Analysis of the brooding and rearing phases of two genetic lines of hillbilly (Campero-INTA type) breeders

Assimilated poultry breeds (Rhode Island, Plymouth Rock, New Hampshire, etc.) are used in genetic improvement programs to produce breeds adapted to the technical, production and cultural conditions of rural areas in different countries (Bonino & Canet , 1999). They have better production indexes than the native breeds but bear the hardiness and adaptability to the environment of the latter (Finzi, 2000).
Poultry production cycle in this area is conventionally divided into phases of growing, rearing, pre-breeding and breeding (Buxadé Carbo, 1988; North, 1993). The first two mark the future of parent stock, as it depends on them achieving the highest number of chicks per hen housed. During the peak of the laying period, the reproductive pattern has been already modeled and little can be done from this point on to influence flock performance (Robinson & Renema, 2003). Although in general the physiological processes involved in reproductive function are the same, there are variations related to breed or genetic line considered as the reason why this paper assesses the growth phases of two maternal breeder lines of poultry for the production of Campero INTA chicken.

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 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. Between 5 and 19 weeks, the feeding program for both genetic lines was based on the allocation of nutrients based on a feed supply curve, which stipulates daily delivery amounts per week, obtained from previous studies with Campero-INTA breeders (Terraes et al., 2010). Then a feeding program was implemented on alternate days (5/2).

At 21 weeks, body weight, uniformity, and length of tarsus-metatarsus were determined from a representative sample of pullets; results are shown as means and standard deviations, and an ANOVA was performed to detect statistically significant differences between the response variables.

The average weights at the end of the rearing period (21 weeks) are presented in Table 1, in which significant differences in favor of line T (p<0.05) are observed.

Table 1. Average body weight and standard deviation of pullets at the end of the rearing period

As suggested by Bruggeman et al. (2005), this result could be attributed to compensatory growth that occurs in birds subjected to severe dietary restrictions between 7 and 15 weeks and that in our test was expressed differently in relation to the genetic line. This finding in the T line is highlighted herein because it constitutes evidence with predictive value in terms of future reproductive performance of the line, at least under conditions in which this work was conducted, taking into account that body weight negatively correlates with productivity performance, a fact pointed out by others (Leeson & Summers, 2000, Kerr et al., 2001).

Table 2 shows the percentages of uniformity of pullets per genetic line at the age of 20 weeks. Such differences were not statistically significant (p>0.05).

Table 2. Percentage of average uniformity at 20 weeks

The lower tendency to uniformity of the broiler breeder (heavier) lines observed in our study coincide with the point made ​​by Hudson et al. (2001) who found that in breeding stock F1 of broiler-producing lines at 21 weeks, heavier birds were significantly less uniform (p<0.05) than the lightest, averaging 90% and 77%, respectively.

The percentages of uniformity found in this assay, are only slightly below those considered optimal by Hudson et al. (2001), although both lines received in relation to their weight, different feed allocations as from 20 weeks, expressed as a percentage of BW, since the amount of feed offered at this stage of the cycle remained constant for both lines. Thus, and in agreement with that indicated by Leeson & Summers (2000), it is likely that the significantly greater percentage of uniformity observed in the birds of line E at 20 weeks, may be due to the fact that they received a greater proportion of feed with respect to their body weight in relation to T (5.30 and 5.00%, respectively).
The length of the tarsus-metatarsus at 20 weeks, according to the genetic line, is presented in Table 3. It shows that, although the differences favored the lighter pullets (line E), values did not ​​become significant.

Table 3. Tarsus-metatarsus length (mm) by genetic line at 20 weeks

It is considered that the different breeding programs in meat producing poultry has resulted in changes in the development of various body tissues, including the skeletal. Thus, heavier birds with higher growth rates, generally show greater tarsus length, which indicates a greater weight of their skeletal structure (Moreki, 2005). It is generally recognized a positive relationship between the tarsus and skeleton size and some authors even make it between skeleton and BW (Almeida Paz et al., 2006). However, it is also accepted that genetic improvement in broiler breeders based on muscle deposition and increased body growth has resulted in an imbalance between development of several systems including the skeletal structure (Williams et al., 2000). Wilson et al. (1995), believe that tarsus length is an indicator of skeletal development. These authors (in agreement with what is reported in our trial) found no significant differences between different feeding programs, body weight and the response of tarsus length.

From the partial results obtained herein, it can be concluded that, although the trial had similar conditions for both lines during the growth stage, birds showed differences in some of the variables analyzed, particularly with regard to body weight at the end of the rearing period and early photostimulation. Given the power of the inferences reached in this trial, it should not be ruled out for this type of genetic material the coherence in the relationship between lower body weight of birds and the higher uniformity found in other lines of heavier birds. Similarly, and if a greater length of the tarsus-metatarsus in lighter birds is seen in future morphometric studies, this could be a phenotypic evidence from the breeds involved in the formation of this type birds.

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