STABILITY OF PERENNIAL RYEGRASS (LOLIUM PERENNE L.) PLANTS CULTIVATED FOR SEEDS AT VARIED LEVELS OF NITROGEN FERTILIZATION

Małgorzata Szczepanek
Department of Plant Cultivation, University of Technology and Life Sciences in Bydgoszcz, Poland

ABSTRACT

The effect of different doses of nitrogen fertilization on the usefulness of perennial ryegrass (Lolium perenne L.) cultivated for seeds for long-term use was studied in a field experiment. A negative correlation of seed yield and the length of use was found. The strongest reduction in seed yielding occurred in the fourth year, which mainly resulted from a decrease in the number of generative tillers and the number of seeds per spike. In the first and second year of full use, nitrogen fertilization in a dose of 60 kg·ha^-1 was sufficient. In the third year, increasing the dose by 30 kg N·ha^-1 prevented a decrease in perennial ryegrass plant productivity, but it did not prevent the yield reduction in the fourth year after sowing. Germination energy and germinability was the lowest in the third year of cultivation; a level of nitrogen fertilization did not affect the seed sowing value.

Key words: perennial ryegrass, seed yield, nitrogen fertilization, years of full use.

INTRODUCTION

Seed plantation of grass can be maintained until the seed yield amounts to at least 50% yield from last year or 30% yield from the first year [8]. The information available in the literature concerning the stability of seed plantations of perennial ryegrass is equivocal. It is commonly reported [8,10,11] that perennial ryegrass gives the highest yield in the first year of use. At precipitation shortage in the first year in spring the maximum yielding is obtained in the second year [15]. According to Martyniak and Domański [10] the seed yield in the second year of use makes 1/3 of the yield from the first year, and in the third year this species usually do not yield. Prończuk et al. [13] report that a decrease in the seed yield of perennial ryegrass amounts to 30% in the second year, and 70% in the third year. Another study indicated [15] that the seed yield of a late cultivar of perennial ryegrass in the third year decreased only by 21.8%, and in the fourth year by 30% as compared with the first one. Kryzeviciene and Zemaitis [8] obtained in the fourth year only 13.7% of the seed yield from the first year of full use.

Perennial ryegrass stability in cultivation for seeds depends on natural habitat conditions [6], cultivar physiological properties (the growth rate, ageing, the time of seed ripening) and the way of utilization [7] as well as on agronomic factors [15] shaping plant growth, development and yielding. Ageing of perennial ryegrass is accompanied by a decrease in new root formation and the increased withering of older ones [7] as well as a reduction in the number of generative tillers [8], spikelets per spike and seeds per spikelet [15]. At long-term use, seed germination energy may also decrease [17].

Perennial ryegrass responds strongly to nitrogen [2], which when applied at the beginning of vegetation affects formation and development of inflorescences [4]. Higher doses of mineral nitrogen result in an increase in number of generative tillers [12]. Apart from generative tillers, also vegetative shoots develop intensively at a high level of nitrogen fertilization of perennial ryegrass [9], which can reduce ryegrass seed yield as a result of competition for assimilates [5]. According Young et al. [18] nitrogen may have a bigger influence on a seed number per spike than on a number of spikes from an area unit. Both Nordestgaard [12] and Rowarth et al. [14] report that mineral nitrogen can cause an increase in thousand seed weight, and thus improve their vitality (for example seedling weight and germinability). However, Goliński [3] found no influence of N on the perennial ryegrass seed weight and their germinability.

In accordance with a working hypothesis it was assumed that a progressive reduction in the root system and impediment in nutrients intake in multi-year use of perennial ryegrass for seeds has a negative effect on plants growth, development and yielding, thus an increase in mineral fertilization level, especially with nitrogen, can prevent a decrease in productivity in subsequent years of use.

The aim of the study was to analyze the effect of nitrogen fertilization level on shaping the structural elements of yielding, yield and seed sowing value of perennial ryegrass in four-year period of use.

MATERIAL AND METHODS

The subject of the study was perennial ryegrass (Lolium perenne L.) that was grown for seeds for four years. An exact field experiment was carried out at the Mochełek Research Station of the University of Technology and Life Sciences in Bydgoszcz. The research was carried out in two series: one was conducted from 1998 to 2002 and the other from 1999 to 2003. The split – plot method was used, and the area of the plots was 14.2 m². The experiment was conducted on grey-brown podsolic soil of the rye good soil suitability complex, in which the humus horizon lies on light loam, deposited below a depth of one meter. The content of the total N in soil at the start of the experiment was low and amounted to 60 mg N in 100 g of soil, which approximately makes 18-36 kg·ha^-1 of mineral nitrogen available for plants in topsoil. The factors in the experiment were: I – the length of use (from the first to the fourth year of full use) and a dose of nitrogen fertilization (0, 30, 60 and 90 kg N·ha^-1). The lawn cultivar of perennial ryegrass Stadion was sown as an undersown crop in barley grown for grain, in the second decade of April, at the amount of 10 kg·ha^-1 and a row spacing of 12 cm. In the years of full use, nitrogen doses were applied in early spring, at the start of vegetation, and then post-emergence harrowing was done. Before seed harvest, measurements of the number of generative tillers were made at a plot with an area of 0.5 m²; the number of spikelets per spike and seeds per spikelet was determined on 10 randomly selected tillers from each plot. Seed harvesting was done in two steps: cutting in the second decade of July and threshing after a few days. Four months after the harvest, thousand seed weight was as well as their sowing value – germination energy and germinability were determined according to ISTA. The significance of differences was evaluated in the statistical analysis according to the Tukey test, at α = 0.05. Variance analyses for the first and second series made the base for making the synthesis where the series were regarded as replications in the years. Correlation relationships were calculated with the Statistica^®.

