Thesis submitted in fulfilment of the requirements for the degree of Magister Technologiae: Agriculture in the Faculty of Applied Sciences at the Cape Peninsula University of Technology / Conjugated linoleic acid (CLA), a fatty acid (FA) present in the meat and milk of ruminant animals, is considered a functional nutrient for humans. The interest in CLA is attributed to its many health benefits, such as having anti-carcinogenic, anti-atherogenic, anti-diabetic and anti-adipogenic effects. Dairy products are a rich natural source of CLA, and preliminary research indicates that the levels in milk can vary based on cattle breed and feeding system. The Fleckvieh is a popular dual-purpose breed recently introduced to the Western Cape. It is used in crossbreeding programs with Holstein and Jersey herds to increase fertility and beef production. Holstein and Jersey cattle are the two most common South African dairy breeds, but little information is available on the effect of crossbreeding on FA content and CLA levels in the milk. The primary objective of this study was to compare milk FA composition and CLA content of these two dairy breeds and their respective Fleckvieh crosses. A second objective was to investigate the effect of lactation stage on CLA levels. Jersey (J) and Fleckvieh x Jersey (F×J) cows were kept in a pasture-based system (PBS) for the duration of the study. Holstein (H) and Fleckvieh x Holstein (F×H) cows were housed in a feedlot system and fed a total mixed ration (TMR) for the duration of the study. The FA and CLA content of feeds from these two feeding systems were also analysed and compared. All cows used in the study were housed at the Elsenburg Research Farm of the Western Cape Department of Agriculture. Milk samples were collected every 35 days from the cows in each feeding group, starting 10 days after calving. Samples were kept in a freezer at -20 °C until laboratory analyses. The total number of milk samples collected was 1044. The milk FA composition and CLA in particular were determined by gas chromatography. Differences between groups were statistically analysed by two-way repeated measures of analysis of variance (ANOVA). Results show that milk CLA levels are higher in Fleckvieh-crossbred cows when compared to purebred cows. The higher CLA content of milk from Fleckvieh crossbred cows demonstrates the feasibility of using Fleckvieh bulls in South African crossbreeding programs, and are also an indication that genetic selection for increased CLA content is possible. The analysis of lactation stage showed that milk CLA is higher during late lactation. A comparison of feed samples from the two feeding systems revealed that pasture feeding resulted in higher omega-3 FA in milk. Key words: Conjugated linoleic acid, milk fatty acids profile, diet, breed, health benefits, Fleckvieh, Holstein, Jersey, diet, pasture based system, total mixed ration system,