No abstract

Variability of development rate estimates across locations and years using the current heat unit system of growing degree‐days (GDD) with maximum and minimum temperature thresholds of 30 and 10°C (GDD30,10) limits predictability of maturity in hybrid maize (Zea mays L.). Data sets of daily maximum and minimum air temperatures and dates of maize development stages were collected for a range of hybrids at locations in Canada and the northern USA (39° to 45° N lat). Data were analyzed to improve the temperature response functions for maize at different stages of development. Results indicate that during vegetative growth, phenological response to mean daily air temperature followed a sigmoidal curve beginning below 5°C, with maximum response to temperatures between 25 and 30°C. During reproductive growth, the temperature response function was flat from 0 to 12°C and rose significantly only with mean daily air temperatures greater than this range. A general thermal index (GTI) based on these two response functions improved estimation of maturity dates by 50% over estimates made using GDD30,10 (SE of 6.7 d for GTI and 13.6 d for GDD30,10 in estimating time from planting to maturity). The greatest improvement using GTI occurred for the reproductive period (SE of 5.8 d using GTI, compared with 12.1 d using GDD30,10). These results suggest that incorporating the temperature response function reported in this paper would improve prediction of maize development.

Several systems are extensively used to rate maize (Zea mays L.) maturity in North America, including growing degree days, crop heat units, and Minnesota relative maturity rating days. Correspondence between the different systems varies with the temperature range of the environment, because of different bases of calculation and unit size. However, general guidelines for conversion from one system to another would aid communication among researchers, producers, distributors, and extension personnel, particularly where regional preference for one system exists, but information developed in another region is shared. Simple regression equations were developed from a data set with 4 years, 28 hybrids, and 19 locations (between 39° and 48° N lat). Coefficients of determination were ≥0.91, indicating that these equations could be used as a general guideline to compare maturity ratings developed using the different systems.

The response of winter wheat cultivars and advanced generation lines (Triticum aestivum L. em Thell.) to barley yellow dwarf virus (BYDV) infection was investigated. In 1976, the yields of 1,200 early seeded cultivars and lines, which were infested with naturally occurring viruliferous Rhopalosiphum padi. L. aphids, were compared with yields of those from a late planting date, which escaped infestation. Yields in the early seeded plots ranged from 63 (for most of the 1,200 lines) to 85% of the yields in late seeded plots.In 1978 and. 1979, the effect of barley yellow dwarf virus (BYDV) infection on yield and certain yield components was measured for 10 cultivars and lines. Treatments were: a control (non‐infested) and a fall, spring, and fall plus spring infestation with greenhouse‐reared viruliferous aphids. In 1979, the effect of three levels of N fertilization on barley yellow dwarf (BYD) symptom expression was also measured.BYD significantly reduced the grain yield, number of heads, plant height, kernel weight, kernel number, and above ground plant weight. It did not affect the harvest index or lysine percentage.In 1978, grain yields were reduced by 58 and 38% when the wheat was infected with BYDV in the fall and spring, respectively. In 1979, yield reductions of 44, 33, and 27% resulted in plots which were infested with viruliferous aphids in the fall plus spring, fall, and spring, respectively. The cultivars and lines differed in response to BYDV with respect to grain yield and number of heads. Of the cultivars and lines tested, Purdue breeding lines P65256A1‐8‐7 and P6376B3‐8‐2 showed the most effective resistance to BYD.

with location and year (Major et al., 1983;Plett, 1992). Most frequently, the GDD concept overestimates the Thermal indices predict and describe development rate more accuheat units required for grain filling (Dwyer et al., 1996). rately than time in days and are commonly used to rate maize (Zea mays L.) for maturity. Separate temperature response functions for This is particularly evident in years with below-normal the vegetative and grain-filling periods predict more accurately time temperatures, when maturity estimates using GDDs to maturity than a single function for the two periods combined. may be several hundred heat units too high (Roth and However, use of two functions requires a priori knowledge of the Yocum, 1997). silking date, which becomes the transition date from the vegetative Recently, separate temperature response functions function to the grain-filling function. The objective of this study was have been developed for the vegetative and grain-filling to evaluate the sensitivity of estimates of silking and maturity dates periods of maize (Stewart et al., 1998). These equations, to the transition date between vegetative and grain-filling functions which were fitted to data from 28 hybrids collected at and to develop a protocol to combine the two temperature response 19 locations ranging from 39Њ to 48Њ N lat over 4 years, functions in a general thermal index (GTI) for maize. Frequencydiffered from those assumed by GDD and CHU and distributions of mean daily air temperatures for five 20-d periods spanning mid-June to late September at 19 locations in the northern also from each other, especially at temperatures below USA and southern Ontario from 1992 to 1995 indicated few days 15ЊC. Use of the fitted response functions reduced stan-(Յ12%) with mean daily air temperatures less than 15؇C before late dard errors in estimating the entire period from plantingAugust. This was significant, as the two response functions diverged to maturity by 50% compared with GDD estimates, significantly at temperatures below 15؇C. Standard errors in estimating with a reduction in the standard errors in estimating the maturity date using different transition dates remained small (Ͻ7.5