The effect of tick infestation of Hereford, Bonsmara and Nguni cows on the weaning masses of their calves in a dipping versus non-dipping situation was investigated. Breed had a major effect on the level of tick infestation, Herefords being most susceptible and Ngunis least. The productivity of Nguni cows,
as measured by the weaning masses of their calves was also least affected. The effect of infestation on the productivity of Hereford cows was the greatest and that on the Bonsmara cows was intermediate. It was estimated that each engorged female tick (predominantly Boophilus decoloratus) caused a reduction of 8,9g, 8,0g and 8,6g in the weaning masses of calves from Hereford, Bonsmara and Nguni cows respectively. The effect of infestation on the productivity of the Nguni cows was nevertheless small
because a limited number of ticks fed to maturity on this breed due to its natural resistance.