Sugar has been consumed by dairy cattle since the beginning of time. They have come from the pastures which are naturally high in sugars. When we feed fermented forages most of the sugars have been converted to fermentation acids. The rumen and the cow have evolved to use plant sugars. The most prevalent form of added sugar fed to cattle has been molasses. The main sugars in molasses are sucrose (65%), fructose and glucose. Historically, the reason for feeding added sugar has mostly been as a sweetener or to improve palatability of feeds that are being fed. This concept has now changed. There are now research papers showing advantages to feeding sugars. More importantly though is the fact that we now have laboratories analyzing total sugars in forages and byproducts. Sugar addition to rations has had a mixed history. It has only been in recent years that consideration has been given to the addition of sugar to the ration as a nutrient to benefit rumen function as well as the metabolism of the cow. We need to start thinking in terms of individual sugars.
The addition of sugar to dairy cattle diets can be very positive. Trials have reported increased milk yield and milk fat percent or increased NDF digestibility (Broderick and Radloff, 2003, Broderick and Smith 2001, Varga etal. 2001, Oldick etal. 1997). Yet, the addition of sugars to dairy cattle diets has not always improved milk yield, ruminal microbial protein yields or milk components (Hristov and Ropp 2003; McCormick etal. 2001, Morales etal. 1989). The reported variation in response to sugars in dairy cattle diets can be explained by four processes that occur in the rumen. These processes are:
- A shift in the end products of ruminal sugar fermentation based on bacterial growth rate and rumen pH.
- Not all sugars are used with the same efficiency by rumen bacteria for growth.
- Establishment of a viable population of anaerobic fungi in the rumen.
- Wasting of energy by rumen bacteria (energy spilling) when the supply of fermentable carbohydrates exceeds the needs for microbial growth.
5 carbon sugars – Arabinose, Ribose and Xylose
6 Carbon sugars – diverse across all plants; Glucose, Fructose, Galactose and Mannose
Sucrose – found in all plants and consists of Glucose + Fructose
Maltose – found in all plants and consists of Glucose + Glucose Lactose – found in milk and consists of Glucose + Galactose
Raffinose – found in cottonseeds and sugar beet pulp and consists of Galactose + Fructose + Glucose
Maltotriose – found in corn distillers grains and consists of Glucose + Glucose + Glucose
Stachyose - found in soybeans, consists of Galactose + Galactose + Glucose + Fructose
Fructans – mainly in grasses and consists of fructose
Galactans – mainly in alfalfa and soybeans and consists of galactose
Pectins – mainly in alfalfa and soybeans and contains arabinose + galactose
Cellulose – all plants and contains long chains of glucose
Hemicellulose – all plants, higher concentration in grasses and contains arabinose, xylose, mannose, galactose, and glucuronic acids
Starch – diverse among plants and contains long chains of glucose
Sugar alcohols – polyol – forage fermentation product - Mannitol – 6 carbon
It would be a mistake to assume that all sugars have the same fermentation rate in the rumen and that all sugars are used with the same efficiency by rumen bacteria. Yet, that is exactly what we do when we formulate dairy cattle diets.
The most common rapidly available sugars in forages and various grains are sucrose and glucose. However when forages are fermented these sugars disappear, leaving residual sugars from the fermentation. These residual sugars are the breakdown products from hemicellulose which are mainly the 5 carbon sugars shown above. There has been some preliminary work that would indicate that the rumen microbes use the different monosaccharide’s with different efficiencies. We need to know more about the utilization of the water soluble disaccharides and oligosaccharides in the rumen and by the cow. The sugars found in cane molasses are mainly sucrose, glucose and fructose. Molasses contains 50% sugar on an as fed basis and 70% of the sugar is sucrose. Cellulose, starch and sugars all end up eventually as hexoses. These hexoses then are metabolized to pyruvate which can be metabolized to acetate, propionate, butyrate or lactate. Ruminal conditions must exist where the majority of hexose is fermented to acetate, propionate, and butyrate but not lactate. The energy generated from this 19 fermentation must be used for bacterial growth if sugars are to be used successfully in dairy feeding programs.
Rumen Diversity Influences the Efficiency of Sugar and Starch Utilization
Table 1 provides a global, but simplified view of the rumen ecology. This is a balance that can be disturbed by many factors:
- Inadequate effective fiber to produce adequate buffering
- High fermentable starch levels in the ration, which lead to periods of low ruminal pH
- Ration sorting, which can lead to rumen acidosis and sore feet
- Excessive rumen degraded protein containing peptides with Histidine
- Over-crowding or high stocking density, which leads to slug feeding
The starch and sugar bacteria have very high growth rates. They can drive ruminal pH down very quickly. There is a balance that can be achieved where there first is a balanced nutrient flow to these bacteria that will stimulate a coupled fermentation – one that drives microbial mass and not microbial waste in the form of VFA. Next, there is the importance of enhancing the secondary fermenters which will use the lactic acid moderating the pH drop that occurs after an ingestive episode. This requires a malic acid source. A major organic acid found in molasses is malic acid.
