Effects of sowing depth and nitrogen on emergence and establishment of a range of important New Zealand pasture grass species

Abstract

The effects of sowing depth and nitrogen (N) on emergence and establishment of prairie grass (Bromus willdenowii Kunth., mean seed weight 12.0 mg); upland brome (Bromus sitchensis L., 6.7 mg); annual ryegrass Lolium multiflorum Lam., 5 mg); perennial ryegrass (L. perenne L., 2.5 mg); tall fescue (Festuca arundinacea Schreb., 1.8 mg); phalaris (Phalaris aquatica L., 1.5 mg); cocksfoot (Dactylis glomerata L., 0.7 mg) and timothy (Phleum pratense L., 0.4 mg) were examined under field and controlled environment conditions. At all sowing depths in all experiments emergence was positively correlated with seed weight. For all species, emergence decreased with increased sowing depth but the magnitude of the effect was greater for smaller seeded species. Poor emergence of smaller seeded species from 30 and 60 mm sowing depths was likely to be due at least in part to insufficient coleoptile length and seed reserves but poor emergence at 10 mm sowing depth indicated that coleoptile length and seed reserves were unlikely to be the main factor reducing emergence. It is proposed that smaller seeds result in a smaller, more fragile seedling which is less able to penetrate the substrate.

Addition of 100 kg N ha⁻¹ at sowing did not affect emergence in the field experiments but emergence was reduced with increased applied NO₃⁻ concentration a to 5 mol m⁻³ in the controlled environment experiments. It is likely that a lack of effect of N in the field is due to a high interstitial soil nitrate (NO₃⁻) concentration in the 0 kg N ha⁻¹ treatment and the small difference between the a kg N ha⁻¹ and the 100 kg N ha⁻¹ treatments. Under controlled environment conditions, coleoptile length was unchanged with increased applied NO₃⁻ concentration. Possible mechanisms for the N effect on emergence are discussed.

In general, survival of emerged seedlings increased with increased sowing depth but was unaffected by applied N in field experiments. At 10 mm sowing depth, survival was positively correlated with seed weight but at 30 and 60 mm sowing depths there was no consistent correlation between seed weight and survival indicating that insufficient seed reserves was not the main factor causing decreased seedling survival. Increased sowing depth is likely to place the root system further down the soil profile where fluctuations in soil temperature and moisture are buffered somewhat by the soil above possibly increasing survival. Increased partitioning of DM to the shoot may decrease survival but species such as cocksfoot and timothy which showed poor survival in the field did not show increased partitioning of DM to the shoot or leaves under a range of applied NO₃⁻ concentrations in glasshouse experiments.

To achieve maximum emergence and establishment it is suggested that farmers should provide a relatively homogenous seedbed to ensure ease of coleoptile penetration and consistency of sowing depth and to sow excess seed to negate high seedling mortality. Also; a sowing depth of less than 30 mm is essential. Increased seed weight is possible through selection for high seed weight and increased ploidy level. This may take some time to achieve and may clash with other plant breeding objectives.