The effects of pregnancy and lactation on growth were investigated in Ayrshire cattle, from data on 12 linear body measurements at 3-monthly intervals up to 4 years of age. In all, 9 888 sets of 12 measurements were obtained. There were 675 first and 518 second calvings.

When expressed as a percentage of the size of animals of the same age but in a non-pregnant, non-lactating state, the total set-back to normal growth due to pregancy and lactation ranged from 2% in withers height to 18% in forerib width in heifers, and from 1% to 3% for the same characters in cows.

All 12 measurements were affected significantly by first pregnancy (including parturition), the average retardation being 3%. For several measurements, a check to growth during pregnancy was largely obscured by progressive distention of the body, especially in measurements of width, until parturition had occurred. Thus a retardation of 9% in forerib width became apparent only after calving.

First lactation affected significantly most measurements, especially the later-maturing six, which were set-back in their normal growth over the lactation period by 3% to 9%. Thus, there was effectively zero- growth in width at forerib and only one-sixth of normal growth in foregirth, whilst growth rate in width at pins and hooks was approximately halved by lactation.

Calving at an earlier than average age affected growth adversely and thereby involved some increased risk of calving difficulties.