Discipline: mastitis; Key words: SSC, summer rise in SCC, mastitis, transitional category, season, temperature humidity index, heat stress.
Dairy farms often experience a summer through autumn rise in somatic cell count (SCC) with some farms affected more than others. The extent to which climatic factors contribute to this summer rise in SCC in South Africa is unknown although weather is blamed anecdotally. Consequently, the value of heat abatement or other preventative intervention cannot be established. The relative effect of season and/or heat stress on milk production and SCC is likely to be unique to South African conditions and therefore this formed the basis of the study cited below.
The first objective was to study the summer rise in SCC as it may be influenced by season, using transitional category (TC) as the measure of interest. The second objective was to study the summer rise in SCC as it may be influenced by the temperature humidity index (THI), using the same measure. The third objective was to identify any farm-specific or regional differences in TCs.
The study was conducted according to a so-called Retrospective Cross-Sectional design. Following data collection, reorganisation and creation of new variables [days in milk (DIM), transitional category (TC) and major region], the proportion of TC and milk production were observed for each month. Transitional categories were defined by the change in udder health status from one milk test to another to include healthy (low, low SCC denoted LL), recovering (high, low SCC denoted HL), new infections (low, high denoted LH) and chronically infected cows (high, high denoted HH). A mixed multivariable regression model was used to evaluate the effect of season on the log10 of total SCC. Cows within herd were a random variable. The predicted total SCC was corrected for milk production and the proportion of TC for that season, allowing for the calculation of total SCC and any increase from winter to summer. The contribution of each TC to the increase in SCC was then expressed proportionally. For the THI investigation, the THI was calculated and categorically converted. Similar to the season model, milk production and TC was observed per THI. In the model, season was replaced by THI. The predicted mean total SCC was corrected for milk production, allowing for the calculation of total SCC and any increase from a low to a high THI. Having adjusted for confounding variables, the predicted SCC was determined for each region. The distribution of TC in herds was also observed.
In the season model, HH and LH cows produce 78% and 50% respectively of the increase in SCC when transitioning from winter to summer. In the THI model, HH and LH cows produced 92% and 24% respectively of the increase when transitioning from THI conditions less than 65 to THI conditions above 70. Compared to herds in the Central region, herds in the Western Cape (WC), Kwa-Zulu Natal (KZN), and Eastern Cape (EC) had decreasing levels of SCC. The overall proportion of TC varied throughout the year. The total amount of SCC produced by each TC varies throughout the year. The proportion of TC cows in each herd varied significantly from 5% to more than 70% for low-low (LL) and high-high (HH) cows respectively. Herds with a high proportion of HH cows experienced a more drastic summer rise in SCC.
Conclusions: Since HH and LH cows were responsible for 78% and 50% of the increase in SCC in summer respectively, it was concluded that season is a risk factor in the causal pathway towards summer rise in SCC. HL and LL cows were responsible for a decrease in SCC and as such contribute -9% and -19% respectively of the change in SCC when transitioning from winter to summer. Since the THI model showed a similar trend in that HH and LL cows produced 92% and 24% of the increase in SCC’s when transitioning from THI conditions below 65 to those above 70, THI is a high risk factor in the causal pathway towards the summer rise in SCC. Compared to the Central region, EC, KZN and WC had an increasing risk of producing more somatic cells in this context, with some farms displaying a very high proportion HH cows of more than 70%, and these farms will most likely experience a more drastic summer rise in SCC.
Lategan, A., 2017. The effect of season and temperature humidity index on somatic cell counts in South African dairy cows. MSc degree in the Department of Production Animal Studies, Faculty of Veterinary Science, University of Pretoria, December 2017.