temperature of heat-stressed dairy cows, suggesting an action on thermoregulatory functions.
• Under heat stress, yeast supplementation may increase nutrient flow to the small intestine and thus dairy cow performance.
• Yeast supplementation stimulates the synthesis of niacin in the rumen. Niacin can stimulate skin vasodilation by acting on vascular prostaglandin receptors and hence increase peripheral heat loss in dairy cows.
• Yeast-supplemented cows have a lower respiratory frequency suggesting that they are more efficient at dissipating heat.
• The improved heat dissipation in response to yeast supple- mentation may reduce the maintenance energy required for thermoregulation.
• Plasma glucose content is increased when heat-stressed dairy cows are supplemented with live yeast. Cows in heat stress have increased reliance on glucose as an energy source.
• The increased glucose availability to the mammary gland, in response to yeast supplementation, apparently in- creases lactose synthesis, resulting in a positive milk yield response. In one study, it was found that yeast supplemen- tation increased milk yield (+1.2 kg/day) during the first 130 days of lactation.
• Yeast-supplemented cows also tend to ruminate longer, have a lower mean rumen temperature, and spend less time with a rumen temperature above 39.0°C. In theory,
a more stable rumen temperature can facilitate heat dissipation under heat stress.
Points to consider • Yeast is high in nucleic acids, particularly RNA. With such high levels of nucleic acids, there is an increased metabolic rate of purine in the liver. Consequently, uric acid is pro- duced in large amounts, leading to urinary calculi and oth- er metabolic problems. These problems could partially be alleviated when the yeast products are used in rations in restricted amounts.
• Yeast could also have toxic effects on animals, especially when produced in culture media containing petroleum de- rivatives. Cattlemen and women should, therefore, obtain the yeast material from trusted sources and ensure that the quality and composition of that material is verified.
• The imbalanced supply of amino acids is also a problem as- sociated with feeding yeast. Yeast biomasses are usually high in lysine and low in arginine. The increased ratio of ly- sine-to-arginine in the ration may have negative impacts on growth and other biological functions, due to improper metabolism and synthesis of protein in the animal body. This ratio should, therefore, be corrected by adding argi- nine upon yeast processing. Additional methionine should also be considered to correct deficiencies and obtain maximum performance.
References are available from the author upon request ▶ ALL ABOUT FEED | Volume 30, No. 5, 2022 13
The use of yeast in cattle diets has resulted in improved rumen fermentation, animal health, milk yield, and adaptation to heat stress.
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