The main goal in broiler production is to maximise the enormous growth potential of the bird, resulting in high body weight and high breast meat yield. However, both of these objectives are negatively correlated with fertility. As a result, the fertility of broiler breeders has become a challenge. The question arises whether the tipping point has been reached, and if altered broiler breeder fertility has become irreversible.
By Lidija Perić, associate professor, University of Novi Sad, Serbia, and Katarzyna Glębocka, technical coordinator, Solutions Deployment Team, Alltech Europe
When discussing chicken meat production, the focus is often on the final product, being the growth and performance of the broiler chick. The actual “creators” of such a powerful bird are breeding companies that have a history of success in breeder management. Their goal is to maintain or even widen their share of the global market with high-performing, easy-to-manage breeders and broilers. Problems arising from inferior parent breeding stock or inferior broiler chicks can lead to regional loss in the market and take considerable time and effort to rectify. It is for this reason that broiler breeding programmes should focus not only on growth and performance, but also on the reproductive performance of the parents, such as egg number, fertility and hatchability.
Nutritionists are constantly looking for ways to improve feed formulation. However, breeders are already pushed to the limit, meaning that any nutritional mistakes could significantly damage performance and therefore profitability.
Genetic selection and fertility
Broiler breeder performance poses a great challenge to the geneticist because broiler breeder selection constitutes a fine balance between growth optimisation and maintaining a competitive level of reproduction. There is a negative correlation between growth parameters and reproductive traits such as egg numbers and fertility. If growth-type traits are of prime focus in commercial strain selection, there is always concern about reproductive traits, which have much lower heritability of 0.05-0.20. In a very detailed study, Reddy and Sadjadi predicted a decrease of approx. 0.5% in fertility in broiler breeder flocks between 1991 and 2000 in the absence of a balanced breeding approach of this trait.
The introduction of high breast yield strains has led to increased concern about male aggression and its negative impact on mating behaviour and fertility. Some authors have reported an increase in aggressive behaviour in broiler breeder males over the past decade, resulting in increased mortality and reduced fertility in some breeder flocks. The problem can be reduced, to a certain degree, by placing fewer males in the breeder house. A notable example was seen at White Plymouth Rock breeders where the reproductive performance of high (HWL) and low (LWL) body weight males was recorded. The data indicated that the HWL males had greater numbers of abnormal spermatozoa, especially around the midpiece and head, than the LWL males.
The fertilising potential of males varies. Within a flock, some roosters are extremely fertile and create high quality sperm, while other roosters are sub-fertile. This variation in rooster quality is caused by management, environment, nutrition, and genetics. It is generally considered that most male fertility problems are related to mating behaviour, although nutrition and obesity can also play a major role. As roosters get heavier there is a higher incidence of fertility problems correlated with sperm motility, sperm density and more dead or abnormal sperm. Heavier males also have a reduced libido leading to lower fertilisation rates. Underweight males are also likely to have problems with fertility. If the expected 21-29 week growth is not achieved, there is no persistency of fertility, and this declines faster with age. This occurs with both over- and underweight males. Breeder target weights and uniformity higher than 80% is recommended.
It is generally assumed that if the hen is capable of producing eggs and the sperm is viable, then fertilisation will occur. Infertility may be due to an absence of semen in the oviduct, mostly as a result of problems with mating frequency or mating success. Studies in females indicate that selection for higher body weight results in greater proportions of erratic ovulations, and that defective egg syndrome will result in a lower number of settable eggs. This is influenced by a variety of factors, such as genetic, nutrition, environment, and behaviour.
The fertility of a broiler breeder flock usually declines after 40 weeks due to aging. Both females and males are responsible, but there seems to be a higher incidence of male infertility due to two main factors, namely mating activity and sperm quality. However, flock fertility is inversely correlated with growth rate.
