As I write this blog, I am enjoying a break from the heat and humidity in Kentucky; today, it is only 84 degrees, and the relative humidity is at 48%. It almost feels like spring compared to the past several days, which have featured temperatures over 90 degrees and oppressive humidity. The combination of high temperatures and humidity can cause dangerous heat indexes for people and cattle. For livestock, the high heat index can dramatically reduce performance. The annual economic impact of heat stress on livestock is estimated to be between $1.69–2.36 billion, with $900 million attributed to losses in the dairy industry.
How hot is too hot for cattle?
Production losses begin to occur at a heat index of 72 degrees Fahrenheit. We humans feel relief when the temperature drops in the evening, but cattle don’t really get a break. Cows under heat stress will hold heat, and their rumen function will add to the heat load. Research has shown that dairy cows reach their maximum core body temperature between 6 p.m. and midnight (Allen et al. 2015). During times of heat stress, cattle will stand more, change their eating patterns to fewer (but larger) meals and ruminate less due to open-mouth breathing, and they are also at an increased risk of periods of low rumen pH, which we define as sub-acute rumen acidosis (SARA). The map below shows the heat index on July 13 — there were not many places for cattle to be comfortable.
Management considerations
Providing shade, fans and sprinklers and making some ration adjustments can help reduce the negative impact of heat stress. These elements are all very important, but I will spend the majority of this blog explaining the impact of heat stress on rumen health and how some unique CRYSTALYX® products can provide relief.
Sub-acute rumen acidosis (SARA)
SARA occurs when the rumen pH drops below the “normal” levels for extended periods of time. A normal rumen pH typically falls in the 6.0 to 6.4 range. The exact pH point for the onset of SARA varies depending on which study you look at, but it typically appears when pH drops below 5.8 to 6.0 for an extended time. Rumen pH will drop under 6.0 after a meal is consumed but normally rebounds quickly. During a SARA event, the pH drop is significantly greater, and the amount of time the cow has a low rumen pH is extended. SARA can be induced by slug feeding, sorting against the fiber portion of the diet, delays in feed delivery or abrupt changes in eating patterns, which are all common during heat stress. When more animals in a herd experience multiple SARA events, the overall performance of the herd will decline.
Rumen function declines with repeated SARA events
Any situation that changes eating behavior can result in a SARA event, and combinations of factors only increase the risk. Sorting the TMR, estrus activity, calving, heat stress, TMR mixing errors and delays in TMR delivery are all examples of events that can trigger SARA in an individual cow. SARA is also a risk for feedlot cattle for these same reasons.
Repeated SARA events increase rumen damage. Many farms believe that their summer milk production is okay, but sometime in late July or August, the herd “crashes”. This is actually indicative of a slow decline until a high percentage of animals are on their third (or more) SARA event. With each SARA event, the pH drops lower and spends more time in the danger zone of less than 5.5. The graph below shows the extent and duration of repeated SARA in a research trial by Dohme et al. (JDS 2008).
What happens in the rumen during SARA?
First, the rumen pH drops due to rapid fermentation of the diet and results in excessive volatile fatty acid (VFA) production. Then, cattle back off feed, and as the rumen empties, the osmolality (the concentration of electrolytes and VFA) shifts dramatically. This causes bacteria in the rumen to die due to rupture of the cell wall, resulting in the release of an endotoxin into the rumen. Endotoxins come from the lipopolysaccharide (LPS) component of the cell wall of gram-negative bacteria. The LPS component is responsible for the fever, swelling and inflammation seen during E coli mastitis, for example. This osmolality and pH and end shift, combined with the LPS release, cause extensive damage to the cells of the rumen villi. These villi are the finger- or shag carpet-like structures that absorb VFA and other nutrients. In the first SARA event, the cells of the villi harden a little and have reduced absorption capacity, but on the second and third SARA events, the villi slough off to 30–50% of their original size.
The images below depict a gram-negative bacteria and the cell wall with the LPS.
CRYSTALYX Buffer-lyx
CRYSTALYX® Brand Supplements offers a unique product that serves as a patented delivery method for an additional buffering agent for cattle diets. Buffer-lyx® contains a blend of buffering agents to support rumen health. In addition, cows must lick the product to consume it, and that licking action stimulates saliva production, a powerful natural buffer source for cattle. Buffer-lyx blocks are fed at a rate of one block per 20–30 head. Intake will vary from 0.25–1.0 lb. per day depending on the SARA risk. Often, cows know something has changed with the ration before we do, and if we “listen” to the cows, their Buffer-lyx intake can be a barometer for the overall safety of the ration. Buffer-lyx can replace free-choice sodium bicarbonate with less waste and more consistent consumption.
Research has shown that Buffer-lyx can reduce the severity of SARA. Research completed at the University of Wisconsin, Madison showed that rumen pH did not drop as low and returned to safe levels more quickly when Buffer-lyx was offered during a SARA challenge. Multiple field trials have proven that pens of cattle on Buffer-lyx maintained their milk production during summer heat stress while, in comparison, pens on the same farm not supplemented with the blocks lost up to 10 lbs. of milk per head per day over a 60-day period.
We can’t control the weather, but we do have the tools to help prevent production losses from SARA.
