The influence of weather factors on the drying rate of cut perennial ryegrass herbage under controlled conditions

The rates of drying of cut perennial ryegrass (Lolium perenne L.) herbage o ver short periods of time were measured in four experiments in a controlled environment room. Standard weights of 33.7 g grass dry matter (DM) were pl aced in half the area of wire-mesh trays (0.5 m long x 0.3 m wide x 0.07 m high with 11-mm-square mesh) which, so as to simulate conditions in a swath , were supported on wooden frames within dark plastic boxes 25 mm above 35- mm-thick wet sponges. The trays of grass in the controlled environment room were weighed hourly for 7 h, drying rate being assessed by the change in g rass fresh weight. Light was supplied from 400-W mercury vapour lamps, whil e an air conditioning unit within the controlled environment room allowed c ontrol of vapour pressure deficit (VPD). Only one particular VPD could be c reated on any one day and resource limitations restricted the study to one replicate per day. The first experiment correlated drying rates under the m ercury vapour lamps with drying rates in the open air under sunshine over 3 d. This work showed that a distance of 200 mm between the tray and lamps e quated to 1081 W m(-2), 400 mm to 432 W m(-2) and 600 mm to 281 W m(-2). Ex periment 2, conducted with previously frozen grass, was a 4 x 4 factorial d esign with light intensity and VPD as factors. The third experiment (Experi ment 3) compared the drying rate of freshly cut grass with grass that had p reviously been frozen in a 2 x 2 x 2 factorial design with the two grasses, two light intensities and two wind speeds as factors. The final experiment (Experiment 4) was a 3 x 2 x 2 factorial design with light intensity, VPD and wind speed as factors. A wind of approximately 3 m s(-1) was simulated using a 22-mm, 30 W fan set in a fixed position 600 mm from each tray plus grass. Fresh grass was cut each morning of the experiment. There were six r eplicates of each treatment. The effect of the three radiation intensities on grass DM concentration in Experiment 2 was highly significant (P < 0.001 ). VPD had less effect (P < 0.05). Results from Experiment 3 showed that pr eviously frozen material dried much more rapidly than fresh grass and as a result would not simulate actual grass drying in the field. Consequently in Experiment 2 fresh grass was used as opposed to previously frozen material . In Experiment 4, light intensity had the greatest influence on grass dryi ng followed by VPD and wind speed. However, the influence of wind speed was variable. A wind speed of approximate to3 m s(-1) increased the rate of wa ter loss from grass with a low initial DM concentration (< 160 g kg(-1)) re ceiving low levels of solar radiation (281 W m(-2)), while at higher initia l DM concentrations (> 210 g kg(-1)) and higher solar radiation levels (432 W m(-2)) the effect of wind was to slow grass drying. The results from Exp eriments 2 and 4 also indicated that high levels of either wind (3 m s(-1)) or VPD (>6 mbars), when associated with low levels of solar radiation, res ulted in large increases in grass DM concentration. However, these extreme weather conditions are unlikely to occur in practice.