By Elena Meganck
If the world wants to survive the Anthropocene, not only humans but also agriculture will need to adapt to the threat of climate change.
Cattle have had a very important role in human evolution, dating back to their domestication some 10,500 years ago in the Fertile Crescent of Mesopotamia. These ruminants have, and still do, offer a variety of uses, including dairy products, meat, leather, hides, plough pulling and wagon pulling. An increase in population, in turn, leads to an increase in cattle population to provide for rising demand of livestock products (FAO, 2013). But there hasn’t only been a rise in human and cattle population. There has also been a rise in global temperatures, and if models are accurate this is likely to have a significant impact on the sustainability and stability of livestock businesses worldwide (Solomon et al., 2007; Robinson, 2001; Westcott, 2011).
Hot and humid air affects cows in subtle ways, causing reduced dry matter intake (Yadav et al., 2016), milk production, milk quality (Wheelock et al, 2010), feed conversion efficiency (Brown-Brand et al., 2005), reproductive performance (Jordan, 2003), and impairing a cow’s immune system via cell mediated and humoural immune responses. Heat stress activates the hypothalamic-pituitary-adrenal axis to maintain homeostasis in a stressful environment (Sejianet al., 2018) and increases peripheral levels of glucocorticoids, such as cortisol, the most important glucocorticoid. During periods of acute stress cortisol acts as a stimulant for the immune system, but during chronic stress it is associated with immune suppression (Juet al., 2014), causing cattle to become more susceptible to disease. Increased circulating glucocorticoids has been shown to inhibit the production of cytokines, regulators of immune response, such as interleukin-4 (IL-4), IL-5, IL-6, IL-12 and interferon y (IFNy) (Elenkov, 2004)
One type of beef cattle is particularly vulnerable, the European-derived Bos taurus breeds, such as Angus, because of their dark, thick coats. In contrast, the Indian-derived Bos indicus breeds have light, short coats and more adapted sweat glands, but their meat isn’t deemed as high grade. Some farmers have ingenuously crossed the two breads to ensure some heat resistance, but this results in random genetic experiment because the passing on of the heat-resistance genes can’t be controlled; an offspring may inherit the heat-resistant gene or the non-heat-resistant gene.
It has also been shown that supplementing cattle feed with minerals, vitamins, antioxidants and yeast can improve the immune system in heat stressed dairy cattle, but is this enough to counter rising temperatures? Some scientists don’t think so, and they’ve taken on a whole new, much more radical, angle to breed heatproof cattle. In Florida, one team of researchers draws samples from more than 800 cows about their coat type, body temperature, sweat extent, temperament, fat levels and frequency of pregnancy. They then extract DNA from each cow, analyse it for 250,000 genetic markers and run statistical tests to determine which genetic markers correlate most strongly with which desired features. But as always, correlation is not causation, so the challenge remained to identify which genetics patterns produce a specific trait.
Another experiment used CRISPR to alter the pigmentation gene PMEL in cattle embryos in vitro, giving silvery grey coats instead of black or brown coats. The embryos were then cloned and implanted into surrogate mothers. The cloning selects for the best adapted animals, giving better herd quality, but it can be risky because the offspring can have defects or suffer from long-lasting health problems. Indeed, the two calves in the experiment didn’t live long enough to determine whether they were more resistant against the heat. Researchers are also looking into the potential gene candidates for biological contributors to heat stress beyond coat colour, such as coat slickness, or introducing genetic variants from tropical cattle that has natural evolved to be more heat and disease tolerant.
In the meantime, and before gene-edited, heat-resistant cows become a reality, shade, proper ventilation and access to clean water can and should be used to reduce heat stress. From the perspective of greenhouse gas emissions, one might still argue that it would be better to eat less steak anyways (IPCC, 2020). It produces eight times as much greenhouse gases as chicken and since 200 has been responsible for 33% of global methane emissions and 66% of agricultural methane emissions (EDGAR 4.3.2 database, May 2018). Nevertheless, the impact of heat on cattle is currently a tremendous stress on their status as an economic engine, and many are dependent on it (St-Pierre et al., 2003); it is the primary cause loss in productive performance of dairy cows.
As such, improving animal selection methods and the development of climate resilient breeds may support the sustainability the livestock industry into the future, keeping in mind that adaptation, whether in agriculture or another industry, is likely to become a constant state of acceleration with no clear end goal. Selective breeding and gene-editing may insulate cattle industry from the effects of global warming for some time, but there will always be a diminishing chasm between what we most want and what we can hold on to.
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