There are several management techniques to maintain fertility and to enhance mating activity. Assuming optimal environmental conditions, several management practices can be implemented, including a body weight control and feeding programme, male placement and sex ratio, spiking and intra-spiking. The first two are designed to ensure good mating efficiency through male fitness and hen receptivity. The other two practices are mainly meant to compensate for the decline in mating interest (libido). Recent research has shown that in the second half of the reproduction cycle, candling fertility decreased in all examined flocks, although the true fertility did not decrease to such a degree. This indicates an increase in early embryo death. The authors concluded that, from a practical point of view, changes in sex ratios, widespread spiking, or the complete exchange of cockerels between flocks, do not improve the fertility level in the second half of the reproduction cycle.
Influence of nutrition
It is generally accepted that nutrients that influence general body size and body composition, such as protein and energy components, can influence female fertility. Research has shown that lowering the crude protein level in the diet from 16% to 14%, 12% or 10%, significantly increases fertility from 91.6% to 95.4% respectively.
It is generally assumed that this is as a result of the decreased body weight as hens on the lower protein diets were smaller throughout the whole experiment even though feed and energy intake were similar for all treatment groups.
Also, in male diets the crude protein ration can be as low as 9-10% for optimum semen production. However, the formulation of low protein diets can improve fertility by up to 2-3%.
Furthermore, vitamins and trace elements can have an important influence on fertility and hatchability, particularly vitamin E, carotenoides and selenium, which act as natural antioxidants and have a protective effect on the sperm and embryo.
Selenium became standard
An element that has been shown to significantly improve reproductive performance is selenium. Its deficiency in breeder diets gives clear symptoms (reduction in fertility, hatchability and embryo survival) and, as such, supplementing diets with selenium has become standard. Research has shown, however, that the form selenium is supplemented in is important as it affects the efficacy of the element. So-called “organic” selenium gives better responses due to its biological composition, whereas inorganic selenium is poorly retained. In contrast, selenomethionine, (such as Sel-Plex®, organic selenium based on a specific strain of Saccharomyces cerevisiae CNCM I-3060), can either be transported to the liver for incorporation into selenoproteins, or directly to other tissues for incorporation into tissue proteins. Selenomethionine, in fact, is a storage form of selenium in the animal’s body. The central advantage of supplying organic selenium is the ability of the animal to store this natural form. Tissue reserves of selenium are critical during times of increased need, such as immune challenge and reproduction.
Better semen possible
Early studies have shown that organic selenium could improve semen production and quality (Table 1). Examination of the individual semen smears revealed that organic selenium supplementation had resulted in superior semen quality as judged by differential counts of normal vs. abnormal forms. The authors also concluded that organic selenium is more effective than sodium selenite in the maintenance of sperm integrity and sperm production, showing that it is the better form of selenium for the avian male.
Trials that looked specifically at mineral supplementation of the diets of reproducing hens revealed the importance of certain minerals. When 50% of inorganic minerals were replaced with a combination of organic zinc, manganese (such as Bioplex®) and selenium, positive effects were recorded on many reproductive parameters. Most importantly, the fertilization of hatching eggs was significantly improved (Figure 1). The results clearly show that in poultry, where reproduction is on the very border of physiological capacity, the use of organic trace elements is a must.
Tipping point reached?
Other studies have shown that organic selenium can improve egg production, fertility and hatchability of settable eggs in females. Here, better transfer to the egg significantly increased the total selenium in embryonic chicks, the total selenium in eggs (yolk and albumen) and the egg shell breaking strength. Based on Eden’s research, as well other trials, it was concluded that this form of organic selenium can improve fertility and increase the duration or persistency of fertility in breeder flocks.
It is interesting to note the results of another recent study examining the quality of day-old chicks taken from parent stocks supplemented with either inorganic or organic forms of selenium. In this case, chicks from breeder flocks fed organic selenium had significantly higher body weight and body length than the chicks coming from selenite-fed breeders. The uniformity of the chicks from these parents was also greater, although not significant (Table 2).
Breeding companies and producers seem to have reached physiological limits in breeder flocks. It is questionable whether this is the tipping point. Close examination of genetic backgrounds will be necessary to secure the future of broiler breeders. Until this happens, very careful management and nutrition practices are crucial for the success of breeder flocks.