Genetic selection has brought amazing progress in broiler performance, as demonstrated by improved growth rate, feed conversion and breast meat yield over the past few decades. This selection has concentrated mainly on the breast muscle, particularly motivated by North American and European market requirements.
Parallel to this growth in performance, however, several metabolic diseases have started to emerge, myopathies being one of them. White striping, woody breast, green muscle disease and cranial dorsal myopathy, for example, can all have negative effects on meat quality and are consequences of the changes in growth, histology and metabolism of muscle fibers.
These conditions all affect the quality and hence value of what is, in many markets, the most important part of the bird. However, it may be possible to alleviate these syndromes through nutrition.
White striping of breast muscle
White striping is characterized by the appearance of stripes parallel to the muscle fiber, especially in breast muscle fillets, but it can also be found in thighs. Mainly seen in the raw meat of broilers slaughtered at a heavier weight, the stripes can be classified according to severity.
The condition is characterized by localized lipidosis associated with necrotic myofibers and developed connective tissues, termed fibrosis. The myodegenerative muscle fibers are mainly those showing the highest mean cross-sectional area, while samples with severe myodegeneration have myofibers of different diameters and without the characteristic polygonal shape. This is likely to be caused by a lower capillary-to-fiber ratio, and greater intercapillary distance is related to the myodegeneration.
White striping causes the protein to have a poor ability to bind marinade solutions and to retain liquid during cooking, in both unprocessed and marinated meat. Consumers have demonstrated a strong distaste for breast muscles showing moderate and severe striping, due to a fatty or marbled look to the product.
Nutritionally, normal fillets have lower percentage lipid and greater protein content when compared to fillets showing severe white striping. Also, normal fillets show greater percentages of saturated fatty acids than fillets with a severe striping appearance, but proportions of all monounsaturated fatty acids as well as linoleic and linolenic acids are greater in severely affected fillets. A reduced level of omega-3 fatty acids is also associated with white striping.
Studies have indicated that nutritional muscular dystrophy occurs in chicks fed diets low in selenium, vitamin E and sulfur-containing amino acids. Adding methionine, cystein or vitamin E may aid in the prevention of muscular dystrophy in chicks, while selenium has been shown to be only partially effective.
However, a trial that increased levels of vitamin E (15 to 400 IU of vitamin E/kg of feed) concluded that dietary vitamin E level is not associated with white striping in broiler breast meat and a combination of antioxidants with omega-3 fatty acids (EPA + DHA) might be more beneficial in addressing the condition.
White striping can be accompanied by another type of muscle abnormality, called woody breast, which is characterized by the macroscopically visible hard, bulging and pale area in the caudal part of the fillet. Woody breast and white striping exhibit similar histological changes consisting of moderate-to-severe polyphasic myodegeneration and regeneration, as well as variable amounts of interstitial connective tissue accumulation or fibrosis.
Woody breast results in much lower marinade uptake and higher cooking losses, compared to white striping. Therefore, woody breast abnormality results in more adverse effects on meat quality traits compared with white striping.
High amounts of liquid losses may also result in increased toughness of meat. This woody breast hardness trait can affect sausages and nuggets, in cases where this type of meat is used for further manufacturing.
Nutritional approaches for preventing woody breast should be the same as that mentioned previously for white striping in chicken breast meat.
Deep pectoral myopathy
Deep pectoral myopathy, also called green muscle disease, is caused by the mortality of tender muscle cells. It is more prevalent in heavier broilers, and the reason for it is that blood flow to tissue is too low (ischemic hypoxia).
Green muscle disease results from the vigorous and excessive activity of the two breast muscle groups, but only the smallest muscle, or tender, is affected.
During contraction, the muscle volume increases, due to an increased blood flow to meet the local demand for oxygen and nutrients. The tenders have a rigid cover and are located in a tight chamber – the keel on both sides and fillet above – and because of this, they cannot expand to accommodate this increased flow. The muscle dies and necrosis is observed.
This disease is very much related to management and excessive movements in the days preceding loading and slaughter.
Lack of feed and water, long lighting programs, and excessive staff activity and noise are the main causes of green muscle disease. Although green muscle disease is linked to management problems, a recent field trial in Brazil has indicated that the use of a mycotoxin binder with an antioxidant mitigated its appearance.
Cranial dorsal myopthy
Heavier birds are more prone to develop cranial dorsal myopathy. It affects the Anterior latissimus dorsi muscle, which is involved in supporting the wings, preventing them from bending forward. The muscle, formed by Type I slow-oxidative fibers, is highly dependent on oxidative phosphorilation to get adenosine triphosphate (ATP). Macroscopically, a section of skin in an area affected by dorsal myopathy shows a yellow gelatinous edema.
Further examinations indicate multiphasic lesions, with hyaline degeneration, flocular necrosis, regenerating fibers and extensive proliferation of fibrous and fiber-adipose connective tissue. Cranial dorsal myopathy resembles a disease with peroxidation origin. In a field trial in Brazil, the use of selenium-yeast mitigated this metabolic disease, from 2 percent down to 0.5 percent.
Live animal performance has made extremely large strides forward over the past several years; however, the nutritional requirements on which we base our dietary formulations is decades old in many cases, and is putting stress on our flocks' muscular growth.
While meat quality issues, such as water-holding capacity, appearance and texture, have traditionally been the responsibility of the processor, the poultry industry should be looking to earlier stages in production for solutions. Achieving optimal muscular health is not possible using a single additive or intervention, but rather a holistic strategy is needed to improve flock health in commercial poultry operations.