Body length to predict broiler performance

Day-old chick quality is an indicator of future broiler performance. In order to enhance the prediction power of chick quality it would better to define as a combination of several qualitative parameters at hatch, and 7-day growth (Tona et al., 2003a). Some measurements done in day-old chicks include body weight and body length. Considering that 7-day weight is a good predictor of market weight (Willemsen et al., 2008), the correlation between pull weight and 7-day weight can predict bird performance in a more precocious manner. According with Molenaar et al. (2008), hatchling body length seems to be the best parameter to predict broiler performance than day-old weight, but the gender of the animal also needs to be considered. In addition, incubation time (total de hatching time) can also be a factor to consider, since embryos with a better development are more able to use the nutrients in the yolk sac and, therefore, they are more precocious than those with a lesser growth. The purpose of this study was to evaluate the relationship between body weight and body length at day of age, as well as total hatching time with 7-day weight in broilers of both genders.

Four hundred (400) broiler breeder eggs containing viable embryos were selected at 18 days of incubation. Eggs were identified and conditioned in  individual permeable bags at transfer time. All eggs were weighed prior to transfer and vaccinated in ovo. The eggs were removed from the hatchers at approximately 504 hours of incubation (21 days). Mean hatching time and hatch weight of each individual chick were recorded. The chicks were sexed based on wing feathers, weighed and identified individually using numbered rings. Body length was measured using a digital calibrator, from beak tip to longest toe end, nail excluded. Once these measurements were recorded, the chicks received a commercial pre-starter feed in the hatchery, t the rate of 1 g/bird. Afterwards, the animals were placed in groups of males and females in  house equipped with baby chick feeders and drinkers, and gas heating. All chicks were weighed at placement, and a commercial starter feed and water were provided ad libitum. At 7 days of age, the body weight was recorded again. Data was subjected to analysis of variance (ANOVA), using the statistical SAS package. Pearson´s correlation coefficients of the variables of interest were calculated. Each chick was considered as the experimental unit.

Chick quality-related variable means and per-gender animal performance are shown in Table 1. Both body length and 7-day weight showed differences between the sexes (P<0.001), the females showing longer bodies and lower body weights at 7 days than males. Egg weight, hatching time, hatch weight and housing weight showed no differences between sexes (P>0.05). The correlation coefficients for males and females are shown in Tables 2 and 3, respectively. Even when the correlations among the variables of interest showed similar responses between bot sexes, correlations among individuals of the same gender are needed, so that different correlation values are shown for males and females. The importance of the specific per-gender study was stated by Molenaar et al. (2008) who concluded that day-of-age body length resulted to be a good predictor of market weight and breast yield in broiler males, but not so in females. In both sexes, a positive correlation existed between 7-day weight, body length, egg weight, hatch weight and housing weight. Both body length and 7-day weight had a negative correlation with hatching time, so that the chickens hatching in a lesser time presented higher body length and higher 7-day weight than those that hatched later. This suggests a possible difference in the metabolism of those birds that require a longer time to hatch. Tona et al. (2003 b) stated that the increased proportion between the thyroid hormones T3 and T4 , and the higher level of corticosterone can help the initiation of the hatching process (internal/external pipping) and chick quality. In fact, Tona et al. (2004) found metabolic differences among strains with different growth rates, and observed that incubation time is shorter for the heavy strains. These authors found a relationship even higher between the above-mentioned thyroid hormones in the chick at hatch for the heavy genotypes. In this study, no differences were found between the sexes in relation to hatching time, but given that a significant correlation existed  between this variable and 7-day weight, it can be inferred that also individual metabolic differences exist. Regarding the productive values of 7-day performance, body length was better correlated with 7-day weight than hatching weight in both sexes, so that this is a useful tool to predict bird performance. The utilization of the hatching time variable is an alternative to minimize prediction errors related with hatching weight, since in agreement with Joseph et al. (2006), the differences in hatching weight are explained by variations in the amount of residual yolk sac. Given that residual yolk sac mass can vary from 0.8 to 10.6 g in the different strains (Wolanski et al., 2006), using only hatching weight can lead to unreliable values.

Table 1. Means of the day-old chick quality-related variables and performance, as per bird sex

 

Table 2. Pearson´s correlation coefficients for male broilers

 

Table 3. Pearson´s correlation coefficients for female broilers

Both hatching body length and hatching time can be used to assess potential 7-day growth, considering the differences within each gender.

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