Green muscle disease (deep pectoral myopathy or DPM), a condition characterized by necrosis of the breast tender (pectoralis minor) muscles of poultry selected for meat production, is a problem of increasing economic significance in the poultry industry.

Breast fillets and tenders are economically the most important muscles of poultry, representing one-fourth of live weight, one-third of carcass weight and one-half of the total edible protein. Severe red, purple and green discoloration of broiler tenders is a frequent cause of trim and condemnation once discovered during the deboning process in the plant. Hemorrhage from necrotic tenders can also extensively stain broiler breast fillet (pectoralis major) muscles, requiring them to be trimmed or condemned since they appear discolored and necrotic.

One or both tenders may be affected. Tenders exhibiting DPM may be swollen and covered by a fibrinous exudate of pink to purple coloration due to hemorrhages, or these may have an overall green-to-tan discoloration and be dried up and friable “like rotting wood” due to complete myofiber degeneration.

Cost to broiler processing

Recent field studies in the U.S., Italy and Poland indicate the likely cost of DPM. Plants producing large broilers for deboning observed DPM in an average of about 0.7% of tenders. For a plant processing a million of these 7- to 8-pound broilers per week, projected losses to DPM are about a ton of condemned tenders and a similar weight of fillet trimmings each week. This would result in an economic loss of $7,000 per week or $350,000 per year.

Total losses to the U.S. industry, taking into account differences in DPM susceptibility due to age, are projected at about $16 million annually.

How and why DPM develops

DPM develops during repeated contraction of the muscles, when circulation to muscle tissue is increased markedly to supply oxygen and nutrients. The muscles swell due to an increase in intracellular fluid. The tenders of chickens increase in weight by about 20% due to this swelling.

Most muscles have room to expand so an increase in size does not impair function. However, because broiler tenders lie in rigid compartments, bordered by the sternum, keel and an inelastic fascia, they cannot expand during activity. Therefore, muscle swelling impairs blood flow due to an increase in intramuscular pressure (it “strangulates” itself) and ischemia rapidly develops.

Why is DPM more prevalent?

It has been hypothesized that intensive selection for breast meat yield once may have contributed to the prevalence of DPM in turkeys, where the defect appears to be inherited. Up until 1990, few broilers were deboned, and most were marketed whole or as bone-in parts. Therefore, DPM typically went largely undetected at the processing plant. The relatively few lesions occurring were sometimes discovered by consumers after cooking, which caused some complaints and product rejects or returns. This generally resulted in relatively minor economic losses or hard to detect losses in consumer acceptance.

Since the percentage of broilers that are deboned, and hence the weights to which they are being grown have increased steadily over the last 20 years, both the incidence and detection of DPM in the plant has increased. Concurrently, there has been an increase in genetic selection pressure for greater breast meat yield in broiler stocks.

Recent field reports from deboning plants in the U.S. and worldwide verify an increasing rate of occurrence of broiler DPM. The incidence is expected to increase in the future, as selection for breast meat yield continues and broilers are increasingly marketed at heavier weights.

Broiler growth and yield studies

Over the last decade, research at Auburn University has monitored the occurrence of DPM through broiler growth and yield studies with commercially available strains of broilers. In these studies, spontaneously occurring DPM varied from 3% to 17%, seemed to be higher in broilers with greater growth rates, decreased in warmer weather, and was more common in males than females.

More recently, an “encouraged wing flapping” (EWF) technique used to assess susceptibility to DPM has shown different results. The gender effect was found to be relatively slight, and the influence of weight when age and sex effects were accounted for was negligible. However, DPM occurrence was greater in breast meat yield than cut-up strains of broilers.

DPM in broiler trials

In a limited preliminary trial on likely causes of DPM in the industry, lesions spontaneously occurred in 12% of control broilers, while only 5% of those driven over migration pipes twice daily from 10 to five days before slaughter developed DPM. However, 20% of those subjected to light intensity increases and caretaker disturbances daily during the same period developed DPM. In research funded by U.S. Poultry & Egg Association, we are investigating the effects of concurrent increases in light intensity and caretaker disturbance in growing broilers.

Influence of broiler activity, lighting

Differences in photoperiod and intensity are known to markedly influence broiler activity levels. Increased activity levels were hypothesized to reduce the occurrence of DPM development by keeping tender muscles in better condition so that they withstand periods of flapping that might otherwise induce DPM.

Recently, the authors saw a reduction in spontaneously occurring DPM when broilers were provided ramps that had to be climbed over and “flown” off of to go from feed to water. However, activity inducing lighting programs had no effect and the incidence of induced DPM was not affected by either ramps or shorter photoperiods of brighter intensity.

These results indicate that while ramps limited DPM susceptibility to naturally occurring inducers, a lighting program believed to increase activity did not, and neither reduced susceptibility to a strong artificial DPM inducer (the EWF technique).

Controlling DPM

Multiple genetic, physiological, and management factors may play a role in the DPM susceptibility of broilers. At present, the key to controlling this problem is to limit sudden and excessive wing activity through reducing stress and flock flightiness particularly after 35 days of age.

Flightiness or excessive wing flapping may result from feed and water outages, human activity in the house, the use of loud equipment in or close to the house, increased day-lengths or light intensity, increasing light intensity episodically for flock inspection, or house preparation for feed and water withdrawal or catching.