ULTRASONIC-SCANNING FOR DETERMINATION OF STAGE OF PREGNANCY IN THE LLAMA (LAMA-GLAMA) - A CRITICAL COMPARISON OF CALIBRATION TECHNIQUES

Measurements of foetal head diameter (HD) and trunk diameter (TD) were made using ultrasonic scanning of 11 pregnant llamas (Lama glama). Ea ch llama was scanned fortnightly to obtain measurements of HD and TD f rom c. 84-271 and 65-168 days since mating respectively. There were ap proximate linear relationships between TD and days after mating and be tween HD and the logarithm of days after mating. Calibration equations for predicting the number of days after mating (d) from foetal measur ements constructed using (i) the inverse method in which d is regresse d on HD or TD and (ii) the classical method in which HD or TD are regr essed on d. These calibration methods were assessed by cross-validatio n, treating each animal in turn as the individual for which prediction s were required. Analysis of the prediction errors showed bias in the classical method, which consistently underestimated d at low values. A components of variance analysis indicated substantial variation betwe en individuals which must be taken into account in calculating standar d errors of prediction (S.E.P.) and confidence intervals. S.E.P. of d from TD can be reduced from 12.5 to 10.4 days by increasing the number of observations on an individual from one to four at fortnightly inte rvals. For prediction from HD, the equivalent figures are size depende nt: examples are from 8.3 to 5.6 days, and from 26 to 18 days, for HDs of 2 and 6 cm respectively. The effect of small positive correlations between residuals of successive fortnightly measurements on the same llamas had a negligible effect on S.E.P.s, increasing them by c. 0.2 d ays. Ultrasonic scanning is suitable for determination of stage of pre gnancy of llamas providing S.E.P.s which are small in relation to thei r long gestation period (335-360 days).