Page 354 - The Welfare of Cattle

CoW CoMfort daIrY housInG sYsteMs 331

extensively raised dairy cattle. Such work would nicely complement existing research programs
exploring how management changes can minimize aggressive and competitive social interactions
in intensive systems.
Given the negative impacts of overstocking, it seems reasonable to consider why a majority of
U.S. dairy farms still maintain a SSD greater than 100% (von Keyserlingk et al., 2012)? We surmise
the answer has to do with the fact that overstocking is profitable. Adding more cows dilutes fixed
costs over a larger number of production units. While this may reduce individual milk production
and negatively impact cattle welfare, it appears to improve overall farm profitability (De Vries
et al., 2016). Without attendant changes in market conditions that currently incentivize overstock-
ing, reducing the inclination to overstock will be difficult.

therMaL StreSS

The modern dairy industry is dominated by continental Bos taurus cattle which are adapted
to mild European climates. The versatile nature of cattle coupled with technological advances in
housing has allowed dairy herds to occupy a diverse array of landscapes from Saudi Arabia to
Scandinavia to Brazil. Although the extent and severity vary, the majority of regions where dairy
production takes place still experience seasonal struggles maintaining cow comfort in the face of
thermal stress. In 2003, it was estimated that the U.S. dairy industry loses $897 million annually
due to decreased production, increased mortality and decreased reproduction due to heat stress
(St-Pierre et al., 2003). In the next section, we explore the current science addressing the negative
effects of thermal stress on cattle and various heat abatement strategies.
Like most mammals, cattle seek to alleviate the discomfort associated with elevated body tem-
perature. Some cattle, particularly Bos indicus type animals, have innate mechanisms for heat abate-
ment. These include respiratory rate modifications, changes in hair coat thickness/length, lighter
skin pigment, more abundant sweat glands, and increased skin vascularity. Behavioral strategies to
mitigate heat stress also occur including decreases in time spent lying and corresponding increases
in standing times that help to dissipate heat. As we have seen, these behaviors also increase the risk
of hoof lesions and lameness (Cook et al., 2007). There is even some evidence that prolonged heat
stress can lead to increased aggression among cattle, especially in competitive feeding situations
(Polsky and von Keyserlingk, 2017).
Other responses to heat stress align closely with our own personal experiences. Heat-stressed
cattle consume more water (West, 2003) and prefer standing in shade versus lying in unshaded
areas, even after being unable to lie down for 12 hours (Schütz et al., 2008). In other cases, cattle
modify their behavior in less seemingly rational ways (see review by Polsky and von Keyserlingk,
2017). Dairy cows, for example, prefer shade to sprinklers, despite sprinklers being more effective
means of cooling as measured by decreasing respiratory rate (Schütz et al., 2011). It is not yet clear
how we ought to address these instances where cow preferences seem to contradict other relevant
lines of evidence.
When cattle are unable to adapt and become heat stressed a rapid depression in feed consump-
tion (i.e., dry matter intake) will occur (West, 2003). As cows begin to enter a negative energy bal-
ance, milk production will begin to suffer. These changes place sensitive populations (e.g., transition
cows) at even greater risk of production diseases like metritis, displaced abomasum and ketosis that
typically require immediate veterinary attention (Esposito et al., 2014). Although milk production,
like all animal welfare indicators, is by itself an imperfect measure of thermal stress, changes in
feeding behavior and intakes are useful indicators of the effectiveness of different heat-stress abate-
ment strategies.
The integration of bovine genetics for high heat tolerance may help to alleviate the severity
of heat-stress effects in the long term. However, even well-adapted breeds of cattle will likely