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Ultrasonic scanning for determination of stage of pregnancy in the llama (Lama glama): a critical comparison of calibration techniques

SUMMARY Measurements of foetal head diameter (HD) and trunk diameter (TD) were made using ultrasonic scanning of 11 pregnant llamas (Lama glama). Each llama was scanned fortnightly to obtain measurements of HD and TD from c. 84–271 and 65–168 days since mating respectively. There were approximate linear relationships between TD and days after mating and between HD and the logarithm of days after mating. Calibration equations for predicting the number of days after mating (d) from foetal measurements constructed using (i) the inverse method in which d is regressed on HD or TD and (ii) the classical method in which HD or TD are regressed on d. These calibration methods were assessed by crossvalidation, treating each animal in turn as the individual for which predictions 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 between individuals which must be taken into account in calculating standard 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 intervals. For prediction from HD, the equivalent figures are size dependent: 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 days. Ultrasonic scanning is suitable for determination of stage of pregnancy of llamas providing S.E.P.S which are small in relation to their long gestation period (335–360 days).