growers and consultants assess wild oat population thresholds for herbicide treat- etition i ments. An overall assessment of the critical interactions of wheat and wild oat would ort-st help us understand the competitive rela- tionships between the two species. For all of these reasons, we undertook a 3-year study David W. Cudney o Lowell S. Jordan o Warren E. Bendixen in irrigated wheat at UC Riverside and an additional field study in the Santa Ynez Jodie Holt CI A. E. Hall D Chris J. Corbett ’ o John S. Reints region of south-central coastal California. Materials and methods Wild oat and wheat were synchro- intouse. Thenew cultivars respond to higher Five field experiments were conducted at nized in their development and nitrogen and irrigation inputs because they the UC Riverside Agricultural Experiment were shown to be equally competi- resist lodging more than the older, taller Station. The five planting dates were Janu- cultivars. But the shorter wheat stature and ary and February in 1985 and 1986, and tive in southern California studies. increased nitrogen fertilization and irrigation February in 1987. The soil was a sandy loam. Competition effects of wild oat have combined to enhance conditions for We established wild oat populations by were most evident in wheat after wild oat. Wild oat competition has reduced uniformly sowing pre-weighed quantities the stem elongation stage. wheat yields by as much as 66%. of seeds into plot areas. The plot areas were To control wild oat successfully, growers then harrowed 2 inches deep to incorporate A serious pest of small grains in California, must time their herbicide applications cor- the seeds. The seeds had been harvested in wild oat (Avenafatua) germinateswithwheat, rectly relative to the growth stages of both previous seasons from commercial fields in oats, and barley and competes with these wheat and wild oat. An accurate way to the region. Plots were then planted withpre- crops throughout their growth cycle. Wild predict development rates would greatly weighed amounts of wheat seeds using a oat can reduce grain and forage yields and enhance a weed management program by small-graindrill. Rows of wheat were spaced quality. reducing the amount of time devoted to a standard 6 inches apart with eight rows per Wheat has been a particular victim to field checking for growth stages. plot. Plots averaged 75 feet long. The wheat wild oat competition in recent years, as new, An evaluation of the competitive ability cultivar ‘Anza,’ a short-statured, hard red shorter-statured wheat cultivars have come of wild oat in irrigated wheat would help spring wheat commonly grown in southern California was used. To reduce interactions of moisture and nutrient competition, we grew all plots with ample soil moisture and nitrogen.Plots were sprinkler-irrigated weekly until the stem elongation stage, at which time we shifted to twice-weeklyirrigation until crop maturity. Each sprinkler application averaged 1 inch of water. Soil nutrient analysis showed ad- equate phosphorus and potassium. Addi- tional nitrogen was applied as urea through a preplant application of 100 lb nitrogen per acre and a side-dress application at the tillering stage of 75 lb per acre. Experiments at Riverside were a com- posite of additive and replacement series designs. The additive series consisted of wheat planted at a density of 20 plants per square foot, averaged over the five experi- ments. Four densities of wild oat were then added to the wheat: 0,8,13 and 21 plants per square foot, average. The replacement series consisted of mo- noculture plots of wheat and wild oat aver- aging 20 and 25 plants per square foot, re- spectively. Mixtures of wheat and wild oat were established in plots to obtain propor- tional ratios of 33%:67% and 67%:33%of wheat and wild oat, respectively. Each ex- periment was replicated four times in ran- domized complete blocks. Six wheat and six wild oat plants from each plot in mixed and pure populations were chosen at random and flagged so we couldmonitor them for stageof development Santa Barbara County Farm Advisor Warren Bendixen shows the height differentiation at maturity throughout the growing season according between wild oats and wheat. to the Haun scale. The Haun scale is a means

