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Somatomedin-like activity (SLA) was measured in plasma samples from cattle using the porcine costal cartilage disk bioassay. In 40-day-old pre-ruminant bull calves, SLA was highest in the plasma of eight Friesians (mean potency 0·97±0·06 (S.E.M.) units/ml), lowest in five Aberdeen Angus x Friesians (0·78±0·03, P<0·05) and intermediate in six Hereford x Friesians (0·89±0·09). Plasma levels of SLA were significantly (P<0·01) lower in eight lactating Friesian (high-yielding) cows (0·53 ± 0·04) than in five dry cows (0·76±0·06). Levels of SLA in the plasma of a further eight lactating Hereford x Friesian (low-yielding) cows (0·76±0·07) were similar to the levels in the plasma of the five dry dairy cows.

Large diurnal changes in plasma SLA occurred in lactating Hereford x Friesian and Friesian heifers. In three Hereford x Friesian heifers plasma levels of SLA were lowest at 19.00 h (0·39±0·07) and highest at 03.00 h (0·77±0·08). A similar pattern was present in plasma samples from three Friesian heifers; the lowest levels of SLA being at 21.00 h (0·37±0·07) and the highest at 05.00 h (0·61±0·13). Throughout the period of 24 h the overall mean levels of SLA were significantly (P<0·01) higher in the plasma samples from the three Hereford x Friesian heifers (0·6 ±0·01) than in the samples from the three Friesian heifers (0·49 ± 0·02 units/ml).