Rye is, as wheat, barley and oats, a member of the grass family, the tribe Gramineae. The divergence of the wheat and rye lineages from the Pooideae happened about 7 million years ago. The common name is rye, feral rye or cereal rye – subsequently as rye described. The use of the name cereal rye reduces confusion with ryegrasses (Lolium spp.) in the English eloquent countries. The scientific name is Secale cereale L. Internationally, rye is called Roggen (German), Centeno (Spanish), rosh (Russian), Segale comune (Italian), zyto (Polish), Seigle (French), råg (Swedish), Rug (Danish), Ruis (Finish), Rogge (Dutch), or Rúgur (Icelandic). It is a typical allogamous plant species showing high degree of self-incompatibility.
Rye can be described as an erect annual grass with flat blades and dense spikes. The habit resembles that of wheat, but usually taller. It has the longest stems of all cultivated small grains [culms (min. 35) 50-120 (max. 300) cm] and these provide most of the photosynthetic area. Average height of older ryes is 150, e.g. the variety “Viatka“ 190 cm, “Wojcieszyckie“ 160 cm, “Dankowske Zlote“ 145 cm, or “Kustro“ 135 cm. Modern, high-yielding varieties are much shorter, in particular hybrid varieties. The latter are distinguished by their long compact ears, possessing seeds of equal size.
During grain formation, stems with sheaths account for 60-80 % of the total plant area. As compared to wheat, the flag leaf is smaller and less important in photosynthesis (blades 4-12 mm wide). Leaves (stems and spikes) have a bluish color by their waxy surface. The spike is longer and more slender [spike (min. 2) 4.5-12 (max. 19) cm], somewhat nodding when mature.
The inflorescence and seeds are as follows: spikelets usually are 2-flowered, solitary, placed flatwise against the rachis, the rachilla disarticulating above the glumes and produced beyond the upper floret as minute stipe. The glumes are narrow, rigid, acuminate or subulate-pointed. Lemmas are broader, sharply keeled, five-nerved, and ciliate on the keel and exposed margins, tapering into a long awn. The lemma and palea that enclose the floret are free threshing, and the lemma often bears barbs on the keel and often has an awn of intermediate length. Anthers are about 7 mm long with light yellow to purple color. The rye grain is longer and more slender than that of wheat, and it shows sometimes a pale but often greenish seed coat. The pale grain is genetically coded by recessive gene loci. Purple seeds were found in mutant plants (Ruebenbauer et al. 1983).
Rye has the best-developed root system among annual cereal crops, as with other grasses, the system is fibrous, with no defined taproot. The extensive root system including its specific rhizosphere enables it to be the most drought-tolerant cereal crop and makes it among the best green manures for improving soil structure. Field studies revealed that decomposition of incorporated 14C-labeled rye residue was accelerated through having rye plants growing in the soil. This was believed to be due to the rhizosphere microbial complex. Comparison of shoot and root dry weight and soil moisture during progressive growth stages for wheat, triticale and rye in both greenhouse, pot and field studies demonstrated significantly higher root dry weight in rye when grown in pots, but wheat showed greater root mass from depths of 20 to 50 cm in the field studies. Root growth is greatest from the seedling to flag leaf for all three cereals. Rye generally roots to a depth of 90-230 cm, thus it is deeper rooting than other small grains.
Rye seeds may germinate in storage or even while still in the ear (pre-harvest sprouting). Seeds will germinate at temperatures as low as 3-5 °C, but optimal range is 25-31 °C. In most northern countries rye can be established when seeded as late as October 1. Minimal temperatures for germinating have been variously given as 1 to 2 °C, and 0.6. In the autumn, rye grows more rapidly than wheat, oat, or various other annual grasses. Although rye is usually regarded as a winter crop, several spring-sown varieties are available. As a long-day plant, flowering is induced by 14 h of daylight when this is accompanied by temperatures of 5-10 °C. Shortened day length can cause rye plants to remain vegetative for up to seven and more years. This behavior is sometimes used for maintaining or multiplication of specific genotypes by cloning under short-day conditions.
In comparison to other cereals, such as wheat, triticale, and barley, the energy content of rye seeds ranges with 13,3 MJ/kg between barley and wheat. The crude protein content is the lowest among the cereals (8-10 % of the dry matter), while the starch content ranges with 50-70 % of dry matter again between barley and wheat. The fat content is about 2 % in seed. Pentosans are playing a particular role in rye utilization. Their content is between 9 to 10 % of the dry matter, depending on the variety. About 25 % of the pentosans are water-soluble. Pentosans showing soaking effects and decrease digestion when fed to animals. The lysine content is about 3.7 %, the methionine content 3.6 % and the threonine content about 3.0 % of the mean crude protein content, respectively. Thus, rye shows the highest lysine content among the cereals mentioned above (wheat ≈ 2.5 %, barley ≈ 3.5 %, triticale ≈ 3.0 %).