Growth monitoring of suckling lambs in Tunisia. II. Loss of precision in calculating performances at typical ages according to a simplified protocol as alternative to the conventional protocol

The performances at typical ages P10 (weight at 10 days), P30, P70, DwG10/30 (average daily weight gain between 10 and 30 days) and DwG30/70 of 9642 Barbarine lambs were calculated according to F2, the conventional protocol, or a simplified protocol based on fitting von Bertalanffy’s function to two measurements at 42-day intervals, the first taking place between 2 and 60 days after birth of the lamb, and the actual birth weight (ABw: controlled at birth), a fixed Bw (FBw), i.e. the same for all lambs (FBw = 3.4 kg), or an estimated Bw (EBw) based on the first measurement (w) and the interval between birth and the first measurement (D) [(EBw = 2.71009 + 0.208313 w – 0.00578 w (60 – D) + 0.00016 w (60 – D)2]. The average loss of precision associated with the calculation of weights at typical ages was high for P10 (40– 45%), moderate for P30 (6–7%) and low for P70 (2–3%). The loss of precision associated with DwG10/30 was lower than that of the conventional protocol for an interval between birth and the first control between 15 and 45 days (15% maximum), and similar between 7 and 15 days’ intervals (20% maximum). These results suggest simplifying the F2 protocol to two or three measurements and a birth weight. The first measurement should be postponed to 46 days after birth, whereas the inter-val between successive controls may vary between 30 and 40 days, depending on an acceptable loss of precision on the estimation of performances at typical ages.