Growth monitoring of suckling lambs in Tunisia. I. Global curve fitting and critical analysis of the conventional protocol

Abstract

Growth monitoring data of two experimental sheep herds of Barbarine breed, collected during 31 years on 9642 lambs weighed at birth, then six times at 21-day intervals, were analyzed. Estimates obtained by fitting a global growth curve were compared to assessments carried within the “F2” conventional protocol of growth. Brody, logistic, Gompertz and von Bertalanffy functions were adequately adjusted to the seven controls. The fitting factors, coefficients of determination (R^²) and residual sums of squares were estimated at 0.997 and 2.9, 0.997 and 3.0, 0.996 and 3.8, and 0.996 and 3.8, respectively, for the functions of von Bertalanffy, Gompertz, Logistic and Brody. The average weights at typical ages, estimated by the conventional method, were 3.4, 5.5, 9.1, 15.5, 18.2 and  22.2 kg at birth, 10, 30, 70, 90 and 120 days of age, respectively. The calculation of performances at typical ages by the conventional method was accompanied by a small loss of precision of 5, 2, 0.8 and 0.5% at 30, 70, 90 and 120 days, and by a sizeable drop of 25, 19, 12 and 10% of DWG10/30 (daily weight gain between 10 and 30 days), DWG30/70, DWG30/90 and DWG30/120, respectively. The loss of precision associated with DWG10/30 exceeded 50% for the interval birth - first control at 21 days because of a lower and imprecise extrapolation of the weight at 10 days.