Western wheatgrass, Pascopyrum smithii, is a native, long-lived perennial, cool season, mid grass, monocot, of the grass family that is abundant on healthy mixed grass prairie plant communities. Western wheatgrass can grow on sandy, shallow, silty, overflow, clay, and thin claypan ecological sites. It grows better on silty and shallow sites. Western wheatgrass is tolerant of cold, drought, and periodic flooding, has a high tolerance to alkali and saline soils, and moderately shade tolerant. Western wheatgrass tillers live for two growing seasons; the first growing season as a vegetative tiller and the second season as a reproductive lead tiller. Early season activity starts by regreening with active chlorophyll in the portions of the carryover leaves that have intact cell walls from the previous growing season vegetative tillers. The green portion of the carryover leaves provides large quantities of carbohydrates and energy for the production of new leaves.
New leaf growth of western wheatgrass lead tillers starts in early to mid April, leaf growth rate increases during May and June, and becomes much slower during July. Second year reproductive lead tillers produce 3.5 new leaves by mid May and 4.5 new leaves by early June. The tallest culm leaves are at 79% of the maximum height during the first week of June. Early flower stalk growth and development begins to swell in the boot stage around early to mid June. Lead tillers progress rapidly through head emergence and reach early first flower before 21 June. About 87% of lead tillers progress to the anthesis stage during late June to mid July. Lead tillers reach about 98% of maximum stalk height during mid July and they drop below the crude protein requirements of lactating cows during the third week of July. No new leaves are produced after the anthesis stage. Early flowering tillers usually produce 5 or 6 leaves and late flowering tillers usually produce 7 or 8 leaves. Late developing lead tillers reach anthesis by late July. Culm leaves and seed stalks reach maximum height by the end of July. Peak herbage biomass of western wheatgrass lead tillers is produced during the last weeks of July. Seeds develop through the milk and dough stages during July and early August with most seeds reaching maturity during mid to late August and a few seeds mature in early September. The phosphorus content of lead tillers drops below a lactating cows requirements during early August. Leaf dryness reaches greater than 50% after mid August.
Vegetative tillers are derived from carryover of the previous seasons fall tillers and the current seasons early spring initiated tillers. Vegetative tillers have slightly slower growth rates than lead tillers during the early portion of the growing season. Vegetative tillers produce 3.3 new leaves by mid May and 3.8 new leaves by late May. During early June, 85% of the vegetative tillers have produced 5 or 6 new leaves and 15% have produced 7 or 8 new leaves. The quantity of vegetative tillers with 7 leaves stops increasing in late July and the tillers with 8 leaves stops increasing in early August. Grazing management practices like seasonlong that have less than 100 lbs/ac of available mineral nitrogen have less than a third of the quantity of vegetative tiller as grazing management practices like the twice-over system that can produce greater than 100 lbs/ac of available mineral nitrogen. When 100 lbs/ac of mineral nitrogen is available, the greater quantities of vegetative tillers provide increased amounts of nutrients, mainly crude protein, that meet the requirements of lactating cows during mid July to mid October.
During growing seasons with normal precipitation, about 5.0% of the vegetative tiller population on the grazed twice-over system develop robust growth and produce 9 or 10 and rarely up to 16 new leaves. The robust vegetative tillers grow amongst the normal tillers with 5 to 8 leaves and there is no obvious difference in habitat characteristics. The mean leaf heights of the robust tillers are no taller than that of the normal leaves. The only noticeable difference is the uncommon number of more leaves.
Secondary tillers are derived from initiated vegetative growth of axillary buds on lead tillers during the growing season. The growth rate of secondary tillers is hormonally controlled by a dominant lead tiller that has proprietary access to all essential nutrients available to the secondary tillers. This arrangement gives secondary tillers far superior survivability than an independent grass seedling. Reproductive lead tillers have a high nutrient demand during the flower stalk growth and development stages causing the lead tiller to restrict nutrient flow to the secondary tillers resulting in very slow rates of growth. Secondary tillers can remain at the 2 or 3 leaf stage for a month or two. During growing seasons with periods of water stress or other problematic conditions, the nutrient restrictions could be great enough to cause secondary tiller termination. If the secondary tiller survives through the seed development stage, the lead tiller releases greater quantities of essential nutrients to the subordinate secondary tillers. After the secondary tillers produce their fourth leaf, they have adequate leaf area to photosynthesize their own carbon energy and develop a large enough root system for self sustaining nutrient resource uptake, they become independent. Many secondary tillers produce leaves 5 and 6 before hard frost.
Improvement of the quantity and quality of grassland forage depends on increasing the quantity of vegetative tillers initiated from lead tiller axillary buds and requires implementation of a specialized grazing management strategy like the twice-over system designed to manipulate the hormones that regulate axillary bud meristematic tissue and that cause the exudation of lead tiller surplus carbohydrates through the roots into the microbe rhizosphere.
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