Summer Feeding – Part One – Heat Stress

December 2016

Heat stress is not only an issue for cows; it also throws up challenges for pastures. From an animal perspective, increases in average daily temperatures and humidity make it difficult for cows to keep cool. As a result, feed intake drops as your girls spend more time resting and looking for shade in the heat of the day. At the same time, pastures become less digestible and provide less total nutrient. The obvious end result of these challenges is declining milk solids production.

Managing heat stress through rations

There are some things we can do with our ration to help cows keep cool and maintain their intake. Firstly, the more fibre we have digesting in the rumen, the more heat being produced from fermentation. So it makes sense to use highly digestible feed sources that create less heat. For example, maize silage will be better than hay or mature grass silage. Cows then switch into using more blood glucose for energy in periods of heat stress, meaning feeds higher in starch and sugars (pelletised wheat/barley as opposed to soy hulls and PKE) will provide more blood glucose precursors and assist in maintaining milk solids output.

Perennial ryegrass toxicosis (PRGT) - Ryegrass staggers

Another factor that can contribute to heat stress is PGRT. The endophyte Ergovaline has the effect of restricting blood flow when ingested by cows, which makes it harder for them to keep cool. Many of you may have seen the effects of another endophyte called Lolitrem B, which is the one responsible for the staggering and shaking behaviour we sometimes observe during December and January.

Longer term, incorporating novel endophytes, such as AR37 and NEA2, into new established pastures can help.

However, if you are encountering issues now, the good news is there is a feed additive, Elitox®, which your Dairy Nutrition Specialist can arrange to have included in your dairy pellet formulation. Independent university studies have shown Elitox® to effectively prevent these endophytes from binding with bentonite based toxins and control the immediate effects of PRGT.

PRGT affects not just milking cows; your replacement heifers are also vulnerable. Elitox® can also be added to pelletised feed for your heifers.

Livestock don’t typically die from the direct effects of PRGT. However, when stock have convulsions caused by PRGT, they are very susceptible to injuries from falling into drains or fences and there is the potential danger of drowning in waterways.

If you’d like more information about controlling heat stress or PRGT through your cows’ ration, contact your local Dairy Nutrition Specialist.


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