The scientific reason has to do with the way the rumen functions, the importance of microbes and the way they function in the rumen. The growth of microbes depends on the energy and protein content of the forage.
In a typical forage, the rapidly available energy and proteins are unbalanced, with energy being lower, which limits microbial growth. If the concentration of soluble sugars is increased, then more energy becomes rapidly available to the microbes, which balances the equation.
But whether you prefer the simple reason or the more complex explanation, in either case to produce high-sugar forages, you’ll have to know what to plant and how to harvest to obtain those higher levels.
In a recent webinar sponsored by Beef Cattle Research Council, Gilles Bélanger (Agriculture and AgriFood Canada, AAFC) and Robert Berthiaume (Valacta) discussed their study conducted in collaboration with other researchers from AAFC and University Laval involving the production and use of sweet (or high-sugar) forages. For purposes of this discussion, keep the formula in Figure 1 in mind.
Morning versus evening harvest
The study included taking cuts of spring-growth alfalfa every two hours, from an hour after sunrise to early evening for a total of eight cuttings, and measuring the carbohydrates as a percent of dry matter.
The result was a noticeable increase (about 3 percentage points) in NSC, which was mostly in the form of starch.
As the day progresses, photosynthesis produces sugars faster than the crop can use for growth. These sugars accumulate through the day, and for the legume species it accumulates in the form of starch.
Maximum sugar concentration content was reached in late afternoon, between 11 and 13 hours after sunrise (roughly 5 to 7 p.m.).
Very similar results were found when evaluating morning versus afternoon cuttings of timothy. Sugars typically began increasing a few hours after sunrise and then tapered off toward late afternoon and evening.
As a grass, the NSC increase was due to an accumulation of sucrose.
The test included eight species of grasses and legumes commonly grown, including reed canarygrass, smooth bromegrass, meadow bromegrass, timothy, tall fescue, Kentucky bluegrass, red clover and alfalfa. In all cases, sugar concentration was greatest during afternoon or evening cuttings.
The greatest change in percentage came from reed canarygrass. The mean across all species was an increase of 1.5 percentage points.
Bélanger, Berthiaume and collaborators also tested the summer regrowth from these species on the same cutting schedule and found a mean change of 2.1 percentage points greater for afternoon or evening harvests.
The greatest increase in sugars from summer regrowth came with tall fescue, with an increase of 5.5 percentage points in late-day harvest.
It is well-known that as the plants continue to respire in the swath, it will use up some of the sugars in that process. This results in an overall decrease in sugars. What Bélanger and Berthiaume found is that the difference in sugar concentrations (not the sugars themselves but the difference in sugar levels) at cutting (morning versus afternoon) was maintained at the end of the wilting period.
In other words, those forages that had been cut in the afternoon still had higher sugars after wilt than those that had been cut in the morning.
The study also looked at the differences in sugar content with narrow windrowing versus wide windrowing in alfalfa. Those forages that had been narrowly windrowed for drydown had lower sugar content than those cut for ensiling.
They also determined that greater sugar levels at ensiling meant better fermentation, due to decreased pH, increased lactate concentration and decreased ammonia.
Species selection
In the eight-species comparison over spring growth and summer regrowth tests, two species consistently had higher sugar concentrations: tall fescue and red clover.
The two species that tended to have lower sugar concentrations were reed canarygrass and alfalfa.
In a subsequent study, one cycle of genetic selection in alfalfas accounted for an increase of 1.1 percentage points in sugars, while afternoon or evening harvesting accounted for an increase of 4.1 percentage points.
The Canadian scientists continue to investigate the potential of genetic selection for increasing sugar concentration in alfalfa.
N fertilization
Bélanger, Berthiaume and collaborators conducted studies two different years to determine if nitrogen (N) fertilization had an effect on sugar concentration in timothy.
They used different rates of nitrogen on timothy, which did not significantly influence sugar concentration at the heading stage. They also looked at the sugar levels at the anthesis stage (later than the heading stage) and again did not see a significant difference, even though the N fertilization rate was quite high.
High-sugar forage effects on cow performance
To study the effects of high-sugar forages in the diet of dairy cows, Bélanger, Berthiaume and collaborators conducted a study in Quebec with three cow groups and forage treatments – early lactation cows with alfalfa haylage, mid-lactation cows with timothy haylage and late-lactation cows with alfalfa haylage.
In each group of 18 cows, eight were fitted with rumen cannulas to assist the monitoring regimen, looking at the balance of energy and protein levels in the rumen.
With that information, their hope was to balance energy and protein to increase microbial production in the rumen. This would, in turn, reduce fecal nitrogen and urine nitrogen, which would increase milk protein production.
For this study, plants were cut in late afternoon (about 12 hours after sunrise) after a sunny day or early in the morning of the next day (at sunrise).
