Fertility of five maternal strains of breeding hens used in the production of free-range chickens. A longitudinal analysis

Embryodiagnosis is a routine tool in poultry, which makes it possible to determine the age of death of the embryos during the incubation process and its possible causes (flat & Di Matteo, 2005). This technique permits to assess both the conditions of the fertile egg (disinfection, storage temperature, feeding in the breeders, etc.), as certain variables related to the operation of incubators and hatchers. The procedure consists in planning a routine according to the needs of the incubation plant and the production system and its application gives information on the procedures that should be modified  on the breeders' farm and/or on the incubation floor (Antruejo et al., 1996). There are three classical indicators to summarize, roughly, the efficiency of the overall production process of BB chicks: fertility (fertile eggs / incubated eggs), the incubability (eclosioned eggs / fertile eggs) and feasibility (eclosioned eggs / incubated eggs). Considering the process of incubation as a complex system, it is clear that the first critical component, which influences the output of this system, is the fertility of eggs to incubate that is affected by genetic and environmental factors associated with the breeder lineage (Plano, 2003). Since it is a variable that is closely related to the biological efficacy, fertility presents a strong non-additive genetic basis and is likely to suffer inbreeding depression if the effective size of the populations of breeding is reduced (Falconer & Mackay, 1996). The objective of this work was to study the dynamic behavior of the fertility of eggs produced by females of five maternal strains of breeding stock throughout their first egg-laying cycle. The evaluated populations present a low effective size (no more than 100 females and 10 roosters) and they are devoted to the production of hybrid hens which, in turn, are used for final crossing with a paternal line enhanced to produce a three-way hybrid free-way chicken.

Evaluations were performed between weeks 32 and 59 of age in five maternal lineages (A, E, CE, DE and ES), intended for the production of simple hybrid hens to be used as breeders in the production of free-range chickens. The stocks kept on floor, together with contemporary roosters from the same genetic group. A total of 5000 eggs from each of the lineages were considered. The collection was conducted five times a day, eggs were disinfected with a 50% (P/V) potassium permanganate in formaldehyde at 40%, eggs were classified by size, unfit eggs (broken, double yolk and deformed) were discarded and the remaining eggs were stored at a temperature of 15 ± 1º C. Every 14 days were the incubators were loaded, alter placing each breed separately, on trays and preheating for 12 hours. On day 18th of incubation the eggs were examined through a light and the clear ones were removed (infertile and early embryonic mortality) and the eggs with viable embryos (fertile) were transferred to the hatcher. Eggs that were considered infertile were opened and the truly infertile ones and the ones with early embryonic mortality were recorded. The fertility of each breed at any time was calculated as the proportion of fertile eggs transferred to the hatcher, with respect to the total number of eggs placed for incubation, without considering those with early embryonic mortality.

Fertility-age data were adjusted by linear regression after confirmation of the hypothesis of linearity with runs test, and the effect of the strain on the values of the slope and the elevation of the respective regression lines was assessed via a co-variance analysis (Graph Pad Software; Sheskin, 2000).

The following table summarizes the values of the estimators of the parameters of the regression lines (slope and origin ordinate) resulting from the linear adjustment of the fertility values of eggs placed in the incubator, depending on the age of females from the breeding stock.

Table 1. Estimators of the parameters of the regression lines (slope and origin ordinate) resulting from the linear adjustment of the fertility values of eggs placed in the incubator, depending on the age of females from the breeding stock.

All the slopes were negative and statistically significant (other than zero). A significant effect of lineage on the slopes of the regression lines was observed (F = 3,879; P = 0,006), preventing overall comparison of the effect of maternal lineage on elevations. Visual inspection of the graphic representation allowed making clusters of specific behaviors. No differences were observed between the slopes corresponding to strains A, E y CE (F = 0,096; P = 0,9086). This made it possible to calculate a common slope for the three groups (b = -0,5733), and calculate the effect of the maternal strain on the elevation of the respective straight lines, which turned out to be statistically significant (F = 7,363; P = 0,0016). This difference is attributable to lineage E, which showed the greatest height (greater fertility over the entire evaluated period), since there were no differences between the elevations of the regression lines corresponding to lineages A and CE (F = 0,011;) (P = 0,9156), which allowed to calculate a common elevation for both (a = 103,39). Lineages ES and DE showed a high fertility of their eggs at the beginning of the period evaluated; nevertheless, due to the slope values, this fertility declined rapidly with growing age.

The evaluated lineages had marked differences in the fertility behavior of their eggs, which, basically, can be ascribed to genetic reasons, since all of them were reared under identical environmental conditions. Secondly, a low fertility attributable, in part, either to low initial values or to a rapid decline in time is observed. Since that fertility is evaluated in the eggs, it is not possible to discriminate the potential differential inputs of breeding males and females at this level. The low levels of fertility recorded would be related to the actual size that these populations have maintained for years, which translates into consanguinity levels consistent with the presence of inbreeding depression. Such situation can be reversed by crossing two of these strains, since desirable heterotic effects could be expected in the offspring. This practice should be used for producing hybrid hens to be used in a scheme with an improved parental line for terminal crossbreeding production of three-way hybrids of free-range chickens.

We thank Ms. Carina Álvarez, student of Veterinary Medicine at UNR, for her cooperation in data gathering.

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GraphPad Software, San Diego, California, USA, www.graphpad.com.

Plano CM & Di Matteo AM. 2005. Atlas de patología de la incubación del pollo. RIA 34(2):75-89.

Plano CM. 2003. Embriodiagnóstico como herramienta para evaluar problemas de plantas de incubación y granjas de reproductoras. pp. 97-104. En: XVIII Congreso Latinoamericano de Avicultura, Bolivia.

Sheskin DJ. 2000. Handbook of parametric and nonparametric statistical procedures. Chapman & Hall. USA.