Abstract

Dairy production is a rather new and not a traditional system in Vietnam. It is mainly based on smallholder dairy farms. The general aim of the studies in this thesis was to improve milk production on smallholder dairy farms in Southern Vietnam and also to create a foundation that could be used in the advisory service or/and in further research for better milking management routines. Studies were done to cover the specific objectives of this thesis. The studies were designed to identify the problems for dairy production on smallholder dairy farms, to investigate which are the management factors that influenced milk somatic cell count (SCC) in lactating cows, identify the prevalence of subclinical mastitis based on SCC and to study the protein degradation caused by Streptococcus (Str.) agalactiae. The survey study indicated that the majority of the farmers kept between 2 to 17 cows (mean = 12). The main breed of dairy cow was Holstein Friesian (HF) crosses. This HF cows produced about 16 kg/day/cow. Around 35% of the farms provided fresh water ad libitum for the cows, while 51 % provided less than 30 L of water per cow per day. Moreover, milk SCC was high (1,300,000 cells/mL milk) in many of the studied farms. The second study found that limited to drinking water significantly increased herd SCC. Str. agalactiae was found to be a predominant species in infected udders. Further investigation showed that the prevalence of subclinical mastitis (SCC > 200,000 cells/mL milk) at quarter basis was 63.2% (285 out of 451) and at cow basis 88.6% (101 out 114). Str. agalactiae was found on 65% farms, 35.6% cows (41 out of 115) and 21% quarters (96 out of 458). Among 96 isolates of Str. agalactiae, 11 different strains were identified. The proteolysis of casein was higher (12-70%) compared with whey proteins (4-12%). The strains of Str. agalactiae in the same phylogenic group did not show the same degradation of casein and whey protein. Str. aglactiae caused proteolytic activity where the proteolysis of αS2-casein was highest, up to 70%, compared with control milk. Proteolytic activity caused by different strains showed a large variation. The lowest breakdown of casein was found to be 30% compared with control milk. Overall, the high milk SCC in this present study showed poor udder health of lactating cows on smallholder farms. The high milk SCC was mainly caused by the infection of udders with Str. agalactiae.