A significant part of the normal rumen ecology is composed by protozoa which, under normal conditions, make up 40-45% of the microbial mass (Table 1). In contrast to the bacterial population, only 10% of this population washes out of the rumen to contribute to the microbial protein supply. This is in contrast to 75 to 85% of the bacterial population washing out of the rumen, with the rest lysing and being predated on by the protozoa. We say that the microbial mass that stays in the rumen contributes to the recycled N pool. It is now suggested that we are under estimating the contribution of this pool to the N needed by the bacteria in the rumen. It should be pointed out that many years ago classic studies conducted by Reis were done in sheep in Australia demonstrating the positive wool growth in sheep resulting from depopulating the rumen of protozoa. These studies pointed out the large pool of bacteria that the protozoa consumed daily in the rumen. The Fungi make up only 3-8% of the microbial mass. It is proposed that they have a key role in opening up the fiber that will enhance colonization by the fiber bacteria. They are stimulated by sugar, which research suggest will result in improved fiber digestion.
It is the understanding of these interactions that will help us in developing better rations going into the future. The balance of the carbohydrates with different fermentation rates and the different protein sources with different fermentation rates must be balanced. Unfortunately we have yet to reach the point where we can effectively do this. It is a dynamic second order system. We have improved our ability to measure and define the amount of each protein and carbohydrate fraction, but in our nutrition models we assume that a cow eats 24 meals a day, the same size and evenly spaced. We know this not to be the case. Does this mean that we should abandon the use of nutrition models? No, but it does require that as nutritionists, we admit that these models do not account for associative feed effects or the impact of ration sorting and meal pattern on nutrient utilization in the rumen.
Impact of Sugar on Animal Performance
There have been several studies over the years that have demonstrated the value of adding sugar to rations. Most of these studies have been with the addition of molasses or sucrose directly. Several of the studies replaced the starch with the sugars, keeping the NFC constant. The results were always positive not only in milk yield but also in components. In studies with fermenters, the work showed increases in fiber digestion. It has been suggested that the fungi which play a role in opening up the fiber, are stimulated by the 6-carbon sugars. This is important because it fits well with the rapidly degraded protein (mostly the soluble) to give an early stimulation to fiber digestibility. Additionally, if we can reduce the starch in the ration, we will have better control of rumen pH, an additional enhancement for fiber digestibility. In the original nutrition models, it was assumed that all of the sugar was broken down in the rumen. This assumption is not correct. This is important because of the positive impact that the digestible sugars can play in the metabolism at the mammary gland. It is important to note that added sugars, in the form of sucrose or sucrose equivalents (glucose + fructose) are about 84% degraded in the rumen.
Figure 1 displays results from some of the work conducted in France by Dr. Rulquin and colleagues at INRA. This suggests that it is important not only to enhance rumen function but also having an optimum amount of digestible sugar that will enhance milk true protein yield. This means that we need to consider the feeding of additional sugars in the rations. Many rations that are fed in this country have silages as their forage base. This usually results in rations with a 3 to 4% total sugar as measured by the 80% ethanol procedure. Unfortunately, this procedure does not identify the individual sugars. A high percentage of these sugars are the 5-carbon sugars discussed earlier. These sugars are not very digestible in the rumen or in the small intestine. It is recommended that we should add about 3 to 5% additional sugar in the form of 6-carbon sugars. This will result in a total sugar in the ration of 6 to 8% of the DM. If we assume that 35% of this sugar will escape then added to the base sugar level plus what we will derive from the escaped starch, we will approach the levels suggested by the work of Rulquin shown above. Research using molasses based liquids shows 84% utilization in the rumen.
Harris and Van Horn (1983) suggested that at 8% or less of the total ration dry matter molasses would contain the same productive energy as ground corn. This would be equal to 4 pounds of molasses on an as fed basis, when DMI was 50 pounds. Feeding 4 pounds of cane molasses would supply 2 pounds of sugar. At dietary concentrations above 8% of ration dry matter, the value of molasses declined relative to corn. Recent trials suggest that Harris and Van Horn were correct. In a recent meta-analysis of 25 published research trials, 3.5% FCM yield, milk protein yield, milk fat yield, were maximized when the diet contained 5 – 7% supplemental sugar (Emanuele et al., 2015). Non-linear analysis of the database indicated that for cows producing 90 to 100 pounds of milk, the ideal dietary sugar content was 7.14% (Emanuele, 2016, Leading Dairy Producer Conference, Wisconsin Dells, WI.). For all cows, the ideal dietary sugar content was 6.75% of diet DM (Emanuele et al., 2015).
