Seed production and loss in white clover (Trifolium repens L.)

Abstract

Two studies of seed production, and loss, and seed characteristics of white clover cultivars 'Grasslands Tahora, Pitau, Huia', and G18, (now 'Grasslands Kopu'), were investigated over the 1984/85 season at Lincoln College, Canterbury, New Zealand (43° 38'S). A third study was initiated to examine the breakdown of buried hard seed.

These trials aimed to compare the seed production and loss of the new cultivars, Tahora, Pitau and G18, with the established cultivar Huia. The seed characteristics and the hard seed content were examined for a relationship(s) which may allow agronomic manipulation to reduce hard seed losses. The effect of herbage mass and harvest delay on seed losses was examined to minimise seed loss; especially hard seed loss. A trial with buried seed was initiated to provide data on the rate of hard seed breakdown and influence of depth of burial on depletion of the soil seed bank. The overall aim was to provide basic information for the development of methods to minimise contamination when cultivars of white clover seed crops are changed.

The first study examined flowering patterns, yield components and seed characteristics for a series of inflorescence cohorts of the four cultivars. The highest total inflorescence number of 1150 m⁻² was produced by Tahora while other cultivars produced 830-850 inflorescences m⁻². However seed yield of 0.11 g head⁻¹ of Tahora was lower than other cultivars, which were 0.14-0.16 g head⁻¹. When harvested 28 days after peak pollination 97 % of all seed was hard. Harvest delay reduced the mean hard seed content by 5 % although reductions of up to 12 % occurred for Pitau during a period of optimal softening conditions with temperatures up to 33°C, and 12 days with >15°C diurnal fluctuations. Thousand-seed weight declined with time of flowering which was attributed, in this season, to reserve depletion.

The second trial aimed to quantify seed yields and seed losses. fol1owing defoliation to 5, 10, or 15 cm at closing (22/11/84) to produce variation in herbage mass at harvest, with four sequential harvest dates, (22/1,5/2,20/2,5/3/85). The harvested yield was examined for changes in seed characteristics, especially hard seed.

The mean of the cutting heights at closing resulted in a harvested seed yield for G18 of 286 kg ha⁻¹ , 28 days after peak flowering (Harvest 1). This was less than Tabora 392, Pilau 396 or Huia 373 kg ha⁻¹. Defoliation to 5 cm at closing of the crop yielded 318 kg ha⁻¹ compared with 380 and 387 kg ha⁻¹ respectively for 10 and 15 cm defoliations.

Seed loss at harvest 1 was greatest for Tabora at 275 kg ha⁻¹, which was 42% of total seed produced. Pilau 232, Huia 165, G 18 136 kg ha⁻¹ lost less seed and these losses were only 32-37% of total seed yield. Seed losses from the 5 cm defoliation (250 kgha⁻¹; 44.5%) were significantly greater than at 10 or 15 cm (both 32 %). No significant interactions occurred in harvest 1.

Harvest delay resulted in a linear decline in seed yield. A 6 week delay from harvest 1 resulted in a yield of only 45 % of that obtained at harvest 1. This resulted in a quadratic increase in seed losses from 35 to 75 % of total seed producted. There were no interactions of cultivar with harvest delay for seed yields or losses. Thousand-seed weight also declined with harvest delay indicating sequential loss of the earliest formed and thus best provisioned seeds.

An eight year investigation of the breakdown of buried hard 'Grasslands Huia' seed, in two soil types, was initiated at Lincoln College in 1986.

Initial results from the burial trial (0-9 months) indicated that increasing burial depth increased the protection of seeds to softening. Highest rates of breakdown (65%) occurred at the surface, with little difference at 10 or 20 cm (20%). Seed breakdown was greatest in the initial period with declining rates of loss with time. This may be related to differences in the intensity of hardness of the individual seeds, with seed having a low intensity of hardness being lost first Very little soft seed has been recovered (<2.5 %), although recovered seed will germinate after scarification (>80%).

The major difference between cultivars was the large number of inflorescences produced by Tahora compared to other cultivars. All cultivars have similar seed characteristics. Virtually all seed was in the hard state at physiological maturity, and the rate of seed softening was low. Seed yield was maximised and losses minimised by harvesting 28 days after peak flowering, compared to later harvests.