low (L). Transitional categories (TC) were defined
by the change in udder health status from one
milk-recording test to another, and included
healthy cows (cows with two consecutive low
recordings, or LL), recovering cows (cows that
The wide range in distribution of HH and LL cows
on various farms would explain why some farms
experience unacceptable increases
in SCC
in
Volume
22 • Issue
02 • 2018
summer, with a resultant loss in premiums and/or
incurring of penalties.
Figure 1: Proportional contribution to change in total SCC by transitional category
THE DAIRY MAIL • APRIL 2018 83
recordings, or HH).Four production regions
were defined: Central, Eastern Cape (EC),
KwaZulu-Natal (KZN), and Western Cape (WC).
Weather data were obtained from the South
African National Weather Service (WeatherSA),
including monthly averages of maximum and
minimum temperature (°C), pressure (kilopascal
or kPa), relative humidity (%), and rainfall (mm).
The temperature humidity index (THI) was
calculated, combining temperature, humidity,
and pressure into an outcome parameter, and
was applied in four categories: <65, 65―69,
70―74 and >75+. The total contribution of
each transitional category to the bulk milk
SCC was calculated, and the increase in SCC
between winter/summer and low/high THI
conditions determined.RESULTSThe wide range
in distribution of HH and LL cows on various
farms would explain why some farms experience
unacceptable increases in SCC in summer, with a
resultant loss in premiums and/or incurring of
penalties.
Data suggest that more than 70% of the increase
in bulk milk SCC, secondarily to an increase in
THI, was due to the cows in the HH category. This
suggests that a summer increase in bulk milk
SCC is not due to a mild or moderate increase in
the SCC of many cows in a herd, but rather to a
moderate to marked increase in the SCC of cows
in the HH category, those suffering from chronic
mastitis. Herds with a higher proportion of cows
with chronic mastitis will be most affected during
periods of high THI, mostly due to decreased
immunity, increased environmental bacterial
load, and suboptimal management practices. The
proportion of HH cows on a farm is a function of
general udder health and this is the first aspect
that needs to be addressed if a farmer experiences
an unacceptable summer increase in SCC.
Conclusion
Results of this study suggest that, compared to the
central region of South Africa, and with all other
factors being equal, producers in the WC, KZN, and
EC have an increasingly smaller risk of producing
high SCCs during the winter to summer period.
This might be a reflection of the more favourable
environmental conditions for dairy cows that
prevail in these main milk-producing regions of
South Africa. Cows with healthy udders (LL) are
less sensitive to higher THI levels, whereas cows
with chronic mastitis (HH) tend to show a marked
increase in SCC, which is why these cows should
be the focus when addressing a summer rise in
bulk milk SCC. Identifying cows with chronic
mastitis and finding the reasons for their elevated
SCC is the first step in devising a strategy to
minimise the seasonal rise in bulk milk
Used with permission from the April 2018 issue of The
Dairy Mail
29