Summer-Dormant Cool-Season Perennial Grasses -

A New Source of Winter-Active and Persistent Forages

for Semiarid Environments of the Southern Great Plains

Rationale

    For at least three decades, agronomists and plant breeders have been attempting to introduce improved cool-season perennial grasses to semiarid environments of the southern Great Plains to complement forage availability from dual-use wheat (Triticum aestivum L.) pastures during the fall-winter-spring (October-March) grazing season and from warm-season grass pastures and native rangelands in the spring (March-May) grazing season. The introduction of improved cool-season perennial grasses has not been successful because of: 1) very limited forage production during winter grazing season, and 2) poor persistence due to inadaptability to severe water deficits accompanied by extreme heat in summer. Nota bene all those grasses originate from dry but cool environments, in contrast to dry and hot environments of the southern Great Plains.

Global warming has an effect on the climate of the Southern Great Plains. Within the last 20 years, mean annual temperature has been constantly increasing, while annual precipitation has been decreasing (www.noaa.com). Although one may argue this may be a part of natural climate cycle, the fact is that such changes will have an impact on decision making in regard to agricultural crops and practices.     

    Breeders in Argentina, Australia, Italy, and New Zealand have recently developed drought resistant cultivars of cool-season perennial grasses, i.e., tall fescue (Lolium arundinaceum), orchardgrass (Dactylis glomerata) and perennial ryegrass (Lolium perenne) based on germplasm originated in the Mediterranean Basin of Europe and Africa. These grasses produce most of forage in winter and early spring and are obligatory summer-dormant, i.e., they cease growth in response to long days and high temperatures even at adequate soil water supply. In contrast, currently recommended cultivars of perennial cool-season grasses in the Southern Great Plains are summer-active, i.e., they continue with grow during summer if soil water is available. Consecutive periods of drought exhaust the plants resulting in high tiller mortality by autumn.  

    In Mediterranean-like environments with prolonged and severe summer drought,  summer-dormant cool-season perennial grasses have been proven more persistent than summer-active types. Their strategy of drought resistance, which is actually similar to drought escape, is superior to that of summer-active types that developed mechanisms of drought tolerance.

    Summer-dormant cool-season perennial grasses are not productive during summer months, therefore, they are not suitable for improved, intensive grasslands in the temperate zones of the United States. This may explain the lack of summer-dormant cultivars developed for the U.S. market. Our 6-year research data indicate that summer-dormant cultivars are perfectly adapted to the transitional semi-arid steppe and warm semi-arid steppe zones of the Southern Great Plains, most likely because the climate here (relatively mild winters and severe summer droughts) is similar to that of their origin.

Objective

    The main objective of our research program is to introduce summer-dormant cool-season perennial grasses to grazing systems of the Southern Great Plains because of their winter-active growth habit and extraordinary persistence in environments with prolonged and severe drought. 

Research Progress

    In cooperation with AgResearch (USA) Ltd., we initiated research on an obligatory summer-dormant type of tall fescue, presently registered as ‘Grasslands Flecha’ at Vernon, TX in 2000. We compared forage productivity and persistence of this cultivar with summer-active cultivars. Vernon is beyond the range of existence of traditional, summer-active tall fescue cultivars. As expected, all of them died during the first growing season (see picture above), but Grasslands Flecha survived very well for the next 5 years. You may like to read more about this experiment here.

    Based on these preliminary results, we established an experiment in autumn of 2002 to determine the importance of summer-dormancy on persistence of cool-season perennial grasses in the Texas Rolling Plains. Many international and domestic seed companies agreed to cooperate on this project and supplied grass seeds. The objective of this study were: 1) to identify the role of summer dormancy on drought resistance and persistence of tall fescue, orchardgrass, hardinggrass (Phalaris aquatica), and perennial ryegrass; 2) to identify soil water dynamics under swards of summer-dormant and summer-active cultivars as a function of defoliation intensity; and 3) to identify mechanisms of summer dormancy. As expected, all summer-active cultivars died during the first summer drought of 2003 (nota bene, the second driest summer since 87 years in this area). All summer-dormant cultivars of tall fescue, cultivars and breeding lines (courtesy of Dr. Eric Hall) of orchardgrass, and cultivars of highly summer-dormant hardinggrass, however, survived the first and all subsequent summer droughts (as of November 2005) very well. This experiment is still going on in order to collect long-term data on grass persistence and productivity, but you may like to read a summary of our preliminary (2002-2004) results here.   

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