22 CALIFORNIA AGRICULTURE, VOLUME 45, NUMBER 1 wheat. of monitoring the main stem development ing wild oat was varied. Similarly,the devel- The results of the removal studies from of grass species.Leaves along the main stem opmental stages of the main stem of wild oat Riverside in 1985, 1986, and 1987 are pre- of wheat and wild oat were numbered and did not differ between pure culture and sented in table 1. The removal of wild oat at recorded as each whole leaf or fraction of a mixed cultures with wheat. Also, the de- tillering of wheat resulted in no reduction in leaf appeared from emergence to the last full velopmental rates of wheat and wild oat did wheat yield. Removing wild oat either at leaf (Haun stages 1 to 9). After the last leaf not significantly differ. ’ stemelongationoratflowering or noremoval emerged, elongation of the eighth internode Inaneffort toprovidemoreuniformityin of wild oat resulted in progressively greater (Haun stage lo), swelling of the ninth leaf our measurements of the developmental reductionsinwheatyield. Themostdramatic sheath or boot (Haun stage 111, emergence rates over differentenvironmental conditions reduction in wheat yield occurred when of the head (Haun stage 12), and elongation (e.g.,location, planting date, year), we sub- wild oat was removed after stem elongation, of the peduncle (Haun stage 13) were re- stituted a measure of physiological time, indicatingthat competitionwas greatestafter corded. We also recorded the canopy height degree days, for chronological time. We ar- this time. of both wheat and wild oat. rived at our base temperature for degree- When we clipped wild oat leaves to Harvests were made at tillering, stem day calculation by taking the rate of devel- prevent them from emerging above the elongation, flowering, and maturity. Leaf opment (Haun stages per day) from wheat canopy, wheat yield increased sig- area and biomass of each species were emergence to Haun stage 13 for the six ex- nificantly. The clipping experiment showed measured along with grain yield at maturity. perimentsandplottingit againstthe average the importance of the shade cast by wild Insubplots,weremovedwildoatfromwheat temperature for that period. A base tem- oat’s higher canopy late in the season. at each of the four stages. An additional perature of 40°F was estimated by least- subplot was included in the 1986 and 1987 squares regression. Regression analysis of Summary experimentsin which we trimmed wild oats plant development for all six experiments The developmentof wheat and wild oat was to the level of the wheat canopy every day taken together gave a better fit when we synchronized from emergence through after the stem elongation stage. used degree days as the unit of time. flowering.This illustrates the ability of wild The shoot dry weight (biomass) of each oat to perform as a crop mimic. When dif- Results and discussion species at harvest declined in direct pro- ferent planting dates and years were com- Analysis of variance of the Haun develop- portion to the increase of the other species’ pared, degree days gave a better fit of de- mental stage data taken for each experiment shoot dry weight (fig. l),so the combined velopmental rates than did calendar days. at UC Riverside and Santa Ynez revealed no biomass of both species at each treatment Utilizing the regressionequation and degree significant difference in the development of was similar to that of either species grown days, the Haun development of both wheat the main stem of wheat in any of the ex- alone. Apparently, both species were com- and wild oat could be predicted. Density periments when the density of the compet- peting for shared resources, and each made and yield comparisons revealed that wheat and wild oat were equally competitive on per plant basis. This was true even though wild oat had less leaf area per plant than did wheat. Wild oat has a height advantageover wheat in late season which results in shad- ing and yield reduction.

David W. Cudney is Extension Weed Scientist, Lowell S. Jordan is Professor of Horticulture and Plant Physiology, A. E. Hall is Professor of Plant Physiology, Jodie Holt is Associate Professor of Plant Physiology, Chris J. Corbett is Staff Re- search Associate, Plant Pathology, and John S. Reints is Staff Research Associate, Botany and Plant Science, all at UC Riverside; and Wawen E. Bendixen is Cooperative Extension Farm Advisor, Santa Barbara County. The research effort reported here was sup- ported by a grant from the Statewide Integrated Pest Management Project.

CALIFORNIA AGRICULTURE, JANUARY-FEBRUARY 1991 23