Broderick and Smith (2001) replaced high moisture corn with dried molasses. Their diets contained 0, 4, 8, or 12% dried molasses. Their diets contained 60% forage with 67% of the forage from alfalfa silage and 33% from corn silage. When high moisture corn was replaced with dried molasses at 4 or 8% of diet DM, DMI increased (P = 0.04) (Table 2). The magnitude of the increase in DMI was 2.4 pounds. At least some of the nutrients from the increased DMI were used for fat synthesis because 3.5% FCM was increased when diets contained 4 or 8% dried molasses. The magnitude of the increase in 3.5% FCM was 4.4 pounds. Fat yield (lb/day) was increased when diets contained 4 or 8% dried molasses but not at 12% dried molasses. Rumen ammonia concentration was decreased when dried molasses replaced high moisture corn (P = 0.05). The magnitude of the decrease was 1.4 units (11.3 vs. 9.9 mM).
Based on this trial, dry molasses should not exceed 8% of diet DM. The amount of forage in the diet may influence the amount of sugar or molasses that can be used in the diet. Broderick and Radloff (2003) fed diets to high producing dairy cows that contained 50% forage on a dry matter basis (Table 2). The forage component of the diet was 60% alfalfa silage and 40% corn silage. They replaced high moisture corn with liquid molasses. Diets contained 0, 3, 6, or 9% liquid molasses. Dry matter intake and milk yield were maximized when the diet contained 3% liquid molasses on a dry matter basis (P < 0.01). Some of the additional energy derived from the additional DMI appears to be used for fat synthesis because 3.5% FCM was increased 4 pounds compared to the control diet. Yield of all milk components was maximized when the diet contained 3% liquid molasses. Based on the reported dry matter intake, the amount of liquid molasses in the diet was 1.75 – 1.84 pounds on a dry matter basis. This would be equivalent to 2.33 – 2.45 pounds of liquid molasses on an as fed basis. The amount of sugar added to the diet from the molasses would be 1.2 pounds on an as fed basis.
Molasses was compared to molasses and animal fat (Oldick et. al. 1997). The treatments in this trial were control without molasses, molasses only, molasses and animal fat at 2, 4 and 6 pounds of ration dry matter. The molasses and fat liquid supplements were included in the diets at 2.5, 4.9 and 7.4% of the diet dry matter. The molasses only diet contained molasses at 3.4% of diet dry matter. All treatments had similar energy density. Cows on the control diet had an average milk production of 71.6 pounds. Milk response to the molasses only treatment was 2.9 pounds greater than the control diet. Molasses did not increase dry matter or net energy intake but did increase milk yield. There are two possible explanations for the occurrence. One possibility is that the energy from molasses was used with greater efficiency for growth by rumen bacteria than the energy from other dietary carbohydrates. A second possibility is the presence of an associative effect. Adding molasses to the diet may improve the ruminal digestion of NDF. This hypothesis is supported by recent observations from Varga and coworkers (2001). They reported that when starch was replaced with sucrose, NDF digestibility was increased. At the greatest concentration of sucrose, 7.5% of diet DM; NDF digestibility was increased 8.5% compared to the control diet, which did not contain supplemental sucrose.
Growth Rate Dependent Shifts in Fermentation Products Can Explain the Variable Response to Sugar Addition in Dairy Diets
In the trials conducted by Broderick and Smith (2001) and Broderick and Radloff (2003), the response to sugar additions to the diet was not linear (Table 2). The response was quadratic because positive responses were reported at low inclusion levels of sugar addition and negative responses were reported at high inclusion levels. One explanation for the quadratic response to sugar addition is that some ruminal bacteria change their fermentation products based on their growth rate. When the rate of ruminal fermentation is rapid and starches and sugars are readily available in the rumen, Strep. bovis and Selenomonas ruminantium shift their fermentation from acetate, propionate and formate to lactate (Russell 1998, Russell 2002 pg.71-72). Both Strep bovis and S. ruminantium can grow very rapidly in the rumen. It is likely that at the higher levels of molasses, these bacteria shifted their fermentation to lactate with a reduction in acetate and propionate production. The shift to lactate fermentation is also influenced by the supply of amino acids in the rumen. When amino acid nitrogen availability is low, these organisms will use ammonia nitrogen as a nitrogen source. When they use ammonia nitrogen as a nitrogen source, the shift to lactate fermentation occurs at a slower growth rate (Russell, 1998). To prevent a shift to lactate production, sugars need to be added to dairy diets in moderate amounts and in combination with protein sources such as soybean meal and canola meal.
