William H. Miner Agricultural Research Institute
Summer’s heat and humidity are fast approaching. As always, we need to be prepared with effective heat abatement systems. Unless a cow is properly cooled, adjusting dietary ingredients and feeding highly digestible forages will not result in the expected responses in rumen fermentation, feed intake, or milk production. Our modern dairy cows have been selected based on heat-producing processes such as milk synthesis and consequently heat abatement has become ever more critical. Recent research indicates that cows become heat stressed at a temperature-humidity index (THI) of only 65 to 68.
Heat Stress Makes Cows Stand and Ruminate Less
As air temperature increases from the mid-70s to about 100oF, eating decreases by 46%, ruminating decreases by 22%, standing increases by 34%, and drinking increases by 30%. Higher producing cows (>70 lb/cow/day) are typically more sensitive to heat stress than lower producing cows, especially for resting, ruminating, and standing activity. Unabated heat stress can easily reduce intake by 10-15% or more and seriously compromise forage digestion.
Body temperature mediates the cow’s standing and lying response to heat stress. Cornell researchers found that during heat stress conditions, core body temperature appears to control whether the cow lies down or stands. The cow will stand up once her temperature reaches ~102.0oF and won’t lie back down again until her core body temperature reaches ~100.9oF. Research published in 2015 by Arizona and Missouri researchers confirmed that cows with elevated core body temperatures stood longer in an effort to dissipate heat and rested less. The bottom line is that cooled cows lie down longer, ruminate more, and have rumen conditions more conducive to efficient fermentation.
Heat Production and Forage Digestion
The cow’s total heat production consists of the heat increment from digestive fermentation and nutrient metabolism plus heat from basal metabolism and activity. For the lactating dairy cow, the heat increment comprises about 67% of the heat generated. Heat generated by fermentation is substantial and it varies with diet – especially the forage component. This heat load is an energetic burden to the heat-stressed cow because otherwise productive energy must be used to cool the cow’s body temperature.
With heat stress, we must focus on high NDF digestibility of forage and nonforage feeds to reduce the heat increment per unit of net energy supplied to the cow. Over the range in diets that might commonly be fed to dairy cows, from high to low heat increment, we can easily observe changes in metabolic heat loads that equate to 3 to 6oF – which means the cow consuming the more digestible diet has far less heat load to dissipate (Chandler, 1994). Basically, forages that contain more undigested NDF (uNDF) plus relatively more slow-digesting than fast-digesting NDF, will result in greater heat production during fermentation.