The influence of ryegrass on vegetative and reproductive growth, yield, and yield components of winter wheat

Abstract

Field and greenhouse experiments examined the interference of semi-dwarf, winter wheat (Triticum aestivum L.) by Italian ryegrass (Lolium multiflorum Lam.). Sharp declines compared to controls in leaf area and above ground biomass of wheat occurred at the lowest tested density of 40 ryegrass plants m$sp{-2}$ in early spring in central Texas fields. The early season advantage of wheat in height and leaf area suggested that below ground interference by ryegrass was likely the most important interaction of the two species. This interference persisted during the season, caused lodging, and substantially reduced wheat yields. While spikes m$sp{-2}$ and kernels per spike were equally reduced by ryegrass interference, mass per grain and harvest index were little affected. Greenhouse experiments using partitions to separate roots only or shoots only of the two species and replacement series studies showed that the primary interference by ryegrass on wheat growth was below ground. Drought enhanced ryegrass interference of wheat growth as assessed by leaf area expansion rate, leaf area extent, and dry masses of leaves, stems and roots. Water stressed ryegrass interseeded with wheat maintained a greater leaf area expansion rate, leaf thickness, leaf area, and above ground dry mass than its companion wheat. Eleven data sets including two from North Carolina, three from Texas, and seven from Oregon were used to model wheat yield loss in response to ryegrass populations. The relatively high correlation coefficient (r$sp2$ = 0.88) from regressing wheat yield loss against ryegrass proportion was very striking considering the differences in wheat cultivars, ryegrass biotypes, soils, and weather among the sites. The best equation describing ryegrass interference was a simple linear regression: percent wheat yield loss = (132 $star$ ryegrass proportion) $star$ 100.