Climate Change Considering Dairy Cattle Susceptibilities to Extreme Heat and Assessment of Plausible Adaptive Strategies for Resilient Dairy Operations on a Warmer Planet

Guillermo Ortiz-Colón is an extension specialist of dairy cattle nutrition at the University of Puerto Rico-Mayagüez. This thought piece was written for the 2023 Perry World House Global Shifts Colloquium, “Living with Extreme Heat: Our Shared Future.” The colloquium was made possible in part by a grant from Carnegie Corporation of New York.

As I write this essay, I am looking through a window at my dry farm. It is starting to look like a desert. Where lush grass used to grow, now I see weak and thin spots of grass, resembling an ill-kept golf course. It has been more than a month without a drop of rain here in Leguísamo, Puerto Rico, certainly atypical for this part of the year. It has been too long even for the drought-resistant grass species that I planted (Brachiaria spp. Mulato II) to thrive.  Last Saturday, the smell of something burning awakened me. As I went outside, I could see smoke and ashes coming down the hill. I went to investigate, figuring that the fire was in a neighboring abandoned farm, and the fire was getting uncomfortably close to our property.  Fortunately, the fire department arrived 15 minutes after I called, and began putting out the brush fire, probably caused by a thrown cigarette. For more than a month, I have been using purchased hay and concentrated feed for my cattle, which is costing me a lot of money. Indeed, climate change is not an abstract concept for me anymore.

Although the Intergovernmental Panel on Climate Change foresees that worldwide normal temperatures could increase between 0.3 to 4.8 °C by 2100, higher temperature increases between 7.5 to 9°C have been projected for Puerto Rico by the end of the twenty-first century.  Even under very optimistic forecasts, Puerto Rico might still experience a considerable escalation in the total number of days that exceed historical temperature maximums, and perhaps an increase of extreme heat events. Moreover, extended dry periods are projected to become more common in Puerto Rico and the rest of the Caribbean, even with a conservative prediction of 1°C increase in average global temperatures. It is then projected that in the near future, the negative effects of drought and extreme heat on dairy production in Puerto Rico and the whole Caribbean will be magnified. 

Amended alimentary approaches to counter the diminished feed consumption of dairy cattle under heat stress usually rely on altering the dairy cattle diet by a) increasing the use of dry grains (i.e. corn and soybeans); b) including citrus pulp in the ration; and/or c) adding rumen inert fats.  All of these strategies depend on imported feedstuffs into Puerto Rico.

Due to the negative effects that climate change and its associated extreme heat will have on dairy cattle productivity, researching the intrinsic differences exhibited by heat tolerant breeds and selecting for them in dairy cattle would be invaluable in the quest to maintain high milk production efficiency in warmer climates. Bos indicus cattle, selected by humans in tropical Asia, epitomize thermotolerance in bovines. However, lower milk productivity and temperament problems have discouraged their use in the dairy industry of Puerto Rico. Nevertheless, after European cattle arrived to Puerto Rico with the Spanish conquest in 1511, for more than 500 years, farmers in Puerto Rico selected for tolerance to the hot and humid environment of the island.  In the 1950s, there was a large influx of European dairy breeds, primarily Holstein cattle, which farmers began crossing with the Creole cattle. Seventy years later, in Puerto Rico we find Holstein cattle with a very short, sleek, and mostly glossy hair coat, which provides them thermotolerance as reflected in lower rectal temperature, lower respiration rates, higher milk production, and better fertility than their wild-type contemporaries under the tropical conditions of Puerto Rico. These thermotolerant bovines exhibit a very similar phenotype to the one described by Dr. Tim Olson of the University of Florida after crossing Senepol cattle with the Holstein breed. It has since then been determined that both Holstein populations, although independently developed, share the Slick mutation that confers them with the shorter hair and makes them better adapted to higher temperatures.  Importantly, these thermotolerant Holstein cattle present milk productivities superior to Bos indicus cattle under similar climatic conditions.

As a warmer climate becomes the norm in our planet, we could expect that extreme heat will become a major challenge to global food security, as severe heat events will become more and more common. Adaptive strategies for resilient dairy operations in Puerto Rico under chronic heat stress can be useful to protect global food security against future extreme heat events.   

