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Summer Heat Stress and Rye Grass Staggers
August 29, 2021

As we move into Jan, Feb and March the hot days of summer will challenge our dairy cows. Changes to the diets can help manage the stresses of heat and humidity.

When cows actually feel heat stress it’s a function of heat and humidity. In very dry conditions, 10% to 20% humidity, cows may not show signs of heat stress until temperatures are in the high 20s. In very humid conditions, above 75% humidity, heat stress can start as low as 22 degrees. Often there is a lag of about 2 days between heat stress conditions and cows showing signs of heat stress.

Indicators that cows are experiencing heat stress are: –

  • Feed Intake is reduced
  • Milk production declines
  • Cows are less active and stand more
  • Panting and open mouth breathing
  • Rectal or milk temperature increases    

   Dietary modifications will help cows cope with stress, but diet has a far less impact on mitigating heat stress than does altering the environment. Sprinklers, fans and shade can be even more important than diet but nutritional alterations can help reduce heat stress. The 3 areas to focus on with diet changes are: –

  • Feed intake
  • Energy intake
  • Keeping cows healthy

Water is the most important nutrient of all to minimise heat stress. Cows need access to plenty of clean fresh water. Water intake can increase 50% above normal levels during heat stress. Drinking water helps cool off cows as it is a heat sink drawing body heat into the water to warm it after digestion giving a cooling feeling to the cows. Cows prefer to drink water that has a temperature of about 20 degrees.

Cows lose potassium and sodium in response to heat stress and large amounts of potassium are lost through milk production. Sodium is excreted via urine to balance the loss of potassium. Increasing salt (Friesian cows up to 35g/day) can help stabilise body temperatures and also encourage cows to drink more which in turn helps reduce temperatures even more.

Total feed intake has a big impact on the amount of heat produced by the cow during the digestion of feed. During hot weather, high feed intake contributes greatly to the heat stress of cows, therefore, the natural reaction of the cow is to eat less. Increasing the energy density and reducing fibre content is one approach to handling the summer months. This needs to be done in balance however as increasing fermentable carbohydrates and reducing fibre puts the cow at risk of acidosis through reduced rumination leading to less saliva being produced to buffer the rumen. In addition, the buffering capacity of the saliva is lowered during heat stress as cows lose bicarbonate, the buffering component in saliva, through urination and increased panting.

Ensure summer feed is of high quality. Provide supplement feed at night when cooler. Use paddocks with shade trees during periods of heat stress. Provide shade at the shed if possible and use of fans in combination with sprinklers to remove water vapour.    

Fats are high in energy and digestibility which result in less heat produced during digestion than other feeds. The addition of 1 to 2%  (150 to 200grams) bypass fats can help maintain energy intake as feed intake decreases.

While normally in NZ we are short of quality protein in the summer months, protein should not be over fed. Feeding protein above requirements increases the workload of the liver and kidney to excrete the extra nitrogen increasing energy requirements and body heat production. Keeping crude protein levels to around 17% with about 60% of the protein being rumen degradable and evaluating amino acid balance is a good heat stress protein feeding strategy.

Other additives. Rumen stabilisers like “Rumen Buff” and digestion aids like yeast cultures “BioSprint” and Levucell SC have shown to be beneficial during heat stress periods by helping stabilising ph, increasing biomass and improving FCE.

Molasses improves digestibility and palatability and can help stimulate appetite and increase Dry Matter Intake.

Ryegrass Staggers is the nervous disorder animals suffer from as a result of eating pasture containing high levels of the ryegrass endophyte chemical Lolitrem B. This can occur as early as late October through to the end of April. Most perennial ryegrass contains naturally occurring endophytes (fungus) and the endophyte chemical is concentrated in the seed heads and base of the pasture. This is the rye grass’s natural chemical protection producing Peramine and Ergovaline to aid against insects such as Argentine Stem Weevil, Black Beetle and Pasture Mealy Bug. Ergovaline can increase heat stress so the goal is to avoid over grazing pasture. By pre mowing, ideal pasture residuals can be achieved while reducing seed heads. This in conjunction with feeding quality supplements such as silage, maize and introducing crops such as chicory, turnips, clover and Lucerne can dilute the proportion of ryegrass and extend the grazing round making it easier to not over graze.

   Young stock appear to be more susceptible to ryegrass staggers and if crops aren’t available consider feeding those most badly effected 100% supplement.  ProCow 15 is a safe product that they can ab lib on.

While not clinically proven, mycotoxin binders such as Fusion Dyad have been very effective in reducing the incidence and severity of ryegrass staggers. The key is to start early enough as this can help avoid production loss and reduce feed costs through better feed utilisation (feed conversion efficiency). It also aids efficient processing of proteins and acts as an appetite enhancer, so increasing feed intake, improving digestion but most of all reduces stress caused by mycotoxin-related problems offering higher antioxidant activity.

     Through cows being on their feet longer under heat stress it’s worth considering Zinc Methionine along with Betaine. Betaine supplies methyl groups to form methionine and assists metabolism in hot conditions. Both these products are found in Liquid Trace8D and also help maintain good hoof health.                                                                                                                                                                    

 Milking in the hottest time of the day should be avoided and OAD milking to reduce energy requirements in walking is a sensible option especially if total DM intakes are restricted. Reducing milking numbers to a sustainable nucleus to weather the summer conditions makes it possible to capitalise on potential Autumn growth while also making it easier to reach the ideal body condition score at dry off and establish a good grass cover going into the winter.        

Ken Winter
Technical Support Manager GrainCorp

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