The rates of drying of perennial ryegrass, subjected to different treatments at mowing and after mowing, were assessed in the field by weight change of grass fresh weight in wire‐mesh trays over 3·5 d (76 h). In a 5 × 3 × 3 factorial experimental design, the effects of five weights of grass per unit area [1·5, 3, 6, 12 and 24 kg fresh material (FM) m−2], three treatments at mowing (no treatment, mower‐conditioned, flail‐treated) and three treatments after mowing (no treatment, inverted, mixed) were examined. The experiment was replicated twice on 16 occasions in 1992 at the Agricultural Research Institute of Northern Ireland. This gave a total of thirty‐two replicates per treatment. The trays were weighed at 2‐h intervals from 09.00 to 17.00 h each day. Data sets were restricted to rain‐free days and also to the first day after mowing (day 1). On day 1, grass weight per unit area was a major factor dictating drying; reducing the grass weight per unit area of unconditioned grass from 6 to 3 kg FM m−2 increased grass drying rate by 47%. There was no significant (P > 0·05) benefit over the untreated grass on day 1 from mixing or turning mower‐conditioned or the unconditioned grass. Mixing of the flail‐treated grass improved drying rate significantly (P > 0·001) over the control. Over the whole 76‐h period, the relative benefit from either mower conditioning or flail treatment over no treatment was dependent upon both grass weight per unit area and initial dry‐matter (DM) concentration. At higher initial DM concentrations (>150 g kg−1) and lower grass weights (<6 kg FM m−2) both mower conditioning using a nylon brush type conditioner and intensive conditioning by flail treatment gave substantial increases in drying over no treatment. Moisture regain of grass exposed to overnight dew was small. Rain had a much greater effect than dew on subsequent moisture regain. Unconditioned grass at 12 kg m−2 retained 82% less water following rainfall than unconditioned grass at 3 kg m−2.

The rates of drying of cut perennial ryegrass (Lolium perenne L.) herbage over 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 placed in half the area of wire‐mesh trays (0·5 m long × 0·3 m wide × 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 grass fresh weight. Light was supplied from 400‐W mercury vapour lamps, while an air conditioning unit within the controlled environment room allowed control of vapour pressure deficit (VPD). Only one particular VPD could be created on any one day and resource limitations restricted the study to one replicate per day. The first experiment correlated drying rates under the mercury 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 equated to 1081 W m–2, 400 mm to 432 W m–2 and 600 mm to 281 W m–2. Experiment 2, conducted with previously frozen grass, was a 4 × 4 factorial design with light intensity and VPD as factors. The third experiment (Experiment 3) compared the drying rate of freshly cut grass with grass that had previously been frozen in a 2 × 2 × 2 factorial design with the two grasses, two light intensities and two wind speeds as factors. The final experiment (Experiment 4) was a 3 × 2 × 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 replicates 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 previously 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 drying followed by VPD and wind speed. However, the influence of wind speed was variable. A wind speed of ≈3 m s–1 increased the rate of water loss from grass with a low initial DM concentration (<160 g kg–1) receiving low levels of solar radiation (281 W m–2), while at higher initial 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 Experiments 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, resulted in large increases in grass DM concentration. However, these extreme weather conditions are unlik...

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