Reproductive characteristics of roosters

A distinct physiological characteristic of female birds is the regression and atrophy of the right ovary during embryo development. In males, the left testicle is usually larger than the right, which may contribute with a greater share of sperm production. The relationship between testicular weight and sperm production raises the possibility of improving gonad development through modifications in nutritional management practices, health status and environmental conditions, e.g. to suppress particular undesired physiological characteristics or features.

Unlike mammals, the gonads of male poultry are located within the coelomic cavity. In most mammals, sperm does not remain viable at body temperature, thus testes remain in the scrotum, where the temperature is two to three degrees below body temperature. Temperatures above or below the range of thermal comfort for birds (18-25°C) directly interfere with their metabolism, predisposing them to secondary disturbances caused by thermal stress. A reduction in sperm production and fertility in birds in summer, especially in hotter climates like Brazil, has been reported (Daghir and Jones, Reference DAGHIR and JONES2008), and although some of this decline is related to birds reducing their dietary intake on hot days, a portion can be attributed to metabolic interference of high temperatures.

As with temperature, photoperiod plays a fundamental role in reproductive efficiency of males, and is a key environmental factor influencing the expression of reproductive traits in broiler breeders. Changes in photoperiod intensity and/or duration alter GnRh production, stimulating spermatogenesis in males and ovulation in females (Rutz et al., Reference RUTZ, ANCIUTI, XAVIER, ROLL and ROSSI2007). Manuals for broiler breeders recommend implementing a light stimulation program one week earlier for roosters than for females in order to avoid delaying production of fertile eggs. Maciel et al. (Reference MACIEL, COTTA, MURGAS, LIMA, LIMA and ALVARENGA2011) found that roosters subjected to continuous or intermittent lighting programs had higher semen production and sperm cell count. Although photoperiod is a key component of proper male breeder function, optimal results are achieved when light stimulation is combined with other elements related to environment, nutrition, health and physiological conditions.

Indicators of reproductive efficiency in roosters

In a broiler breeder flock, fertility rates are often related to male performance being the modifying agent in productivity parameters. The diagnosis of fertility in poultry would be more accurate if sperm-quality information was evaluated. However, often only phenotypic characteristics are routinely used to phase out non-standard males when hatching rates are lower than expected. Although comparatively inexpensive and easily applicable, visual assessment is not error-free and often leads to the disposal of reproductively capable birds and/or retention of individuals with low reproductive potential. Staining and comb size, leg thickness and colour, body weight and conformation (fleshing) and wattle length are some of the physical attributes that are used for visual selection of breeding roosters, with birds that have poorly developed and pale combs, colourless and thin legs, a ‘U’ shaped or striped chest, or those that are overweight or underweight or poorly developed normally being discarded. Thus, direct evaluation of sperm quality and reproductive capability should be an integral part of rooster management, which would improve the economic efficiency of ‘spiking’ farms. Routinely checking quality by examining fresh semen under a microscope is a low cost and simple method that could be often used in poultry systems.

Sperm production in roosters reaches its maximum between 24 and 30 weeks of age and begins to decline after about 40 weeks (Adjanohoun, Reference ADJANOHOUN1994). This range varies between males and is influenced by exposure to different environmental conditions and management as well. Some conditions, such as oxidative stress, are known to contribute to reductions in sperm motility. According to Halliwell and Gutteridge (Reference HALLIWELL and GUTTERIDGE1999), the first evidence that oxidative stress affects the viability and function of male gametes was observed in 1943, when it was noticed that human sperm incubated under conditions of high O₂ concentrations quickly lost their motility, and catalase addition under the same conditions, avoiding the reduction of this index.

