Sugars are non-structural carbohydrates that are soluble in water. They can be classified as simple sugars or monosaccharides, having one sugar molecule, and disaccharides, having two monosaccharide molecules. Monosaccharides include glucose, fructose, and galactose, while disaccharides include sucrose (glucose + fructose), lactose (glucose + galactose), and maltose (glucose + glucose). It ‘s necessary to clarify that a structural carbohydrate known as cellobiose also contains two glucose units, but it differs from maltose by the type of bond that links the glucose units; maltose is made up of α-linked glucose units while cellobiose is made up of β-linked glucose units. The importance of these bonds lies in the ability of animal/human enzymes to break them down. Human and animal enzymes can digest α-linked glucose units but cannot digest β-linked glucose units. The β-linked glucose units are broken down by microbes. Glucose, fructose, and sucrose are the most abundant sugars found in plants (Hall, 2002). Complex sugars which include polysaccharides (multiple units of simple sugars) like starch and glycogen won’t be discussed in this article.
Sugars are important sources of energy in the diet, like starch, but in ruminant animals sugars are fermented in the rumen at a faster rate than starch. According to Van Amburgh et al. (2015), the digestion rate (kd) of sugar in the rumen is 40-60%/h, compared to starch and soluble fibers with a slower digestion rate of 20-40%/h, and available neutral detergent fiber (NDF) which is even slower at 1-18%/h. The rapid rate of sugar fermentation might pose the risk of lowering the rumen pH, but there hasn’t been any conclusive evidence in the literature to prove this, so sugar inclusion in the diet isn’t directly linked to ruminal acidosis (Oba, 2011). Studies have shown that high sugar diets produce more butyrate compared to other volatile fatty acids in the rumen, and this is advantageous because butyrate provides energy for the functioning of the inner lining of the rumen and the cells that line the large intestine, aiding the absorption of fermentation products across the rumen wall (Hall, 2002; De Ondarza et al., 2017). Inclusion of dietary sugar has also been shown to increase dry matter intake (DMI; Broderick, 2008), milk yield (De Ondarza et al., 2017), and milk fat content (Broderick et al., 2000; Firkins et al., 2008; Penner and Oba, 2009). On the flip side, NDF digestion has been shown to decrease with high sugar diets. A study by Huhtanen and Khalili (1991) showed depression in ruminal NDF digestion when they fed a diet with included sugars of up to 15.9% of diet dry matter. Regarding the effect of sugar inclusion on enteric methane emissions, Hindrichsen et al. (2005) noted that there was no statistical difference in enteric methane emissions when they fed six diets with dietary sugar ranging from 6.2 – 17.7 % to Brown Swiss dairy cows. This was supported by Staerfl et al. (2012) who observed no effect of feeding a high-sugar ryegrass containing 193 g/kg water-soluble carbohydrate (DM) on enteric methane emissions in primiparous Holstein cows.
A major question is how much sugar is ideal for optimal rumen function and productivity? Hall (2002) suggested 5% of the ration should be sugars, while De Ondarza et al. (2017) observed from the review of several studies that the inclusion of 6.75 – 8.0% dietary sugar in the diets of lactating dairy cows had the highest effect on fat-corrected milk yield and milk protein yield. They also recommended the inclusion of 21.6 – 27% of diet DM of dietary starch, and 6 – 8.5% of diet DM of soluble fiber when adding sugars in dairy diets. Furthermore, Hall (2002) advised the provision of adequate effective fibers and rumen degradable protein in the diet as considerations for sugar inclusion in dairy diets.
Sugars can be considered as possible alternative sources of energy, however, the impact of other components in the ration like NDF, protein, and starch has to be considered for optimum benefit.
— Gift Omoruyi