The influence of fertilizers on the carbohydrate composition and nutritional value of forage crops

The energy value and digestibility of forage grasses are directly linked to the accumulation of carbohydrates during photosynthesis. In grain legumes, storage substances are deposited as assimilation starch, whereas in grasses (Poaceae family), they are stored as fructosides, whose molecules contain from 3 to 30 fructose residues. The localization sites of these reserves differ across plant organs.

• Assimilation starch is concentrated in the leaves and root system.
• Fructosides accumulate in the leaves and the lower thickened part of the culm.

The total amount of these polysaccharides in forage grasses accounts for about 6–8% of the dry matter. Plant cell walls also store 10–15% hemicelluloses, which perform structural and storage functions. Cellulose content fluctuates significantly depending on the age of the stand and growing conditions, ranging from 20 to 35% of dry matter.

In addition to polysaccharides, the vegetative organs of grasses contain easily soluble carbohydrates — monosaccharides and sucrose. Their total amount determines the energy value of green forage.

To increase the carbohydrate nutritional value of legume grasses, it is important to create optimal conditions for symbiotic nitrogen fixation. Nitrogen fertilizers are usually not applied to them, while phosphorus and potassium rates are calculated based on plant requirements. In well-cultivated soils, balanced phosphorus-potassium nutrition significantly enhances the intensity of photosynthesis.

When growing grasses and corn for green forage or silage, phosphorus-potassium fertilizers are necessarily supplemented with nitrogen. Nitrogen deficiency leads to undesirable consequences. When it is lacking, the proportion of fiber in the vegetative mass increases sharply, and the sugar content decreases.

• Cellulose limit for good feed digestibility — 30%
• Sugar content of sugar beet roots — 16–20%
• Proportion of sucrose in sugar beet sugars — 80–90%
• Sugar content of carrots and fodder beet — 7–12%

Mineral Nutrition and the Carbohydrate Complex of Root Crops

In root crops, up to 70–80% of carbohydrates are represented by easily soluble forms — sucrose and monosaccharides. The highest concentration of sugars is characteristic of sugar beet, in which the proportion of monosaccharides (glucose and fructose) does not exceed 1% of the fresh mass, and small amounts of maltose and raffinose are also present. In the roots of fodder beet, table beet, carrots, and turnips, the total amount of sugars is 7–12%, and in turnip and radish — 6–8%.

Excess nitrogen stimulates excessive leaf formation, delays beet ripening, and hinders the sugar accumulation process. As a result, the sugar concentration in the roots decreases. Simultaneously, the content of harmful nitrogen and ash increases in the tissues, which complicates the processing of raw materials and reduces the total sugar yield at the factory. This negative effect is particularly noticeable with a one-sided increase in nitrogen rates.

Phosphorus fertilizers in optimal doses always increase the sugar content of sugar beet. However, their application must be carried out strictly taking into account the content of available phosphorus in the soil. Applying excessive rates without considering natural soil fertility can reduce both the yield and the sugar content of the roots.

Potassium plays a key role in the accumulation and transport of sugars in beet roots. Under the influence of this element, soluble nitrogen compounds accumulate in plants in insignificant amounts, while the sugar content of the beet and the sugar yield per unit area increase. Micronutrient fertilizers act similarly: on soils with a low content of available forms of micronutrients, their application at optimal rates increases both yield and root quality.