As the other parameters related to sperm quality, sperm motility has several factors that influence their values. Bongalhardo et al. (Reference BONGALHARDO, DIONELLO, LEDUR and RUTZ2000), while studying the effects of aging on sperm motility, observed a sharp decline in sperm motility in light roosters as they aged. From a nutritional standpoint, Aghaei et al. (Reference AGHAEI, TABATABAEI and NAZARI2010) observed that low concentrations of trace minerals led to a reduction in sperm motility rates in breeding birds. As well as motility, knowing sperm concentration per millilitres of semen can further add to the evaluation of rooster efficiency. According Paulenz et al. (Reference PAULENZ, GREVLE, TVERDAL, HOFMO and BERG1995), sperm concentration should be determined in a standardised way. Therefore, the use of a spectrophotometer is more reliable and less subjective than is the Neubauer chamber, for example.

One of the most commonly used methods of predicting fertilisation potential in poultry is to test sperm penetration in the perivitelline membrane. For this, it is necessary to separate the inner membrane of fresh and non-fertile eggs and add a semen sample. The entire process occurs in an environment that simulates female oviduct conditions, allowing sperm to pass through the membrane. The visualisation of ‘holes’ in the membrane is made possible after staining (Robertson and Wishart, Reference ROBERTSON and WISHART1997), and represents an effective marker for semen fertility. The number of holes in the perivitelline membrane is an accurate way of predicting sperm fertilisation capability (Tyler and Bekker, Reference TYLER and BEKKER2012). However, to be useful at a commercial level, the sperm penetration test must be easier to apply, practical and with high sensitivity in identifying fertility rates in breeder systems. Sperm Binding-Assay (SBA), for example, is a test based on the sperm's affinity for binding to a synthetic substrate. This test has already being used to estimate fertility in rooster's in vivo (Barbato et al., Reference BARBATO, CRAMER and HAMMERSTEDT1998), and while less laborious to perform, it does require specialised equipment to do the reading.

Several tests to evaluate seminal content have been described previously (Wishart, Reference WISHART1982; Bayyari et al., Reference BAYYARI, COOK, HARRIS, MACY, SLAVIK and SKEELES1990; Donoghue et al., Reference DONOGHUE, GARNER, DONOGHUE and JOHNSON1995; Froman and McLean, Reference FROMAN and MCLEAN1996; McDaniel et al., Reference McDANIEL, HANNAH, PARKER, SMITH, SCHULTZ and ZUMWALT1998). However, the sperm cell must be able to bind to and penetrate the perivitelline membrane in order to accomplish fertilisation. Thus, the penetration test is the last and decisive criterion for discarding roosters based on their reproductive potential.

Importance of individualised diets in commercial breeder systems

Dietary management of breeding males in commercial broiler breeder farms in Brazil is often inadequate or entirely absent. Generally speaking, the unique dietary requirements of males are not specifically addressed, and they are often fed diets formulated according to the dietary needs of females from the rearing period until the end of the reproduction phase.

Temperature, photoperiod, disease incidence and nutritional deficiencies are factors that can negatively impact semen production, and thus must be controlled. In terms of nutrition, Khan (Reference KHAN2011) reported that deficiencies in some micronutrients essential for proper functioning of metabolic processes within the testes decrease both sperm production and sperm quality.

Dividing the production period into different stages reduces the gap between body weight and allows the design of specialised feeding diets that most closely meet the unique nutritional requirements of birds in each phase, reducing the excretion of unabsorbed nutrients and thereby lowering feeding costs. However, four or five diets are usually used throughout a breeders’ life, mainly due to operational and logistical costs. Unlike broilers, feeding and nutritional management in broiler breeder systems focus primarily on controlling body weight throughout the productive period in order to maximise egg production and fertility.

Despite consistent advances in genetic manipulation of poultry, which contributed to market recognition of Brazilian chicken production and exportation, searching for the best Gallus gallus model whose aggregate high fertility in female breeders and fast growth rates in the progeny continues. Interbreeding the two commercial strains most commonly used in broiler production, for example breeding strain ‘X’, which exhibits more efficient reproduction, with strain ‘Y’, which exhibits more rapid growth traits, may contribute in part to improve production efficiency. At the moment, however, such interbreeding is unfeasible, as the two strains are physiologically incompatible.

