GENETIC CHANGE IN THE NIGERIAN HEAVY LOCAL CHICKEN ECOTYPE THROUGH SELECTION FOR BODY WEIGHT AND EGG PRODUCTION TRAITS
The study was carried out to determine the genetic change in the Nigerian heavy local chicken ecotype (NHLCE) through selection for body weight and egg production traits. Progenies (G0 generation) generated from breeding parents randomly selected from the parent stock of the NHLCE formed the materials for the research. On hatching, the chicks were grouped according to sire families using colour markers. The chicks were brooded and reared according to standard management practices. They were fed a starter mash containing 18% crude protein and 2800 Kcal/kgME from 0 – 8 weeks and a growers mash containing 15% crude protein and 2670 Kcal/ kgME from 8 weeks to 20 weeks. At 20 weeks, all pullets were moved into individual laying cages for short-term (16 weeks) egg production. From then the birds were fed layers mash containing 16.5% crude protein and 2600Kcal/kgME. Data were collected on body weight, egg weight and egg number. A control population was maintained for each generation and was used to measure environmental effects. At the end of the 16 weeks egg production period, hens were subjected to selection using a multiple trait selection index incorporating body weight at first egg (BWFE), average egg weight and total egg number. The relative economic weights of the traits and their heritabilities were used to weight the phenotypic values of each trait in the index. The index score of each bird became a univariate character, which enabled the hens to be ranked for purposes of selection. Males were selected based on their individual body weight performances at 39 weeks of age using mass selection. Selected parents from G0 generation were used to generate the G1 generation which in turn yielded the parents of the G2 generation. Data on body weight, BWFE, egg weight and egg number were subjected to statistical analysis to obtain means, standard error of means and standard deviation using the SPSS 2001 statistical package. Analysis of variance yielded sire comrponent of variance from which the additive genetic heritabilities of the traits were calculated. Genetic, phenotypic and environmental correlations between pairs of traits in the index were estimated. Indicators of selection response, namely, selection differential, expected, predicted and realized genetic gains were determined for each trait. There were significant increases (P ≤ 0.05) in all the traits selected. Body weight performances (sexes combined) increased across the age periods (0 – 20 weeks) from the starting mean values in G0 generation to the final values in G2 generation. The body weight at hatch increased from a mean of 30.30g in G0 generation to 33.48g in G2 generation. Body weights at 4th, 8th, 12th, 16th and 20th week of age also showed similar increases. Body weight of males and females were similarly significantly improved. Mean body weight of males at 12, 16, 20 and 39 weeks of age were 791.40 ± 8.79g, 932.25 ± 7.83g, 1112.60 ± 11.98g and 1693.75 ± 19.91g, respectively for G0 generation as against 825.28±7.54g, 1027.83 ± 9.90g, 1156.69 ± 11.74g and 2000.00 ± 31.34g, respectively for G2 generation. For females, body weights at 12, 16 and 20 weeks as well as BWFE were 667.98 ± 6.30g, 791.52 ± 6.24g, 911.59 ± 6.33g and 1330.44 ± 2.141g, respectively in G0 generation. The corresponding values for G2 generation were 673.94 ± 6.48g, 812.54 ± 7.72g, 939.64 ± 7.28g and 1428.48 ± 3.051g, respectively. For egg production, significant improvements were also made. Total egg number and average egg weight increased from 75.60 eggs and 41.27g, respectively in G0 generation to 79.38 eggs and 43.18g, respectively in G2 generation. Selection differential values were positive and high for 39 weeks body weight in males across the three generations (mean, 302.19g) as well as for total egg number (mean, 10.74eggs) and average egg weight (mean, 0.47g) in females. It was, however, negative on the average for BWFE (-5.41g). Selection intensity values for mass selection in males were 2.11, 1.75 and 1.16 for G0, G1 and G2 generations, respectively. Mean selection intensity values for total egg number, average egg weight and body weight at first egg were 0.729, 0.106 and -0.277, respectively. For index values, selection differentials (∆SI) were equally positive across the three generations and selection intensity (iI) remained relatively stable viz. 0.703, 0.989 and 0.890 for G0, G1 and G2 generations, respectively. Direct selection responses namely, expected, predicted and realized genetic gains were mostly positive for all traits selected. Expected average direct genetic gain per generation for egg number, egg weight and BWFE were 12.58 eggs, 2.98g and 25.04g, respectively. For gain in index traits due to selection on index score, a mean value of 1.705 eggs was obtained for total egg number, 0.949g for average egg weight and 43.93g for BWFE. The ratio of realized to expected genetic gain were positive across the three generations. Specifically, a mean ratio of 0.61 was obtained for 39 weeks body weight in males, 1.58 for BWFE, 1.70 for average egg weight and 1.75 for total egg number, for females. The estimate of additive genetic heritability (h2) ranged from 0.12 to 0.24 for egg number, 0.34 to 0.43 fgor egg weight and 0.57 to 0.70 for body weight. Estimates of genetic correlation (rg) in whole populations across the three generations ranged from -0.01 to 0.01 for EN-EW, -0.06 to 0.01 for EN-BWFE, and 0.002 to 0.02 for EW-BWFE. For phenotypic correlation (rp), a range of -0.12 to 0.09, -0.04 to 0.08, and 0.21 to 0.23 were obtained for EN-EW, EN-BWFE, and EW-BWFE, respectively whereas, for environmental correlation, a range of 0.55 to 1.31, 0.52 to 0.69, and 0.38 to 0.85 were obtained, respectively for the same pairs of traits.