Cattle sweat at only 10% of the human rate and this limits their ability to regulate body temperature under warm conditions. The combination of environmental heat with heat produced from their own activities (exercise and digestion) overrides the cow’s ability to lose heat. As a result the cow must compensate for this excessive heat loading in over ways which manifests in the form heat stress.

What conditions lead to heat stress?
Heat stress is linked to ambient temperature but also the relative humidity. The temperature humidity index (THI) is used to determine the risk of heat stress. Until very recently a THI equal or greater to 72 was used to define onset of heat stress but researchers from Arizona in 2011 demonstrated a THI of 68 can also cause heat stress in dairy indicating a renewed risk to cows in more temperature regions of the world.

Table 1: Relationship between Temperature Humidity Indexes (THI) and air temperature (^oC)

THI	68	70	72	74	76	78	80	82	84
Air temp oC (Low humidity)	24	25	26	29	30	32	34	36	38
Air temp oC (High humidity)	21	22	23	24	25	26	27	28	30
Cow comfort level	No stress		Mild stress			High stress		Severe stress

What effect does heat stress have on lactating dairy cows?
Numerous physiological changes occur in the digestive system, acid-base chemistry and hormones in cows with elevated body temperatures. Ultimately this leads to a range of physiological, anatomical and/or behavioural changes in the cow including reduced dry matter intake, increased respiratory rate, lethargy and sweating. Milk yields, components and feed efficiency are reduced and the cow also becomes at risk of developing acidosis, mastitis, lameness, elevated SSCs and reduced fertility.

Effect of feeding live yeast to dairy cows suffering from heat stress
Research has demonstrated that supplementation with live yeast can alleviate some of the symptoms associated with heat stress and improve performance in dairy cows during summer months (Schingoethe et al., 2004; Moallem et al., 2007; Bruno et al., 2008, Marsola et al., 2010).

In these studies the authors reported increased milk production, dry matter intakes, feed efficiency and fat components.


Figure 1 - Effect of heat stress on lactating dairy cows

As heat stress is a risk factor for acidosis, the ability of live yeast to improve pH and reduce lactate concentrations in the rumen may explain the performance benefits with supplementation under heat stress. The live yeast improves rumen function and stability and provides the cow with digestive relief.

Reducing the risk of acidosis in summer grazing
Acidosis is typically associated with a high cereal content in TMR’s but the potential risk from summer grazing should not be overlooked. Grass is high in rapidly fermentable sugars and very low in NDF which if not appropriately balanced for may result in SARA.

As with any dietary change the first challenge occurs with initial turn out to pasture in the spring and requires careful management. Throughout the summer grazing period the levels of rapidly fermentable sugars vary according to weather conditions, though they are typically at the highest levels in spring and autumn months. The sugars are more highly concentrated in the tips of the grass and so the feeding behaviour of cows may also be a predisposing factor to increased risk of SARA.

Aside from grazing management some producers choose to buffer feed to sustain high milk yields at grazing. Providing cows with a buffer feed containing a high level of NDF helps to reduce the risk of SARA in summer grazing. Buffer feeding also provides the opportunity to supply the cows with live yeast.

Live yeasts scavenge oxygen and encourages the growth of lactate-utilizing bacteria in the rumen, thereby, reducing lactic acid accumulation from the rapid fermentation of highly digestible sugars in grass. This creates a more stable rumen environment for fibre-degrading bacteria enabling more efficient digestion of the buffer feed.

In summary, the approaching warmer months pose a risk to rumen health and milk production in the form of heat stress and high sugar content of grass if not carefully managed. Supplementation with live yeast improves rumen function and stability and can be used alongside other management strategies to reduce risks associated with warmer weather and summer grazing.