Like all vertebrate species, in addition to genetics, environmental and health factors influence a breeders’ reproductive capacity and the composition of the diet also contributes significantly to fertility rates. Depending on the production phase, the deficiency or excess of certain nutrients can directly affect reproductive performance in a commercial flock. In the growth phase, for example, the peak of Sertoli cell proliferation occurs between the 4th and 12th weeks of age (Cobb-Vantress 2008). These cells play an important role in the maturation of sperm cells. Thus, the specific dietary requirements of roosters must be met, since rearing contributes to the appropriate development of important reproduction structures.

The National Research Council (1994) recommends female breeders to be fed 15.6-23.1 g of protein/bird/day, and for heavy roosters, 9-15 g of protein/bird/day, with the maximum recommended for male protein intake roughly equivalent to the minimum suggested for females. Buckner et al. (Reference BUCKNER, RENDEN and SAVAGE1986) suggests that 10.9 g of protein/bird/day is sufficient to maintain production and sperm quality in mature roosters. Tyler and Bekker (Reference TYLER and BEKKER2012), for instance, observed a negative effect on rooster fertility when crude dietary protein levels were reduced by 12.6%.

Due the production of sperm and eggs by males and females, respectively, makes it obvious that females require higher levels of supplemental protein, and that these levels, when fed to males, may negatively influence their performance. McDaniel (Reference McDANIEL1985) demonstrated that feeding excessively protein-rich diets designed for females to roosters reduced sperm production and viability, which, in turn, had negative effects on fertility and hatch rates in commercial breeder flocks. In a recent study with New Yangzhou breeding birds, Ling and Zhiyue (Reference LING and ZHIYUE2010) reported better reproductive performance (and improved economic efficiency) when roosters were fed diets containing 13.04% crude protein. Rostagno et al. (Reference ROSTAGNO, ALBINO, DONZELE, GOMES, OLIVEIRA, LOPES, FERREIRA, BARRETO and EUCLIDES2011), factoring in productivity, metabolisable energy (ME) and feed intake (FI) of the breeders, recommended protein levels of 12.4-15.0%. For roosters, when diet ME and daily FI are taken into consideration, protein levels ranging between 11.7% (360 kcal/d of ME and FI of 130g/bird/d) and 12.6% (385 kcal/d of ME and FI of 140g/bird/d) were suggested. Using an average cost of BRL1.20/kg for soybean meal, the protein level in a diet for breeding males would decrease from approximately 16% to 10.9%, representing a savings of BRL 1,716.00/hen over the average lifetime of 20-60 weeks. As breeder farms are generally arranged in a circle formed by six poultry houses, each containing up to nine thousand roosters, it may be possible to expand economic savings even more by using diets that meet the specific nutritional requirements of males.

Implementing a nutritional management program that includes provisioning roosters with a diet that meets their specific needs can improve efficiency and increase production in broiler breeder systems. Roosters fed diets specifically designed for females (i.e. high-protein), on the other hand, experience a comparatively early decline in fertility rate, requiring their disposal at an earlier age (Hocking, Reference HOCKING1989). Additionally, specialised diets for cockerels can reduce overall feeding costs. Daily caloric intake recommendations range from 385-467 kcal/bird/day for females and 360-385 kcal/bird/day for heavy roosters (Rostagno et al., Reference ROSTAGNO, ALBINO, DONZELE, GOMES, OLIVEIRA, LOPES, FERREIRA, BARRETO and EUCLIDES2011). As with protein intake, the recommended maximum value for total caloric consumption for males is roughly equivalent to the recommended minimum total caloric consumption for females. Excessive weight is one of the most important factors associated with rooster infertility, not only because of physical aspects (e.g. over-enlarged chest volume development) that impairs copulation (Tyler and Bekker, Reference TYLER and BEKKER2012), but may also be due to metabolic changes caused by higher levels of lipid circulation (Hocking and Bernard, Reference HOCKING and BERNARD1997). The diets of breeding females are rich in calcium, a mineral essential for eggshell formation. Excessive intake of this mineral by males has been linked to decreased performance, mainly due to the interference with inorganic manganese, zinc and magnesium uptake, making them unavailable for gut absorption (Waldroup, Reference WALDROUP1996).