Wk Stage Research Articles

Bioavailable phosphorus in runoff from alfalfa, smooth bromegrass, and alfalfa-smooth bromegrass.

Runoff from sloping landscapes cropped with established alfalfa (Medicago sativa L.) may contain bioavailable P (BAP) which accelerates eutrophication of surface water bodies. Such BAP exists as either dissolved reactive P (DRP) or bioavailable reactive particulate P (BPP). We hypothesized that before and after harvest, sod-forming smooth bromegrass (Bromus inermis Leyss.) or alfalfa-smooth bromegrass mixtures would have less BAP, DRP, and BPP runoff losses than taprooted alfalfa. Swards established in 1992 near Lancaster, WI were subjected to a 72 mm simulated rainfall applied for 1 h in 1993 and 1994 to forage regrowth at 4 and 6 wk after first harvest and immediately (0 wk) after second harvest. Hourly BAP losses for all sward types were 82% less when 1.5 Mg ha(-1) of forage dry matter was present. High DRP losses (>0.050 kg ha(-1)) were associated with high DRP concentrations (>7.1 micromol L(-1)) and high surface soil P concentrations (>59 mg kg(-1)) resulting from broadcast maintenance P fertilizer. High BPP losses (>0.035 kg ha(-1)) were associated with high runoff volumes (>24 mm) and sediment concentrations (>2 g L(-1)). Summed over all 6 rainfall simulations, total BAP loss was only 0.07 kg ha(-1) at the 6 wk stage of regrowth compared with 0.35 at 4 wk, and 0.41 at 0 wk. Moreover, there was no significant difference between sward types for DRP concentration, DRP loss, or BAP loss. We conclude that avoiding excessive defoliation was more effective at reducing BAP losses than specific forage species selection.

Adaptation of winter cereal species to shade and competition in a winter/spring cereal forage mixture

Spring planted mixtures of spring and winter cereals maximize dry matter yield and provide fall pasture by regrowth of the winter cereal. However, delay of initial harvest may reduce the winter cereal component and therefore subsequent regrowth yield. Research was conducted at Lacombe, Alberta to investigate the effect of time of initial cut (stage), winter cereal species (species) and cropping system (monocrop and mixture) on winter cereal shoot weight, leaf carbon exchange efficiency and shoot morphology. These parameters may be related to adaptation of winter cereals to growth and survival in the mixture. Winter cereal plants were grown in pails embedded in monocrop plots of fall rye (Secale cereale L.), winter triticale (X Triticosecale Wittmack) and winter wheat (Triticum aestivum L.) and in binary mixtures with Leduc barley (Hordeum vulgare L.). The plants were removed when the barley reached the boot (B), heads emerged (H), H + 2, H + 4 and H + 6 wk stages. Shoot weight was generally smaller in the mixture than in the monocrop and wheat was reduced more than fall rye and triticale in the mixture compared to the monocrop. Dark respiration rate (r = −0.54) and carbon exchange (r = 0.36) under low light intensity were correlated (P < 0.05) to shoot size in the mixture. Fall rye and winter triticale had lower dark respiration rates than winter wheat. Leaf area index (LAI) was closely correlated (r = 0.83 and 0.84) with shoot weight in both the mixture and monocrop. While species failed to exhibit clear cut differences for LAI, fall rye and winter triticale were reduced less than winter wheat in the mixture relative to the monocrop. Stage was the dominant factor affecting winter cereal growth in both cropping systems, but fall rye and triticale exhibited superior morphological features, and their carbon exchange responses to light were more efficient than wheat, which should allow them to be sustained longer under the shaded conditions of a mixture. Key words: Delayed harvest, shade, spring and winter cereal mixtures, adaptation, carbon exchange, respiration

Delay of harvest effects on forage yield and regrowth in spring and winter cereal mixtures

Spring-planted mixtures of spring and winter cereals extend the grazing season and maximize dry matter yield if the initial defoliation is delayed until the milk stage of kernel development of the spring cereal component. However, fall regrowth may be less than spring-planted winter cereal monocrops. Research was conducted at Lacombe, Alberta to determine the effect of time of initial cut and winter cereal species on initial yield, regrowth yield and factors affecting regrowth (tiller density, water-soluble carbohydrate (WSC) and etiolated regrowth immediately post-cutting) of the winter cereal component of spring-seeded monocrops of fall rye (Secale cereale L.), winter triticale (× Triticosecale Wittmack) and winter wheat (Triticum aestivum L.) and in binary mixtures with spring barley (Hordeum vulgare L.). Treatments were planted in early May and harvested initially when the barley reached the boot (B), heads emerged (H), H + 2 wk, H + 4 wk and H + 6 wk stages. Three weeks after the initial cut a regrowth harvest was conducted. Initial yields of both mixtures and monocrops increased until approximately H + 4 wk and H + 6 wk respectively, with no differences due to species in the mixture. The effect of crop stage at initial harvest on regrowth was much larger than the species effect. Monocrop regrowth decreased almost linearly with delay of defoliation, while that of the mixture (winter and spring components combined) declined at a faster rate. Tiller density, WSC and etiolated regrowth also followed decreasing trends with time of initial cut in the monocrops and mixtures. These trends were due directly or indirectly to very low light intensities in the lower levels of the canopies of both cropping systems after H + 2 wk. Both initial and regrowth yields of the winter cereal component of mixtures involving rye and triticale were generally superior to those involving wheat. The ability of the winter cereal component to regrow in mixtures may be related to plant size and therefore ability to compete during initial growth. In general, the initial harvest of the mixture should occur no later than H + 2 wk of the spring cereal component in order to allow sufficient time for recovery of the winter cereal component after harvest. Otherwise, a spring-seeded winter cereal would be a better alternative for fall pasture. Key words: Delayed harvest, spring and winter cereals, forage yield, regrowth

Effects of Poa trivialis, Stellaria media and Rumex obtusifolius on the growth of Lolium perenne in the glasshouse

SUMMARYIn glasshouse experiments shoot dry weight, leaf area and tiller number of Lolium perenne, cv. S. 24, were compared when grown in monoculture and in competition with Poa trivialis, Stellaria media or Rumex obtusifolius at a range of densities and for different time intervals. The results showed that S. media and R. obtusifolius were more competitive than P. trivialis. Competition effects with S. media and R. obtusifolius appeared between the 4 and 6 wk stage, coinciding with the production of the first S. 24 tillers. Spraying with 2,4‐D (0·84 kg acid equivalent/ha) for control of R. obtusifolius did not decrease grass yield whereas mecoprop (2·80 kg a.e./ha) for control of S. media damaged young L. perenne. It appeared that the optimum time for mecoprop treatment was between 4 and 6 wk after sowing when tillering had commenced. At this stage weed competition was only beginning and crop tolerance to the herbicide was adequate.