Under some climatic projections, by the year 2100, more than two-thirds of the world’s land mass could be regularly exposed to heatwaves, exposing the majority of the world’s livestock to dangerous or even deadly temperatures. To avoid this grim scenario’s catastrophic consequences on global food security, we must focus our research on different methods to construct resilience to extreme heat. Firstly, we must understand the intrinsic differences that confer Slick bovines their thermotolerance, but specifically, understand how they are able to have better fertility under severe heat stress. We must re-engineer building shading so they can withstand extreme weather events, like hurricanes, that will become more common with global warming. These farm structures must also be better insulated against heat gains with the use of heat reflective materials and better ventilation. Integrating trees in grazing lands (silvopasture) could be an important management alternative. Trees will help the soil conserve its moisture, minimizing impacts of seasonal water shortages over the productivity of the forages. Moreover, if we plant between 11-20 trees per acre spaced between 46’ x 46’ and 62’ x 62’, we could decrease the temperature under the canopy by 12℃, protecting dairy cows from extreme heat. We must aim to use native trees resistant to extreme weather events like hurricanes. Another challenge is to protect the trees from the bovines, especially during their establishment. 

Dairy farming is crucial to the food security and economy of Puerto Rico. Climate projections for the Caribbean suggest that dairy farmers will have to build resilience to extreme heat in order to maintain the productivity and well-being of their animals in the near future. If global warming continues as anticipated, the usage of thermotolerant dairy animals like the Slick-Holstein will necessarily be combined with silvopasture to lower heat stress in the animals. Moreover, silvopasture can serve as a soil moisture conservation measure, diminishing the effects of drought on the productivity of forages and perhaps making dairy farms less susceptible to fires.

With an annual revenue of over $200 million (22.9 percent of the total gross agricultural income), dairy farming is a key agricultural activity in Puerto Rico, occupying in excess of 50,000 acres and sustaining more than 20,000 jobs. Though it is a country that imports 85 percent of its food from the US, Puerto Rico is self-sufficient in fresh milk production. With a production of more than 200 million liters of milk per year, the dairy community certainly represents an important component of Puerto Rico’s food security. Unfortunately, dairy farming is highly susceptible to climate change, and in particular to extreme heat.

A paramount threat to dairy farming in Puerto Rico and the whole Caribbean basin is heat stress.  In the tropics, high humidity and severe and virtually constant radiant energy through all seasons drastically augments heat gains in cattle. As worldwide temperatures rise, extreme heat will have direct deleterious effects on the productive performance and well-being of dairy cattle. For example, heat stress directly and significantly decreases fertility in dairy cattle while concomitantly decreasing milk production. Moreover, high temperatures increase the proliferation and survival of parasites and disease pathogens. Indirectly, extreme heat will devoid soils of moisture, which coupled with irregular precipitation patterns, could negatively affect the growth and productivity of the grasses and legumes that serve as the basis of nutritional rations for these animals in the Caribbean. 

The most productive dairy cattle breed in the world, the Holstein, was developed in the Netherlands, increasing their susceptibility to extreme heat because they were bred in a cooler climate. Animals selected for high milk production generate a sizable quantity of metabolic heat during lactopoiesis. To diminish the effects of chronic heat stress and extreme heat, three adaptive strategies have traditionally been considered: a) physical alterations to the cattle environment; b) amended alimentary approaches; and c) genetic selection of cattle resistant to high temperatures.

Customary physical alterations to the cattle environment are usually costly, and some require the intensive use of energy, mostly derived from nonrenewable resources. Cooling the cows uses considerable amounts of energy and water, especially when fans are used in combination with water to promote active cooling.  The Puerto Rico dairy community has traditionally depended on building shade to protect cattle from solar radiation. However, in the aftermath of Hurricane María, which hit Puerto Rico on September 20, 2017, the general buildings and shading structures of the island’s dairy farms suffered catastrophic damages, estimated at more than $41.5 million. These damages to farm structures meant that dairy cows, overnight, lost the roofs that protected them from solar radiation. Moreover, Puerto Rico lost an overwhelming number of trees after Hurricane María. Without this natural protection against solar radiation, dairy cows were exposed to tremendous amounts of solar radiation, definitely magnifying heat stress. The impacts were reminiscent of an extreme heat event. Indeed, although certainly multifactorial, Hurricane María killed approximately 4,500 heads of dairy cattle representing an additional loss of $6-9 million. 

The statements made and views expressed in this article are solely the responsibility of the author.