Moving from first-crop haylage to second-crop haylage, finishing up last year’s corn silage and moving into this year’s new corn silage, changing from high-moisture corn to dry corn and changing the protein source from soybean meal to canola meal are realistic changes that may occur. But no matter how diligently we try to maintain consistency, dietary changes occur throughout the year, not just during certain periods or seasons.
We see these changes outside the cow. The new haylage is greener and more finely chopped. The new corn silage has a brighter yellow color and obviously has much higher grain and starch content. The high-moisture corn has a strong smell of alcohol. If we observe changes in feedstuffs from outside the cow, it most certainly will be noticed inside the cow once it drops into the rumen. One widespread strategy to manage these changes is to blend the new and the old feed for a couple of weeks – in some instances, it may be for only a few days – in the TMR to help the cow’s microbial population adjust to the change. Hopefully the cow won’t notice, but what happens next is truly amazing.
The modern, high-producing dairy cow is an incredible walking fermentation vat loaded with an extensive microbial population, and this varies from cow to cow. Digestion in the ruminant is a complex process. The main substrates of digestion in the rumen are non-structural carbohydrates (starch, sugar and pectin), structural carbohydrates (hemicellulose and cellulose, both known as fiber) and nitrogen-containing compounds (protein, peptides and amino acids).
It is at this point that complex fermentation begins, which is highly dependent on a diverse and dense microbial population. Consistent ration contents digested by a massive microbial population are critical to maintaining this process of nutrient flow for high-producing cows. Although the cow eats, we are essentially feeding this vast pool of microorganisms.
Factors affecting microbial growth and yield
In order to optimize microbial growth, a constant supply of nutrients is required. At any given time, the rumen will contain variable levels of peptides, amino acids, ammonia, carbohydrates, polymers, isoacids, lipids, vitamins and minerals. The level of these substrates will vary with time after feeding, the frequency of feeding, amount of rumen fill and the source and quality of feed consumed. We know the major nutrients needed by microbial species in the rumen, but we know little about the amounts and combinations needed to optimize microbial growth. We also know when the supply of nutrients to the rumen is reduced or limited, microbial growth and yield is also reduced. Delayed or late feeding or extended time held away from feed in the holding area or palpation rail also limits nutrient supply. The take-home message: Don’t let high-producing dairy cows run out of feed.
Carbohydrates are the major source of energy for rumen microbes. Studies suggest following the intake and hydrolysis of polysaccharides, the resulting hexoses and pentoses are readily fermented and support microbial growth and yield, but at different rates. For example, barley is more rapidly fermented in the rumen compared to corn. When carbohydrate levels in the ration change, there is a direct effect on the microbial population. When carbohydrate changes are desired in a ration, make small changes over time to allow the microbial population to adjust to the changes.
Ruminal pH has a major impact on fermentation and microbial growth and yield. Changes in ration composition, such as concentrations of grain, impact the rumen microbial population and affect rumen pH. When sources of readily fermentable carbohydrates are added to predominantly forage diets, fiber digestion may be impaired. While fiber digestion may be severely depressed, fermentation of starches and sugars may remain very high. This is the perfect precursor for large amounts of lactic acid to be produced and accumulate in the rumen – thus, causing low rumen pH. Numerous studies have shown low rumen pH decreases fiber digestibility and milk yield.
Prolonged effects of low rumen pH not only impacts fiber digestion and milk production, but also rumen health. Energy is not only spent fighting off an insult, but also to repair the damage inflicted via scarred tissue and reduced nutrient absorption.
Rate of passage has a significant influence on the microbial population. Liquid and solid turnover in the rumen is not the same from cow to cow and varies with intake. Since microbes are present in all compartments within the rumen, their survival will depend on their ability to reproduce at a rate equal to or greater than turnover within that compartment. Attachment to feed particles is necessary for survival of microbes that have a slow rate of growth, such as protozoa and fiber digesters. Both total microbial yield and growth efficiency are influenced by liquid and solid turnover in the rumen. This microbial yield is extremely important because it influences the amount of microbial protein made available to the cow each day. Since microbial protein can supply a significant proportion of the daily protein requirement of the lactating cow, the significance of maximizing microbial yield is obvious. Intake, rumen fill, rumen turnover and microbial growth and yield are all positively and intricately related.
Feed change challenge
As milk production per cow continues to increase, the demands on this massive microbial fermentation system are extraordinary. This complex system cannot afford a bad day. To maintain high daily production, there are several keys to optimizing feed transitions:
- Don’t restrict feed availability if your goals include high production and feed efficiency.
- Manage feed changes by diluting the new feed with the old and allowing time for rumen microbes to stabilize as they adjust to the feed change.
- Adjust dry matter of new feeds if they are forages to account for moisture content. This will help avoid overfeeding carbohydrates that could lead to low rumen pH and the onset of rumen acidosis.
- Monitor intake as an indicator of rate of passage. If organic matter digestion in the rumen is reduced, it will have a negative impact on microbial growth and yield, resulting in lower milk production.
- Consider feeding a direct-fed microbial (DFM; probiotic) to stabilize microbial growth and yield. A DFM is a live microbial feed supplement that beneficially affects the host by improving digestive microbial balance. Be sure to consider a DFM that is supported with sound science and research. Be sure it is compatible with other ration ingredients, specifically monensin.
In addition to management considerations, microbial challenges due to feed changes can be mitigated effectively with DFM support. 
References omitted but are available upon request. Click here to email an editor.
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Keith A. Bryan
- Technical Services Manager
- Ruminant DFM & Silage Inoculants
- Chr. Hansen Animal Health & Nutrition
PHOTO: Consistent ration contents digested by a cow’s massive rumen microbial population are critical to maintaining nutrient flow for high-producing cows. Staff photo.
