Simmentaler cattle of the Highveld Region Development Institute were used in this study to determine the genetic and envi¬ronmental effects on birth mass, weaning mass and the mass gain from birth to weaning. During winter the animals were kept in four groups of roughly 40 cows each. The groups were: (i) Group 1— animals were fed maize silage (Silage group), (ii) Group 2 — animals were fed foggage and hay as well as maize crop residues (Hay group), (iii) Group 3 — animals were kept only on veld (Veld group) and (iv) the Control group. Starting in 1982, data were collected over a period of nine years. The General Linear Models (GLM) procedure of SAS was used to analyse the data. The random effect of sire of calf was, for all traits investigated, highly significant (P < 0.01). Birth mass, weaning mass and the mass gain from birth to weaning was significantly (P < 0.01) lower for the silage group calves. There were no significant differences for birth mass, weaning mass and mass gain from birth to weaning between Groups 2 and 3. The combined mass gain of the cows and calves from birth to weaning for Group 3 was significantly (P < 0.05) higher than for Group 2.