Circadian and Seasonal Variation in Pasture Nonstructural Carbohydrates and the Physiological Response of Grazing Horses
Environmental conditions including temperature, solar radiation (sunshine), water and nutrient availability influence the amount of sugar and starch in plants. Sugar and starch fluctuate through the day and by season.
5 x 36 hour studies were carried out in April, May, August, October and January in Middleburg, northern Virginia.
A 5 hectare field seeded with ~ 71% tall fescue (cool season), 22.5% Kentucky bluegrass (cool season) and 6.5% white clover, grazed until March and then topped regularly to keep sward height at 14 to 20 cm (and reduce seed head production), was divided into 4 equal quadrants.
Grass samples from each quadrant were cut no more than 2.5 cm from the grass base over a 36 hour period in April, May, August, October and January. Samples were collected hourly from 06:00 to 22:00, then 24:00, 02:00, 04:00 then hourly again from 06:00 until 18:00 (33 sample times per month).
The grass samples were either oven dried at 70'C or frozen in liquid nitrogen.
The oven dried samples were tested at Dairy One (LAB1) using proximate analysis - NSC was sugar plus starch, sugar was WSC and included fructans.
The frozen samples were tested using hydrolytic enzyme analysis (Megazyme) at a USDA-ARS laboratory (LAB2) - sugar was simple sugar, fructans included framinans. LAB2 techniques are stated as requiring further validation, but good agreement is shown between LAB1 and LAB2.
NB all white clover was removed from the April and May samples, and biomass yield was not determined for May.
Mean forage biomass yield did not change between trials - but the grass was topped regularly to keep sward height at 14 to 20 cm.
LAB 1 - April, May and August had daily fluctuation in NSC and sugar, but there was no significant fluctuation in October and January, when conditions were overcast and rainy. Starch levels did not fluctuate through the day in any month
LAB2 - similar daily fluctations in sugar, no daily fluctuation in fructan or starch. Fructan was higher in April than other months, when the minimum temperature was 2.8'C - all other months had a minimum temperature >6'C.
In every month there was a positive correlation between ambient temperature and solar radiation and NSC, and a negative correlation between relative humidity and NSC.
NSC ranged from 2.3% to 25.3%.
Other research reported:
NSC ranged from about 4% to 23% DM in mixed grass/legume pastures in Northern Virginia (study??).
NSC averaged 10.3% to 15.7% DM in October & December over 3 years in North Carolina, with maximum NSC 21.6% (Burns 2000).
WSC ranged 95 to 560 g/kg DM, fructan 32 to 439 g/kg DM depending upon temperature - higher values associated with cooler (5-10'C) temps and lower values associated with warmer (15-25'C) temps (Chatterton 1989).
Hoffman et al 2003 reported NSC peaks in April and November.
Lechtenberg 1971 reported that NSC tends to increase throughout the day from 6 am to 6 pm.
April - LAB1 circadian fluctuation in sugar & NSC. Most extreme sugar/NSC fluctuation was in April. LAB2 fructan highest in April. Min temp 2.8'C. Greatest difference between day and night temperatures - ambient temp ranged 2.8 to 16.7'C. Correlations between environmental variables & NSC/sugar but not starch. Environmental conditions favoured photosynthesis - sugar production was high. Plants were young and vegetative (tiller to flower) and generally higher in NSC than other months.
May - LAB1 circadian fluctuation in sugar & NSC. Correlations between environmental variables & NSC/sugar but not starch. Plants were young and vegetative (tiller to flower) and generally higher in NSC than other months.
August - LAB1 circadian fluctuation in sugar & NSC. Ambient temp 26.4'C, maximum 31.6'C. Correlations between environmental variables & starch.
October - LAB1 no significant fluctuation. Overcast & rainy (low light intensity & rain have been shown to result in lower NSC content - Ladyman 2003, Longland 1999). Temperatures were mild with little difference between day and night (which is a major factor in the accumulation of NSC in cool season plants - Chatterton 1989). Correlations between environmental variables & starch. Although humidity was high, light intensity was low and resulted in decreased photosynthesis and therefore decreased sugar content.
January - LAB1 no significant fluctuation. Overcast & rainy. Correlations between environmental variables & starch.
In all months, pasture NSC and sugar were lowest between 4 and 6 am, and highest between 4 and 6 pm. Starch content was minimal but seed head production was minimal due to regular mowing.
Daily and seasonal variations in NSC in cool season pastures are a result of environmental influences on the plants (related to the regulation of photosynthesis and respiration (growth)), and the plants themselves (stage of maturity, species).
Photosynthesis - the process by which plants synthesize sugars. Stomata in leaves uptake CO2 - this is regulated by the opening and closing of guard cells, whose activity is coupled with environmental factors e.g. light intensity, water status and humidity. In high light intensity conditions, the stomata open, CO2 diffuses into the leaf cells and sugars are synthesized. Under conditions of low light intensity, low humidity and low water availability, stomata opening may be reduced, reducing photosynthesis.
Sugars are stored as fructans in cool season grasses, and starches. Plant maturity affects NSC in grass. Immature plants are higher in NSC and lower in fibre, mature plants are lower in NSC and higher in fibre. Chatterton et al. 2006 - simple sugars in vegetative tissues of oat forage were higher (15% DM) in young plants than in mature plants (1-2% DM), starches increased with maturity (3-4% to 10-15%) and fructans were not affected by maturity. LAB2 results showed that only 1/3 of WSC was fructan, simple sugars therefore around 2/3.
Conclusion: NSC, sugar and starch varied throughout the day and by season. Environment affects NSC in forage. Simple sugars were high and fructan low.
"Simple sugars rather than... fructans may be important in the pathogenesis of the metabolic and digestive disorders (e.g. laminitis) that occur in grazing horses."
36 hour studies in April, May, August, October and January in Middleburg, northern Virginia. 5 hectare field, grasses seeded approx 71% tall fescue (cool season), 23% Kentucky bluegrass (cool season), 6% white clover. Grazed until March, then topped regularly. Grass samples were collected hourly from 06:00 to 22:00, then 24:00, 02:00, 04:00 then hourly again from 06:00 until 18:00 (33 sample times per month).
Lab 1 - proximate analysis at Dairy One - NSC was sugar plus starch, sugar was WSC and included fructans.
Lab 2 - hydrolytic enzyme analysis at USDA-ARS- fructans including framinans.
Mean forage biomass yield was 2,612 kg/hectare DM with no differences between the 5 trials.
Greatest range in temperatures from day to night occurred in April (2.8 to 16.7'C).
August was hot (ambient temperature 26.4'C, maximum 31.6'C).
October and January were mostly overcast and rainy.
Treiber KH, Kronfeld DS, Hess TM, Byrd BM, Splan RK, Staniar WB
Evaluation of genetic and metabolic predispositions and nutritional risk factors for pasture-associated laminitis in ponies
J Am Vet Med Assoc. 2006 May 15;228(10):1538-45 (PubMed)