New genetic parameters to exploit genetic variability in low input production systems

Under low input production systems, low heritabilities for growth traits have been a major limiting factor for recommending selection to improve animal productivity. Heritabilities low values were usually due to a large error variance leading to a dilution of the genetic variability. A total of 11,802 Barbarine lambs, from 606 breeding rams and 2428 breeding ewes, born during the period 1972–2002 and raised in a low production system were used in this study. The objective was to derive new genetic parameters that reflect in a proper way the available genetic variabilities for growth traits even under a low production system. Three classical heritability estimators and two new proposed genetic parameters (ratios) were computed and compared: the additive (h2a), the maternal (h2m), the total (h2t) heritabilities, the additive genetic ratio (a2) and the genetic maternal ratio (m2). Main results of this study showed that the direct animal model has led to higher heritability estimates than the animal maternal model. Estimates were 0.30, 0.30 and 0.31 for weights at 10 days, 30 days and 90 days, respectively. For average daily gains, estimates were 0.22 and 0.26 between 10–30 days and 30–90 days, respectively. Under maternal model, h2a varied from 0.05 to 0.08, h2m varied from 0.08 to 0.12 and h2t varied from 0.11 to 0.17. Maternal heritabilities were higher than additive heritabilities, but both remained relatively small. Values of additive genetic ratios were 0.20, 0.19, 0.20, 0.32 and 0.39 for W10, W30, W90 ADG13 and ADG39, respectively. The maternal genetic ratios were 0.41, 0.45, 0.37, 0.41 and 0.40 for W10, W30, W90, ADG13 and ADG39, respectively. These results showed that a2 and m2 describe better the contribution of the additive and maternal effects to the available genetic variability compared to classical heritability estimates because they remain unaffected by the residual error variance even under low-input production systems. These new parameters (ratios) represent, consequently, appropriate indicators for the contribution of the additive and maternal genetic effects to the total genetic variability and encourage breeders to find ways to exploit them.