Plants were field-dried and conserved as haylage into large rectangular bales harvested at 50 to 60 percent dry matter, individually wrapped in plastic. Afternoon-harvested and morning-harvested haylages were offered to the cattle once daily.
First trial – early lactation cows with alfalfa haylage
For the early lactation group, the difference in morning-cut and afternoon-cut alfalfa was 1.3 percentage points in sugar concentration after fermentation.
This was fed in a complete diet with forages as 59 percent of the ration. Overall, there was an increase of 0.7 percentage points of sugar concentration in a TMR of 41 percent concentrate diet due to the late-afternoon cutting of alfalfa.
In terms of production for early lactation cows, the late-afternoon cutting had no significant effect on dry matter intake or milk yield. This held true among primiparous cows as well as multiparous cows.
The only significant finding was an interaction among primiparous cows in relation to milk urea nitrogen. With the late-afternoon-cut forages, these cows were able to more efficiently use the nitrogen and produced less milk urea nitrogen. There was, however, no change for the older cows.
Second trial – mid-lactation cows with timothy haylage
In this trial, mid-lactation cows were fed a TMR containing 65 percent timothy haylage and 35 percent concentrate where the crude proteins and neutral detergent fiber percentages were nearly the same (not significantly different).
The sugar levels, however, in the two diets between the late-afternoon cuttings and early-morning cuttings, had a difference of 0.9 percentage points.
In looking at the energy-corrected milk between the two groups, the primiparous group increased by 0.88 pounds per day (0.4 kg) with late-afternoon-harvested forage, while the multiparous group increased 3.3 pounds (1.5 kg).
High-sugar timothy haylage, then, appeared to be a very economical proposition based on this result.
Third trial – late-lactation cows with alfalfa haylage
This trial included late-lactation cows (around 200 days in milk) which were fed high-sugar haylage (12.8 percent sugars in late-afternoon harvests after fermentation) and low-sugar haylage (10.5 percent in early morning harvests after fermentation).
This was fed straight with minerals. The results were that dry matter intake increased significantly by 2.2 pounds (1 kg) per day with the late-cut forage, and energy-corrected milk yield increased 2.2 pounds (1 kg) per day in the same group.
Those cows fed the late-afternoon harvest of alfalfa haylage also had significantly less milk urea nitrogen than those fed the early morning harvest.
This indicates that the cows fed the forages with higher sugars were able to make better use of the available energy. In this study, primiparous and multiparous cows reacted similarly.
Beef cattle
The study also looked at cattle preference for forages using beef steers. Their study corroborated what several other trials have established in that cattle clearly preferred forages with higher sugar content, resulting in an increased intake of 2.2 pounds (1 kg) of dry matter per day.
However, steers preferred the high-sugar forage during the first two hours, after which the consumption rate leveled out over the two forages for the remainder of the bunk time.
Berthiaume said that while they didn’t have a good explanation for this behavior, they could hypothesize (as a number of Australian studies did) that animals will try to balance the supply of sugars and proteins in the rumen.
But what about grazing cattle? In a second trial, instead of bunk feeding, Bélanger, Berthiaume and collaborators sectioned a pasture into three areas and used the strip-grazing method.
Every day a new grazing strip was offered to cow-calf pairs at 7 a.m., 1 p.m. or 7 p.m. The composition of the pasture was a mixture of native grasses. Forage tests during those same grazing times showed an increase of soluble carbohydrates during the later part of the day.
The research found that cow liveweights decreased by 0.15 pounds (0.07 kg) per day over the summer season of those offered grass in the early morning.
Cows offered fresh grass in the evening saw a weight gain of 0.6 pounds (0.28 kg) per day during the same period. Calf weight gains were 0.2 pounds (0.1 kg) per day less in those offered grass in the early morning as opposed to those that grazed in the evening.
Conclusions
Forage sugar concentration can be increased an average of 1.8 percentage points during the day, with maximum sugar levels reached 11 to 13 hours after sunrise.
To achieve high-sugar concentration, harvest can be planned for those hours using a wide swath (no windrowing) with no increase in harvest costs.
This increase in forage sugar concentration can improve dry matter intake and milk production of dairy cows (up to 5 percent, depending on lactation stage) and daily gain in beef cows and growing calves. FG
To view the webinar, go to How to produce and use sweet forages on youtube.
How does sugar content affect RFV?
Sugars are not included in the RFV calculation. The only way sugars would affect RFV is if the increase in sugar is significant enough to affect the fiber (ADF and NDF), and then it would have an indirect effect, but otherwise you would not see an effect.
While we often see a small decrease in NDF or fiber concentration in high-sugar forages or a reduction in protein, it would not necessarily be consistent.
