Longitudinal study of inbreeding effect on growth traits of Marwari lambs

This study aimed to assess the impact of inbreeding on growth traits and genetic diversity in a closed flock of Marwari sheep. Data from 11 126 lambs born between 1981 and 2020 at ICAR-CSWRI-ARC in Bikaner, Rajasthan (India), were used to analyse trends in inbreeding, its impact on growth performance, and its effect on genetic diversity. The mean inbreeding coefficients for the entire population, males, and females were 1.55 %, 1.57 %, and 1.53 %, respectively. The overall proportion of inbred animals was 67.18 %, with the majority (38.21 %) exhibiting inbreeding levels below 5 %. The linear regression analysis revealed that, for every 1 % increase in the inbreeding coefficient, there was a corresponding change of 6, 79, 121, 146, 116 g, 0.78, and 0.38 g d^-1 in birth weight (BWT), weight at 3 months (3 MWT), weight at 6 months (6 MWT), weight at 9 months (9 MWT), weight at 12 months (12 MWT), average daily gain from birth to weaning (ADG1), and from weaning to 6 months (ADG2), respectively. The regression analysis showed highly significant and positive effects on most growth traits except for KR3, GE3, and RGR3, with significant and negative effects. The ratio of the effective founder genome to ancestor ( $f_g/f_a$ ) revealed that 36 % of the original ancestral genetic diversity was retained in the reference population. A loss of 3 % in genetic diversity was attributed to the uneven contribution of founders, while bottleneck effects were due to the imbalanced contribution of breeding animals to the population's gene pool. Under a specified criterion, the rate of inbreeding gradually rose. The study revealed that, although a low level of inbreeding positively influenced all growth traits, it also impacted genetic diversity within the flock. To sustainably manage the population, utilizing a broader base of unrelated breeding rams and implementing scientifically guided mating strategies to improve performance while conserving genetic variability are recommended.