.

The organophosphate feed-through does have a label for the control of fecal flies in the manure of treated cattle. It prevents development of face flies, horn flies, houseflies, and stable flies in the manure of treated cattle. Methoprene is only labeled to prevent the breeding of horn flies in the manure of treated cattle only. Consider the facts that house and stable flies are only a problem in areas where manure accumulates. As a result, they are primarily pests of confined rather than pastured cattle. They also breed in materials other than cattle manure. Therefore, just treating cattle manure may not control these two species of flies. Face flies are strong fliers that tend to travel from one group of cattle to another. Therefore, treating cattle via manure may not ensure freedom from face flies as they may migrate to neighboring herds.

Development of Resistancy
Chemical resistance would develop most rapidly when adult flies are being treated rather than during immature stages. Under these methods of control, adult flies that tolerate a chemical pass this resistancy to their offspring. This explains the failure of some ear tags, which contain pyrethroids as their active ingredient, to provide adequate fly control. In an effort to prolong the use and effectiveness of these tags, manufacturers use different classes of insecticides in addition to pyrethroids in tags, including carbamates and organophosphates. In some cases, this has resulted in low-level cross-resistance to that class of compounds as well.

In addition, in the late 1970s organophosphates were used exclusively in ear tags. Failures of these products were noted in Florida and Georgia in the early 1980s. These failures were attributed to resistance development of the horn fly population. While a feed-through product containing organophosphate does not treat adult flies, some resistance to organophosphates may exist due to previous treatments of the fly population with this class of compounds. When cattle were fed organophosphates over an 18-month period, researchers from Georgia demonstrated development of resistance to organophosphates.

Methoprene used in IGR products has only been used as a feed-through to treat immature flies. No incidence of resistance has been documented by users since it was introduced to the market in 1977.

*Not a trademark of ADM.