Why Heat Stress Matters

Dairy cattle have historically been selected for traits which contribute to productivity. In general, body mass has increased to accommodate a large mammary system and other internal organs that contribute to milk synthesis.1 As a result, this selection strategy has decreased heat tolerance of dairy cattle because the body heat produced to meet maintenance needs is directly proportional to the body weight and surface area of an animal.2 Therefore, as a cow increases in size, metabolic heat production increases. Metabolic heat production also escalates as the productive capacity of a dairy cow increases. In the United States, approximately $1 billion is lost annually as a result of poor performance during periods of heat stress.3

Temperature Humidity Index (THI)

It was traditionally thought lactating cows become heat stressed when conditions exceed a THI of 72.4 However, recent climate-controlled experiments indicate milk yield starts to decrease at a THI of 68.5,6 Table 1 shows human and dairy cattle experience heat stress under conditions that are not typically thought of as stressful and the consequences can be severe.7

Table 1. Dairy cow and human temperature humidity index.7

Chromium and Heat Stress Literature

Literature Review: The Potential Benefits of Chromium Supplementation for Dairy Cows During Heat Stress

Effect of chromium supplementation in lactating dairy cow diets on response in daily milk yield and dry matter intake, lbs/h/d under heat stress conditions.

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Evaluation of Chromium Propionate on Milk Production of Holstein Cows Under Heat Stress Conditions

A field trial was conducted on a commercial, 800-cow Holstein dairy in southeastern Pennsylvania to evaluate the effect of chromium propionate on milk yield by 2nd lactation and greater cows under heat stress conditions. The results may suggest chromium propionate supplementation from -21 days pre-fresh through peak production to receive full benefit on milk yield and supplementation of chromium propionate through 150 days in milk will help maintain milk yield during periods of heat stress.

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The Impact of Heat Stress on Fertility in Dairy Cattle

Heat stress can compromise a lactating cow's performance in many different ways, such as decreased feed intake, altered metabolism, reduced milk production, impaired reproductive performance and increased disease incidence. Chromium supplementation has been shown to improve energy utilization. A field trial was conducted with chromium propionate during one of the hottest summers on record. The cows in this trial herd were less severely impacted by negative energy balance in early lactation and were more fertile under the timed AI program.

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Leaky Gut's Contribution to Inefficient Nutrient Utilization

There are a variety of situations in an animal's life when nutrient utilization is re-prioritized from productive towards agriculturally unproductive purposes. Two well-known examples that markedly reduce production are heat stress and ketosis. Decreased feed intake, experienced during both diseases, is unable to fully explain decreases in productivity. Additionally, both diseases are characterized by negative energy balance, body weight loss, inflammation and hepatic steatosis. While the metabolism of ketosis and heat stress have been thoroughly studied for the last 40 years, the initial insult in the cascade of events ultimately reducing productivity in both heat stressed and ketotic cows has not been identified. To that end, we have generated preliminary data strongly implicating a metabolic disruptor, endotoxin, as the etiological culprit in each case.

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Heat Stress Biology: Practical Management and Nutritional Strategies

There are a variety of situations in an animal's life when nutrient utilization is re-prioritized from productive towards agriculturally unproductive purposes. Two well-known examples that markedly reduce production are heat stress and ketosis. Decreased feed intake, experienced during both diseases, is unable to fully explain decreases in productivity. Additionally, both diseases are characterized by negative energy balance, body weight loss, inflammation and hepatic steatosis. While the metabolism of ketosis and heat stress have been thoroughly studied for the last 40 years, the inital insult in the cascade of events ultimately reducing productivity in both heat stressed and ketotic cows has not been identified. The focus of the presentation is to review practical management strategies that can be used to help mitigate the impact of heat stress.

Effects of Heat Stress on Gut Health and Immune System Nutrient Requirements

Heat stress and ketosis reduce efficiency. Decreased feed intake experienced during both situations is unable to fully explain the suboptimal productivity. Heat stress and ketosis affect herds of all sizes and almost every dairy region of the U.S. Dr. Baumgard hypothesizes the common etiological origin of both metabolic disorders as "leaky gut." Leaky gut and the resulting endotoxin infiltration alters nutrient partitioning and is a causative agent in metabolic disruption during heat stress and ketosis. Identifying dietary approaches that can improve gut barrier dysfunction is paramount in developing nutritional strategies aimed at improving intestinal health.

References

1Britni M. Brown. 2016. PLoS One. 11(2):e0148234.

2Renaudeau, D. 2012. Animal. 6:707-728.

3St-Pierre, N. R. 2003. J. Dairy Sci. 86:E52-E77.

4Armstrong, D. V. 1995. J. Dairy Sci. 77:2044-2050.

5Zimbleman, R. B. 2006. J. Dairy Sci. 92(E-Suppl. 1):347. (Abstr).

6Ravagnolo, O. 2000. J. Dairy Sci. 83:2120-2125.

7Image used with permission by Jeff Reneau, University of Minnesota Dairy Extension, April 2016.

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