Ruminant Nutrition when the Heat is On

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The modern dairy cow is a high performance animal, and of all domesticated animals arguably the most at risk from heat stress. In our Victorian climate we are not immune to the impacts of heat stress.

Heat stress events in our temperate climate are generally characterised by acute increases in temperature and humidity, ranging from one to several days. This sporadic thermal stress can contribute to significant production and health issues in the cows as they have not had opportunity to acclimatise to chronic high temperatures and humidity.

A recent study performed at Ellinbank Research Centre (Garner et al. 2017) aimed to mimic these conditions and measure the physical and metabolic response of the cows. The experiment was conducted in climate-controlled chambers with a Control Group and a Challenge Group. The Control Group were exposed to a temperature of 14oC and a relative humidity (RH) of 70 per cent at all times. Alternatively, the Challenge Group were exposed to four consecutive days of a daytime temperature of 33oC and night temperature of 25oC.

Compared to the Controlled group, the Challenge group showed a 48 per cent reduction in dry matter intake (DMI) and 53 per cent reduction in milk production by day four, demonstrating the significant production and financial impact that these type of weather events can have.

So what can we do to help ameliorate the effects of heat stress from a nutrition and feed management point of view?

Increase nutrient density of the diet. The increased maintenance energy requirement and reduced DMI of heat stressed cows mean that they need to get more nutrition from less fed. The aim is to get the diet as close to 30 per cent neutral detergent fibre (NDF), 25 per cent starch and 18 per cent crude protein as possible. Provide the cows with the highest quality fodder source on hand on hot days. NDF on fodder sources should be less than 50 per cent, and ideally closer to 40 per cent for lactating cows.

Provide more true protein. Impaired rumen function in heat stressed cows as well as reduced DMI’s mean less microbial protein production. Providing more true bypass protein can help to offset this.

Provide extra starch as maize. Compared to wheat or barley, the starch in maize grain is slower to be broken down, with some of it bypassing rumen breakdown and being absorbed in the small intestine as a source of glucose. Providing extra starch as maize allows for an increase in the energy density of the ration, without adding to the cow’s heat load created by ruminal fermentation.

Increase buffering. Heat stressed cows are more prone to acid accumulation in the rumen. Natural buffering is impaired due to reduced rumination and saliva production. Rumen buffers such as Acid Buff or sodium bicarbonate can help to reduce the risk of sub-acute ruminal acidosis, improve fibre digestion and DMI.

Yeast products. There are yeast products that are proven to reduce fluctuations in rumen pH, improving fibre digestion and DMI.

Betaine. A natural product extracted from sugar beet, betaine helps cows to maintain osmotic balance in heat stress conditions. In the typically hotter climate of Northern Victoria betaine supplementation is a good strategy to use throughout the summer months. In the typically cooler climates of Western Victoria and Gippsland, betaine can be used more strategically, being added to the diet when a heat stress event is anticipated. The betain takes three days to build up in the cow’s system to beneficial levels.

Feed Management. Cows will consume a greater percentage of their daily feed at night in heat stress conditions. Adjusting feed allocation from 50:50 day:night to 40:60 or 30:70 allows the cows to make up for reduced daytime intakes at night. Feed cows close to a shade and water source and reduce the distance walked as much as possible.

Dry Cows. Don’t forget about the impact of heat on dry cows. Studies have shown heat stressed dry cows also consume less feed and produce less milk in the subsequent lactation. Heat stressed dry cows show lower immune function, putting them at a higher risk of infections such as mastitis and metritis as fresh cows. Calves from heat stressed dry cows have lower birth weights and poorer efficiency of adsorption of immunoglobulins from colostrum, putting them at higher risk of disease and poorer growth rates.

Speak to one of the qualified nutritionists from Reid Stockfeeds to prepare a customised ration to minimise the impacts of heat stress on your herd.

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