Gradual grazing switch pays dividends (March 25)

Understanding the rumen’s microbial shift when herds transition from silage to grazing-based rations is key to maximizing milk production from grass.

TEXT LAURA YATES

Focusing on rumen microbiota can help support cows – and milk production – through dietary changes at turnout, while promoting fibre digestion and milk solids.

Grazed grass is a natural diet, but the challenge that turnout poses to the cows’ digestive system should not be underestimated, according to Lallemand Animal Nutrition’s Mark McFarland.

“Spring grass is around 20% dry matter – representing a stark difference to winter rations,” he says. “And when what goes into the cow’s rumen is changed, the microbial environment of the rumen also changes. The rumen microbiota population shifts to adapt to the new diet.”

Cows fed a TMR, with between 40% and 45% dry matter, through the winter will be challenged when turned out to grass. “Suddenly changing to a wetter diet, with less than half the dry matter, will see cows often struggle to physically eat enough,” he explains, adding that the challenge is even greater during a wet spring.  

In addition to the dry matter differences between grazing and a fermented forage-based ration, lush spring grass is also higher in rapidly fermentable sugars and crude protein compared to a TMR.

Rumen microbiota

“It can take as long as six weeks for the rumen and its microbiota to adapt to the new diet, with the biggest change in microbial populations among structural carbohydrate degraders, which break down fibre,” explains Mr McFarland.

He says one study has shown that a change of diet from TMR to pasture resulted in the relative abundance of structural carbohydrate degraders in the rumen dropping from 29% to just 6% of the rumen microbiota classified within a functional group.

“At the same time, rapidly fermentable carbohydrate degraders in the rumen were shown to increase from 68% to 87%, which is another dramatic shift,” adds Mr McFarland. He explains that this can result in excessive lactate production, which reduces rumen pH and fibre digestion, increasing the risk of acidosis.

"Early-spring grass is rich in soluble sugars, with a high acid load and low fibre index,” he explains, adding that there is a significant increase in lactate producing bacteria, such as Streptococcus, which puts the cow at risk of sub-acute ruminal acidosis (SARA) and bloat.

Fresh grass: grazing has a high acid load and low fibre index

Nutritional challenge

Another nutritional challenge posed by spring grass is the additional crude protein load. Early-spring grass is often high in rumen degradable protein (RDP), which is a low-quality protein that is broken down by the rumen microbes into ammonia.

“When RDP levels are high, ammonia levels can exceed what the rumen microbes can handle, leading to an oversupply. A typical symptom of that is loose manure,” says Mr McFarland.

The excess urea requires the cow to expend energy for excretion and this is at a time when she is already struggling to consume enough dietary energy. “As a result, we typically see milk urea levels rise during the early grazing period, while cows can be losing body condition,” adds Mr McFarland.

“In addition to dung looseness, for herds with rumination collars, a drop in rumination time is another indicator of poor rumen function.”

The microbial shift in the rumen caused by a change from a TMR to grazed grass diet also hampers digestive efficiency and ruminal absorption. Not only does the microbiota change, but also what the bugs produce – volatile fatty acids (VFAs) – changes.

“This change in VFAs affects the rumen wall. The rumen papillae, which are the absorptive surface area of the rumen wall, reduce in length and can take up to 10 weeks to fully adapt.”

Mr McFarland explains that these changes limit the feed value of the grass, resulting in a missed opportunity to make the most of this forage. “With milk prices relatively good, producers should be motivated to try and maximise milk production from grass because the extra litres are worth so much more now.”

Diet transition

The rumen needs time to adapt to any new diet, so a gradual introduction to grazing is recommended. Mr McFarland says grass should be introduced slowly at turnout, gradually building up the number of hours that cows are out grazing, and feeding a forage-based buffer ration indoors to support the rumen through the change – particularly during wet days.

“Within the buffer ration producers must provide structural fibre, because that’s what’s lacking in the grass at turn-out, to help stimulate rumination and maintain a more stable rumen pH,” he explains. During the transition, he recommends feeding the rumen-specific live yeast Saccharomyces cerevisiae CNCM I-1077, known as Levucell SC, ahead of and during the grazing season.

“The yeast helps cows to manage dietary changes at turnout from the inside out. It helps raise and stabilise rumen pH, by balancing the microbiota, increasing the abundance of lactic acid-utilising bacteria and fibre degraders.

“The yeast also competes for sugars with lactic acid producing bacteria – effectively competing for their food source – so the production of lactic acid is reduced.” By promoting a positive microbial environment, Mr McFarland adds that the rumen-specific yeast promotes the production and absorption of volatile fatty acids from the rumen, leading to increased milk and milk solids at grass.

“Cows can make better use of the RDP and fibre in the grass, maximising its feed value,” he explains. Commercial trials have demonstrated the significant productivity benefits that can be achieved from including the rumen-specific live yeast in rations. Adding Levucell SC to the diet can increase average daily milk yield by up to 8%, or 2.3kg.

“By managing rumen function and productivity, producers can help to ensure their herds remain in good body condition and, consequently, cows have a greater chance of getting back in-calf while, at the same time, also producing milk from one of the cheapest feeds available.”