Big benefits of sugars in rations

There’s growing evidence of the benefits that ration sugars bring to rumen function and milk production, and a subsequent need to formulate rations to meet specific minimum sugar levels.

Aside from the effects that high-sugar, sweet feed ingredients like molasses can have in simply encouraging the cow to eat more, feeding additional sugars is now thought to have a significant effect on both fibre digestion and microbial protein production, says KW technical manager Dr Michael Marsden.

“Based on research work carried out in the United States, for example, it appears that the fungi important in opening up fibre in the rumen ready for bacterial fermentation are stimulated by the addition of sugars. The fast rate at which sugars are broken down in the rumen also tends to more closely match the release of effective rumen degradable protein (ERDP) in the rumen, particularly when compared with other energy sources like starch.

“It’s like lighting a fire, where you need paper (sugars), sticks (starch) and coal (forage/fibre),” he adds.

Research has shown that molasses is more quickly fermented than high-starch wheat or fibrous silages. The result is a substantial “kick-start” to the rumen fermentation when sugars are present in the ration, and this maximises the capture of ERDP – energy to incorporate this protein into microbial growth is available almost immediately after each meal.

In addition, a US study showed that the replacement of starch with sucrose produced a significant increase in milk fat percentage (from around 3.2% to 3.6%), while milk protein percent stayed relatively constant (Table 1). Compared to starch breakdown in the rumen, sucrose fermentation supplies more of the building blocks needed for milk fat production, while also reducing the risk of acidosis disrupting feed digestion.

“There are also differences in response between the type of sugar that need to be taken into account,” Dr Marsden continues. “Compared to the sucrose, fructose and glucose most commonly found in plant materials, laboratory studies have shown that lactose fermentation, for example, yields a lot of volatile fatty acids – the primary energy source for ruminants – but slightly less microbial protein. Individual sugars also vary in terms of their digestibility within the rumen.”

The issue of additional sugars in the ration has been particularly relevant this winter, with grass silages containing on average just 1.4% sugars, compared with more than double this in the previous two years (2.8% in 2009, 3.0% in 2010)*. It means that unless other feed ingredients are adjusted accordingly, ration sugar levels will have been substantially lower for many herds.

“On a dry matter basis, typical forage-based rations that don’t include any liquid feeds often contain as little as 3-4% sugars overall. That compares to recommendations coming out of the US that suggest on overall ration content of 5-7% sugar is needed to provide the necessary digestible sugars to optimise rumen fermentation (Table 2),” Dr Marsden adds.

“At present, there’s little concrete evidence that these are the absolute levels required, but it appears to be a reasonable starting point. The key point to note, however, is that in most cases these levels can only be achieved by adding additional sugars, usually in the form of one of the molasses or distillery co-product liquid feeds.

“It’s also important to remember that these US recommendations are referring to total fermentable sugars, not just the sucrose content usual quoted as the sugar value in molasses products.”

Few traditional straights contain enough sugars to raise rations levels sufficiently, though rations including alternative feeds (typically co-products of the human food industry) are more often sugar-rich. Of the non-liquid feeds, sugar beet feed and citrus pulp contain good sugar levels in addition to digestible fibre (19-24% sugars), while the confectionery and breakfast cereal blends range from 14-22% sugars. However, all fall well short of the 35-64% sugars found in the various molasses – and distillery syrup-based liquid feeds and liquid blends (Table 3).

“Liquid feeds also have a very low substitution rate, meaning that the extra sugar can be added without displacing other feed ingredients; you get the added benefit of more dry matter intake and a higher energy intake,” states Dr Marsden. “Just be careful to maintain a starch:sugars ratio of around 3:1 to keep a balance of energy supply in the rumen, and introduce any additional sugars slowly to allow time for the rumen to adapt.”