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Two oat cultivars, Clintland 60 and Garry, were grown to maturity in controlled environments under two temperature regimes with daylength held constant and under two daylengths with a constant temperature pattern. Plants were moved between environments at panicle initiation and at anthesis, dividing the growth period into three phases.Growth and development were hastened by warm temperature and by long days. Temperature treatments caused greater variation in the duration of growth phases in Garry than in Clintland 60, but Clintland 60 was affected more by daylength. Higher plant dry weights were obtained from cool temperature treatments; response to daylength was inconsistent.Grain yield was not closely associated with duration of growth phases. The effect of temperature and daylength on yield components was markedly different in the two cultivars. The greatest grain yield response in Clintland 60 resulted from cool temperature during panicle initiation to anthesis, attributable to increased fertile tiller development. Daylength had limited effect on components. In Garry the greatest yield response was from cool temperatures during anthesis to maturity, with increases in all components. Short days before anthesis increased grain size and number.Implications of observed responses relative to cultivar performance in the field are discussed.

Ethephon (2‐chloroethyl phosphonic acid) reduces lodging in barley (Hordeum vulgare L.) by shortening straw, but the optimal rate and growth stage of application, both of which may interact with cultivar and environment, are unclear, especially for two‐row barley in North America. Field studies in Quebec in 1985 and 1986 evaluated the response of ‘Leger’ six‐row and ‘Birka’ two‐row barley to ethephon applied at varying rates, growth stages, and times of day. The two‐row cultivars Rodeo and Micmac were also included in one experiment. Lodging was slight or absent in all experiments. The taller Leger was usually shortened more than Birka, largely due to greater shortening of the peduncle. Late boot applications of ethephon consistently shortened straw, but the window of effective application began at the flag leaf stage and extended for more than 1 wk for Birka, while for Leger it was about 4 d, but differed phenologically between years. Heading was delayed by ethephon. In Rodeo and Birka, ethephon often reduced the degree of spike emergence from the flag leaf sheath. Incomplete spike emergence may have contributed to reduced kernel and test weights, particularly with applications from the flag leaf until the late boot stage. Ethephon increased Leger yield by up to 15% in one experiment under the favorable conditions of 1985. In 1986, a wet year, yield in one experiment was reduced by 11% by the higher ethephon rate (480 g a.i. ha−1), due to reduced spikes per square meter. Birka yield was reduced (P = 0.06) by 6% by the higher rate (480 g a.i. ha−1) in the 1986 application date study. Harvest index of Rodeo and Birka was reduced due to reduced tiller survival following application of ethephon at 480 to 500 g a.i. ha−1. Yield component compensation usually occurred between spikes per square meter and kernel weight, particularly for two‐row cultivars. Height was reduced more by applications at 0400 h and 0600 h than later in the day. Birka kernel weight was reduced with morning or evening applications, while Leger kernel weight was little affected by application time of day. In the absence of lodging and under conditions of environmental stress, ethephon applied at rates over 240 g a.i. ha−1 may reduce yields, particularly of two‐row barley, for which spike emergence may be severely reduced.

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