Heat Stress: Combatting Costly Losses in Milk Fat
Dairy cows tend to have a reduction in both milk volume and milk fat concentration during times of prolonged heat stress. Why?
It has been well documented that heat stress will decrease dry matter intake (DMI) – up to 50% in severe situations. This reduction accounts for approximately 50% of the milk production decrease. The other 50% of the decrease is associated with other heat stress induced changes such as increased maintenance energy cost and the energetic cost of dissipating the heat (Baumgard and Rhoads, 2012).
Cows under heat stress generate conditions for sub-clinical and clinical acidosis. The main precipitating factor is the increase in panting which reduces carbon dioxide in the blood, and hence bicarbonate, which us used to maintain blood pH. This results in lower availability of bicarbonate for the saliva. In addition, heat stressed cows tend to drool more, losing saliva that would be destined to buffer the rumen. This is coupled with the reduced DMI and less rumination which further reduces the saliva supply to the rumen.
Rumen buffers are used in times of heat stress to help feed efficiencies and improve digestion, acting to support fibre degradation and maintain milk fat production. Adding rumen buffers such as sodium bicarbonate and magnesium oxide are essential during heat stress periods to replace the buffers lost in saliva help maintain a healthy rumen pH. Lack of buffering will reduce pH which inhibits fibre digesting (cellulolytic) bacteria which are most active between pH 6.2 and 6.8. These bacteria degrade fibre, producing acetic and butyric acids which are major substrates for milk fat production. A reduction in activity will reduce the supply of these precursors for milk fat as well as slowing down passage of fibre out of the rumen, which will also decrease feed intake.
Yeast has been shown to stabilize the fluctuation in rumen pH, while research from Lallemand has demonstrated increases in the number of fibrolytic micro-organisms in the rumen as well as increased activities of the fibre degrading enzymes hemicellulase and cellulase of these fibrolytic bacteria and fungi. The positive effect on fibre digestion could lead to a stabilization of milk fat concentration. Doubling up on yeast inclusion is a common practice to support digestion during heat stress.
Another nutritional heat stress strategy is to increase the concentration of potassium (K) in the diet. Higher levels of K, often provided through potassium carbonate have been shown to increase milk fat percentage during the summer months. The recommendation is to increase K levels in the diet to at least 1.5% with the addition of potassium carbonate.
BALANCING FORAGE FIBRE AND DIETARY FAT
To combat reduced DMI and energy intake, nutritionists will typically increase the energy density of the ration. This can be accomplished with extra concentrate and reducing forages but this can be a dangerous strategy resulting in lower rumen pH and potential rumen acidosis. An acidotic rumen often results in laminitis, milk fat depression and depressed DMI.
However, the there is an additional issue with fibre since this has a higher heat increment compared to digestion of fat or non-fibre carbohydrates, meaning that high forage diets during summer months can increase the risk of heat stress. The key is to ensure that adequate levels of effective fibre are supplied in the diet – fibre which maintains rumen health while minimising the amount of rumen volume occupied and heat generated as well as limiting the risk of acidosis.
Increasing levels of fat (specifically dietary fats that are high in palmitic fat such as APF+) can significantly improve milk fat production rather than grain will allow you to achieve the optimum energy balance without sacrificing rumen function. The fat sources used should not be rumen active, since this may exacerbate the risks of milk fat depression.