When feeding trials have been conducted, it has been assumed that all sugar sources would support the same amount of microbial growth and have similar fermentation rates. We now know that this is not a correct assumption. Bond and coworkers (Bond et. al. 1998) reported that Streptococcus bovis cannot utilize pentose (5-carbon sugars) and the growth rate of Strep. bovis is 40% slower on lactose than on glucose (Figure 2). Ruminococcus albus and Ruminococcus flavefaciens are the major species of cellulolytic cocci in the rumen. These cellulose fermenting cocci do not grow on pentose, growth on glucose is slow but they will grow well on cellobiose (Russell, 2002, pg. 19). Cellobiose is a disaccharide made up of glucose units with a beta 1-4 linkage. Ruminobacter amylophilus, a starch digesting rumen bacteria will ferment maltose but not glucose (Russell 2002, pg. 21). It appears that certain sugars will stimulate the growth of specific rumen bacteria and that some sugars will not support the growth of major ruminal bacteria species.
All sugars are not equal when it comes to supporting microbial growth in the rumen (Van Kessell and Russell 1995). Strobell and Russell (1986) examined the effect of pH and carbohydrate source on yield of microbial protein from in vitro fermentation. They reported that the yield of microbial protein declined as pH was reduced from 6.7 to 6.0. There was an interaction between pH and carbohydrate source. When pH of the fermentation was 6.0, the yield of microbial protein was lowest on pectin and xylan compared to cellobiose, sucrose, starch or a mixture of carbohydrate sources. When the pH of the fermentation was maintained at 6.7, the yield of microbial protein was greatest on cellobiose, sucrose or a mixture of carbohydrate sources, and intermediate on starch or pectin and least on xylan. This trial suggests that 5-carbon sugars (xylan) will support less microbial growth in the rumen compared to 6-carbon sugars. McCormick and coworkers (2001) reported differences in fermentation between cornstarch, lactose and sucrose. Their diets contained 50% forage and 50% concentrate. They replaced ground corn with either lactose or sucrose at 2.5 and 5.0% of diet DM. Total organic acid production and fermentation pH was not different for any of the diets. Ammonia N concentration in mg/dl was lower on the sucrose supplemented diets compared to the other diets (P = 0.06). This would suggest that the rate of fermentation was faster on the sucrose supplemented diets compared to ground corn or lactose diets. The rate of protein fermentation would have been rapid on all diets because the major rumen degradable protein source in these diets was freeze-dried fresh ryegrass. In this study, treatment differences between ground corn and lactose were not significant for the parameters reported.
Impact of Sugar and Molasses on Ruminal pH and Fiber Digestion
If neutral detergent soluble carbohydrates (NDSC) differ in their rate and pattern of fermentation, we can indirectly measure these differences by measuring ruminal pH and volatile fatty acid production. The impact of NDSC on ruminal pH will depend on the amount of NDSC in the diet and the type of forage. When molasses or sucrose were fed at amounts greater than 12% of diet dry matter, rumen pH was depressed within one hour after feeding (Moloney et al. 1994, Khalili and Huhtanen 1991a). The reduction in ruminal pH lasted for up to four hours after feeding. If sodium bicarbonate was fed in the diet along with sucrose, the depression in ruminal pH was prevented (Khalili and Huhtanen 1991a). When molasses-based liquid supplements or dry sugar are used in dairy rations and fed at amounts less than 8% of diet dry matter rumen pH was not depressed compared to the control diet (Table 3; Piwonka and Firkins 1993, Maiga et al. 1995, McCormick et al. 2001, Varga et al. 2001).
Molasses-based liquid supplements and sugar are readily digestible sources of energy for dairy cattle. When added to dairy rations at 6 to 8% of the total ration dry matter, molasses-based liquid supplements and sugar may increase dry matter intake, fat-corrected milk yield, milk protein and milk fat yield and NDF digestibility. The mode of action appears to be through enhancing NDF digestibility, altering the ruminal microbial population and possibly providing an increased supply of nutrients for fat synthesis. Sugar or molasses, when fed at less than 8% of diet dry matter, can be used with the same efficiency as corn for milk production. Physical factors of the ration can influence responses to molasses or sugar. In rations with less than 19% ADF, and small particle size, use of sugar and molasses based liquid supplements may not increase feed intake and milk production. Response to liquid supplements and sugar has been greater when the ration contains adequate amounts of rumen degradable amino acids and peptides. Research trials published since 1983 suggest that molasses and sugar do more than just increase ration palatability, they can play a greater role in dairy rations by altering ruminal microbial populations and possibly increasing microbial growth in the rumen of dairy cattle.
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