Cooling, dietary CP influence milk production during heat stress

Tim Lundeen

September 11, 2020

Lactating dairy cows are constantly exposed to environmental factors -- such as temperature and humidity -- that can take a toll on their production, health and well-being.

Heat-stressed cows undergo a series of homeorhetic adaptations that alter cellular homeostasis and nutrient utilization, according to University of Tennessee researchers Jeffrey D. Kaufman, Hannah R. Bailey, Amanda M. Kennedy, Frank E. Löffler and Agustín G. Ríus.

Kaufman et al. recently conducted a study to determine the effects of cooling and dietary crude protein (CP) on the fermentability of rumen inoculum, milk production and health of heat-stressed cows. They reported their findings in Livestock Science.

The researchers hypothesized that cooled cows — but not heat-stressed cows — would support productivity in response to an increase in dietary CP content.

Kaufman et al. randomly assigned 36 multiparous Holstein cows to one of four treatments in a 2 x 2 factorial arrangement (nine cows per treatment). Treatments consisted of cooling or moderate long-term heat stress and diets containing low CP content (LCP) of 12.5% CP or moderate CP content (MCP) of 16.1% CP of dry matter.

The cooling treatment provided heat abatement with sprinklers and fans that came on when temperatures exceeded 20°C, while the heat stress treatment did not provide sprinklers and fans, the researchers said. Cows were housed in pens in a freestall barn, and treatments were imposed for 21 days during July and August.

According to Kaufman et al., cooled cows had decreased afternoon rectal and vaginal temperatures (0.60°C and 0.70°C) and afternoon respiration rate (27.6 breaths per minute) compared to the heat stress group.

Treatment interactions showed that, compared with the LCP treatment, MCP increased milkfat yield (23%) in cooled but not in heat-stressed cows, the researchers reported.

Kaufman et al. also noted that increasing dietary CP decreased the milk protein percentage (0.14 units) in cooled cows but increased the percentage (0.09 units) in heat-stressed cows.

Rumen inoculum collected on day 21 showed approximately a threefold increase in total gas production in vitro in the cooling treatment compared with the heat stress treatment, the researchers said, adding that the higher CP level increased plasma glucose and insulin and decreased total free fatty acid concentrations compared to the low-CP diet.

Kaufman et al. concluded that, under the conditions of this study, cooled cows — but not heat-stressed cows — responded to a CP stimulus by increasing productivity.