The objectives of these studies were to examine the effects of sowing date and plastic film on the yield and quality of forage maize (Zea mays L.) in Ireland, a marginal maize‐growing area. In 1989–95, comparisons were made of maize sown through clear plastic film (PP treatment), and maize sown in the conventional manner without plastic (NP treatment), in terms of dry‐matter (DM) yield and quality (cob, grain and starch contents). The PP treatment significantly increased DM yields in five years, with a mean yield increase of 3·10 t ha−1. Whole‐plant DM content increased (significant in four years) by a mean of 0·15; cob content increased (significant in five years) by a mean of 0·32; grain content increased (significant in all of the three years examined) by 0·75; and starch content increased from 236 to 318 g kg−1 DM (P < 0·01) in 1995 where the plastic film treatment was used. Organic matter digestibility values were similar for both treatments in 1995. Sowing date in the April‐May period had no consistent effect on DM yields, but early sowings did increase cob (significant, except in 1994), grain (significant in one (1995) of the two years tested) and starch (P < 0·05) contents (only tested in 1995). Sowing on 11 April 1995 through perforated plastic gave lower yields than the NP treatment sown on the same date, due to severe frost damage in early May.

The objectives were to compare the effects of two plastic film systems [perforated (PP) and complete-cover (CC) systems] on the yield and quality of forage maize, and to study the effects of seeding rate on maize grown with, and without, plastic film. Between 1995 and 1998 (Experiments 1 and 2), comparisons were made of maize without plastic film systems (NP), sown through clear plastic film (PP) and covered with clear plastic film (CC). In the CC system, the plastic was removed manually from the crop. Two dates of removal (CC1 and CC2) were compared in 1995, while four removal dates (CC1, CC2, CC3 and CC4) were examined in 1996-98. In 1996-98, three sowing dates (early, midApril; mid, early-May; late, mid-May) were also examined. Four seeding rates (78 200, 93 900, 107 000 and 126 000 seeds ha )1 ) were examined in two experiments (Experiment 3 in 1996 and Experiment 4 in 1996-98). No plastic was used in Experiment 3, while three plastic treatments (NP, PP and CC) were examined in Experiment 4. In the CC treatment, the plastic film was removed at the six-to eight-leaf stage.In Experiment 1, the PP treatment gave lower DM yields than the NP treatment due to frost damage which killed 0AE30 of the plants in the PP treatment. Plants were undamaged by frost in both CC treatments, and the CC2 treatment (plastic removal at the six-leaf stage) gave significant increases in DM and grain yields. In Experiment 2, averaged over years and sowing dates, DM yields were significantly increased by all plastic film treatments except CC4. All plastic film treatments gave significantly earlier tassel emergence than the NP treatment, and significantly increased grain yields and contents of DM and grain. The highest yields of DM and grain were obtained from the PP system. Within the CC treatments, leaving the plastic film intact until the tenleaf stage (CC4) gave lower yields of DM and grain than removing the plastic film at an earlier stage. Averaged over years, the largest DM yields were obtained from the CC system with early-sown material, while the yield differences between treatments with late-sown crops under this system were not significant. Highest DM and grain yields were obtained with early sowing, while late sowing gave lower contents of DM and grain, and later tassel emergence.In Experiments 3 and 4, DM and grain yields increased as seeding rate increased up to 126 000 seeds ha )1 with NP and CC plastic treatments. With the PP treatment, DM yield reached a plateau at 107 000 seeds ha )1 . Seeding rate did not significantly affect DM content in either experiments but grain content declined in both experiments as seeding rate increased.

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