An international team of researchers from the University of Florida and the State University of Maringa in Brazil examined the use of supplemental yeast culture (YC) in rumen function and dairy cow digestion when cows had differing levels of starch in their diets.

The group published its work in the Journal of Dairy Science​​​.

The research team was interested in exploring if adding supplementary yeast to the diets of dairy cows would have more of an effect or benefit for diets that were high in starch, said Jose Santos, corresponding author and research foundation professor with the department of animal sciences at the University of Florida.

“There has been some suggestion that yeast culture might benefit diets that are high in starch because of the risk of rumen acidosis and the negative associative effects of low pH on fiber digestion,” ​he added.

“Nevertheless, that was not what we observed,” ​he told FeedNavigator. “There were positive effects in both [high and low starch] diets, with 23% starch and with 29% starch.”​

Why additive yeast and varying starch diets?​

Yeast cultures and other yeast-based products have been found to improve rumen or total tract fiber digestion, said Santos. They also have demonstrated the ability to modulate rumen pH and reduce the risk of rumen acidosis.

While it may be common to add yeast to the diets of dairy cattle because of the potential to improve milk yield, the influence that additive yeast has can be inconsistent, the researchers said.

One aspect that may influence response to supplemental yeast is the makeup of the diet, they said. Diets with larger amounts of concentrate maybe more likely to have a negative influence on the rumen, which can lower pH, while yeast products can limit that reaction.

It is possible that adding yeast to dairy cattle diets with large amounts of readily fermentable carbohydrates may reduce lactate concentration, improve rumen pH and boost cow performance, they said. There also is a possibility that benefits from adding a yeast culture to the diet may be heightened in diets with a larger amount of starch.

Those diets would have a higher acid load, which needs more “physically effective fiber” ​to generate a healthy rumen environment, they said. Providing a more stable rumen environment by adding dietary yeast to a high starch diet could limit the need for physically effective fiber and explain the jump in dry matter intake that helps improve milk and milk component yield.

Yeast cultures also may aid rumen fiber digestion as they could stimulate microorganism growth, the researchers said. Improved digestion in the rumen and throughout the digestive tract might predict increased benefit from feeding yeast with diets that have more forage.

Yeast cultures also have been linked to increased dry matter intake when given in the early stages of lactation, they said.

However, one challenge in diet formulation is the variability in nutrient content, they said. Errors in carbohydrate formulation with high starch (HS) diets may lead to health challenges and milk fat depression.

“The hypothesis of this experiment was that YC influences dairy cow performance by improving digestion and attenuating fluctuations in rumen pH, particularly in diets rich in starch or those prone to rumen acidosis,” ​the researchers said. “Therefore, the objectives were to evaluate rumen metabolism, digestion, and performance in response to supplemental YC in Holstein cows fed diets varying in starch content.”​

Methods and materials

In the study, four lactating cows were fed one of four diets for a period of 28 days before cycling to a new diet, the researchers said. Data was collected on the final 9 days of each feeding rotation.

The diets included a low starch diet with 23% of the diet dry matter (DM) and no yeast (LC-control), that diet with 15g of yeast a day (LC-YC), a high starch diet with 29% of DM and no yeast (HS-control) and that diet with 15g/d of yeast (HS-YC), they said.

Cow production, digestibility and nutrient flow were established on days 20-24 and on day 25 3 kg of corn grain DM was added to the rumen prior to morning feeding and milk yield and components were determined, they said.

Blood, rumen pH and rumen papillae were sampled on days 24 and 25 of the rotation, they said.

Results​

Adding the YC was found to improve milk yield for both diets, energy-corrected milk (ECM) production, fat, true protein and ECM compared to dry matter intake, the researchers said. Overall, dry matter intake was not altered.

“The fact that yeast culture improved production in both types of diets indicates that the mechanisms of action are not dependent on level of forage or level of starch in the ration, at least within the constraints of the experiment we conducted,”​ said Santos. “This suggests that if you supplement yeast culture, cows will respond to it even if there would be variability in forage and or starch contents.”​

Based on literature related to the use of supplemental yeast, there was an expectation of seeing some improvement in lactation performance when YC was added at 1.5 to 2 kg/d, he said. Part of that improvement likely stems from the additional feed intake, but some also could be attributed to the improved digestion of dietary nutrients and reduction in risk of rumen disturbances.

“Our data corroborates with that,” ​he said. “However, it also showed that responses to yeast culture do not seem dependent upon the structure of the diet.”​

Cows getting the HS diet saw an increase in milk true protein percentage and yield when compared to those getting the LS diet, the researchers said. The HS diet also tended to increase the amount of propionate and limited the ammoniacal nitrogen (NH3-N) in rumen fluid when compared to control.

The HS diet also increased the expression of genes for receptors and a transporter of short-chain fatty acids in the rumen papillae, they said.

The HS-YC diet boosted the proportion of dietary nitrogen in the milk true protein when compared to all other diets, they said. It also tended to have more microbial nitrogen synthesis than the LS-YC diet produced.

The HS-YC diet also reduced plasma haptoglobin and the concentration of rumen lactate, while raising the average rumen pH and limiting the time that pH was less than 6, they said. It also halted the drop in rumen neutral detergent fiber digestion otherwise caused by the HS diet.

Production post-challenge was similar to that established before the corn challenge, they said. YC boosted the yield of ECM and post-challenge it tended to limit rumen lactate concentration and reduced haptoglobin for cows on the HS diet.

Source: Journal of Dairy Science​

Title: Effect of supplemental yeast culture and dietary starch content on rumen fermentation and digestion in dairy cows​

DOI: doi.org/10.3168/jds.2017-13241​​

Authors: A. Dias, J. Freitas, B. Micai, R. Azevedo, L. Greco and J. Santos