The relationship between liveweight at mating and calving probability was examined in cattle grazing subtropical pastures in a 4 year study. The aim was to establish whether breed type, lactation, protein meal supplements, and/or pasture quality affected the relationship. Cows (n=242) of Hereford (H), Brahman (B) and Brahman×Hereford (BH) breed types were selected as maiden heifers, 20–;27 months of age, and grazed pastures of low digestibility (D), (0·45–0·62) and medium (D, 0·47–0·67) quality which are typical of subtropical grassland areas. The maiden heifers were supplemented with cottonseed meal (D, 0·67, 61 g nitrogen/kg) at 0, 750 or 1500 g/day for 130 days and mated during 90 days in the first year of the study and as cows in the following three years. There were 633 mating records of cows collected during the study.

A generalized additive model was used to describe the relationship between calving probability and mating liveweight. There was no relationship between mating liveweight and calving probability when the model included the main factors. At the mean herd mating liveweight of 430 kg the calving probability (predicted mean±S.E.) of B cows (0·59±0·034) was less than that of H (0·74±0·029) or BH cows (0·81±0·026). There were significant reductions in calving probability with lactation and from medium to low pasture quality. Calving probability was not increased significantly in BH cows by supplementation but was for B and H cows. On low quality pastures, lactation reduced predicted calving probability from 0·77 to 0·54 for non-supplemented H cows and from 0·61 to 0·35 for non-supplemented B cows. The reductions were smaller in H and B cows when supplemented at 1500 g/day. On medium quality pastures the effect of lactation and supplementation on the calving probability of cows was less than for those grazing low quality pastures.