Analysis of the pre-lay phase in two genetic lines of hillbilly (Campero-INTA type) breeders

Management of parent stock is the poultry activity demanding the highest level of care, as mistakes made here are multiplied and have effects on the entire chain of production, including very especially the performance of progeny (North, 1993). Regardless of the criteria that are managed to divide the cycle of these birds, the important thing is that the early stages of life mark the production future (Buxadé Carbo, 1988; North, 1993). In this sense, sexual maturity and the onset of egg production are early indicators of the evolution of the previous process, thus allowing further inference on the future production performance of the flock(Robinson et al., 2007). One aspect of relevance within the cycle is the definition of the time in which sexual maturity occurs in the flock and the physical state in which they achieve it. It is a biological fact and as such, takes place at some point in life and under specific circumstances (Leeson & Summers, 2000). If the phenomenon is left to chance, often the conditions under which will be produced, will not be appropriate to maximize production yields (Reddish, 2004). Because the physiological processes involved in reproductive performance of birds reveal variations related to race or the genetic line considered, is the reason why the present study evaluates the breeder starter phase in two different parental lines (E & T) from which the Campero-INTA chicken is obtained (Bonino and Canet, 1999).

The study was conducted at the Poultry Multiplication Center of the Agricultural Experimental Station of INTA Corrientes, located on National Route N º 12, km 1008, El Sombrero - Corrientes, Argentina. The behavior of two genetic lines (E and T) of females for the production of the Campero-INTA chicken, was analyzed in a completely randomized design. Rearing (0 to 6 weeks) was performed in a closed poultry house with windows and smooth cement floor, in which divisions were made to accommodate the trial birds (boxes of 4 m deep x 3 m wide). At the end of the sixth week, pullets were transferred to a semi-open rearing house, divided into 6 pens (3 for each genetic line). In the first four weeks of life, feeding was controlled, with amounts set according to a protocol developed based on previous work (Terraes et al., 2010), with a daily feed distribution system. During this study, three commercial type feeds were used: starter (0 to 6 weeks), rearing (6 to 20 weeks), pre-breeding (20 weeks to sexual maturity).
The study covered the period of the onset of photostimulation (20 to 24 weeks of the cycle) determining the age at sexual maturity (collection of the first egg laid in the flock), comb index (comb length times height expressed as mm2 ) and the percentage of production at the onset of lay (24 weeks). The results were expressed as mean and standard deviation, and an analysis of variance to detect statistically significant differences between the response variables with a significance level of 5% was carried out.

The age at sexual maturity per genetic line is shown in Table 1. The analysis of variance for sexual maturity showed no significant differences.

Table 1. Mean and standard deviation of age at sexual maturity (days) per genetic lines

It should be noted that the lack of significant differences in the onset of sexual maturity was found, taking into account the genetic lines, in which  different breed crosses are involved. Our results are consistent with those observed by some authors, including Robinson & Renema (2003), who observed no differences during the pre-breeding phase despite being different genetic lines. However, the same authors reported variations in relation to age at first egg, body weight and significant differences in egg production at 58 weeks of the cycle (among other variables), in a paper in which three feeding and nutrient allocation programs were used to evaluate the effects of overfeeding on previous stages to start breeding (Robinson & Renema, 2003).
In this study, significant differences were found in comb index for the E line (p<0.05) at 20 weeks of the cycle. These results are presented in Table 2. 

Table 2. Mean and standard deviation of the comb index (mm²) per genetic lines. Age: 20 weeks

Consistent with these results, Eitan et al. (1998) found a relationship between the size of the comb and the genetic line at the time of sexual maturity. These authors analyzed the effects of genetics, photoperiod and feeding on the development of the comb in an experiment with broiler breeders and layer breeders pullets. The two experimental groups were subjected to restricted feeding during the rearing, showing an effect of genetics and feeding on body weight and comb development before the onset of sexual maturity. The size of sexual attributes adequately expressed the interaction of environmental and genetic factors that led to the deluge of events involved in the onset of sexual maturity. The percentage of production at 24 weeks (laying onset) is shown in Table 3, which it can be seen that the genetic line E showed percentages higher than T, although these differences were not significant (p>0.05).

Table 3. Mean and standard deviation of the production percentage (%) per genetic lines. Age: 24 weeks

Although no significant differences in age at sexual maturity and the oviposition percentage at the beginning of this phase were found, under conditions of this trial, we can conclude that the comb index behaved as a variable with predictive value of the onset of sexual activity in the genetic type that was analyzed. Beyond the evidence associated with genetics of each line of birds, results reported herein could be differentially expressed later, during the pre-breeding and reproduction.

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Robinson FE, Renema RA, Zuidhof MJ. 2007. The pullet to hen transition: Why this is the most critical time in female broiler breeder management. Disponible en: http://www.poultryindustrycouncil.ca/pdf/HC%2007%20PDF/Robinson%20-%20PIC%202007
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