RESULTS AND DISCUSSION

On the basis of the multi-year averages of the Sielianinov coefficient one may conclude that the weather conditions in the area of Mochełek are usually favorable for perennial ryegrass generative development from April till June; small precipitation shortage may occur in August (Table 1). At the research time, in three out of five years of utilization the weather conditions were unfavorable for perennial ryegrass grown for seeds. In 2000 and 2003 a low hydrothermal coefficient in the first three months of vegetation indicates the occurrence of periodical water deficit. There was also too little rain in April and June 2002. The most favorable weather conditions occurred in 1999 and 2001, when the spring rainfall was usually sufficient for perennial ryegrass growth and plant generative development.

In the present study carried out on light soil the cultivar Stadion generated not more than 390 generative tillers per one square meter (Fig. 1). In other studies conducted on medium-heavy soil (class III b, good wheat complex) the same cultivar generated 3-10 times more such tillers [15]. A mean number of generative tillers for different doses of nitrogen fertilization (Table 2) and within each of them was the lowest in the fourth year of seed harvesting. Perennial ryegrass generated significantly more generative tillers in the third year as compared with the fourth, and the most in the first and second. Also the negative correlation coefficient (r = - 0.77*) indicates a decrease in the number of generative tillers along with the prolongation of utilization period. Similar relations were shown in the experiments by Kryzeviciene and Zemaitis [8] and earlier studies of the present author [15]. Decreased ability to form generative tillers in the fourth year indicates a decrease of plant vigor, which may result from a diminution of root systems. According to Troughton [7] perennial ryegrass older roots begin to wither from flowering, and young ones form poorly, which may even lead to plant dying out. Moreover, in the present study the fourth year of use in both series fell on the years when periodical precipitation deficiency occurred during the generative development from April to June, which had a negative effect on the plant growth. The previous studies [15] indicated that the effect of hydrothermal conditions in the first three months of vegetation on perennial ryegrass yielding was even stronger than the effect of the length of use.

Perennial ryegrass responds to an increase in nitrogen fertilization level with a growth in number of generative tillers [2,12]. In the present study, perennial ryegrass plants fertilized with nitrogen generated more generative tillers as compared with not fertilized ones, irrespective of the year of use, and on average nitrogen doses did not diversify significantly this feature (Table 2). Similarly, in the first year of use non-fertilized perennial ryegrass plants generated fewer generative tillers than those fertilized, but the differences were not proved statistically. In the second year of seed harvesting there were more generative tillers after applying 60 and 90 kg N·ha^-1 as compared with 30 kg, and the least – when N was not applied. In the third year nitrogen had the similar effect, and the generative tiller number at the lowest applied dose of 30 kg·ha^-1 and at 0 kg was not statistically different. In the fourth year of perennial ryegrass full use a dose of 30 kg N·ha^-1 proved to be the most favorable for generative tiller formation. The results obtained show that an increase in nitrogen fertilization on a four-year plantation does not prevent the reduction in the number of generative tillers.

In the study by Nordestgaard [12] the seed number per spike in the second year of use was from 16.7 to 22.7 % less than in the first year. In the present study, a number of seeds per spike, as well as a number of generative tillers decreased in the subsequent years of full use (r = - 0.53*.). The seed number per spike on average for nitrogen fertilization level applied, also at a dose of 90 kg N·ha^-1, was smallest in the fourth year, significantly higher in the third year, and highest in the first and second year of seed harvesting (Table 2). A similar effect of the length of use was shown for ryegrass fertilized with nitrogen in a dose of 60 kg, but the difference of the seed number per spike in the third year as compared with the first and second years was statistically insignificant. Perennial ryegrass fertilized with 30 kg N·ha^-1 formed more seeds per spike in the first and second years of full use than in the third and fourth year.

In the present study, nitrogen fertilization in doses ranged from 0 to 90 kg N·ha^-1 stimulated the number of formed seeds per spike. On average for years of full use and independently in each year (Fig. 2) it was indicated that an increase in a dose by another 30 kg increased significantly the seed number per spike, only in the fourth year such relation was found to a dose of 60 kg N·ha^-1. Young et al. [18] only in one of the two series of experiments indicated an increase in seed number per spike for plants fertilized with nitrogen in comparison with not fertilized ones. Nordestgaard [12] did not found the effect of the amount of nitrogen applied within the range of high nitrogen doses 70-130 kg·ha^-1 on the seed number per spike in the first and second year of perennial ryegrass full use.

Thousand seed weight was on average 1.60 g and was by 30.7% higher than in other studies concerning the same cultivar [16]. A significant effect of the length of use on thousand seed weight was not found, although in the fourth year of use it was usually lower than in the other years, which might have resulted from rainfall deficiency during seed filling. Thousand seed weight on average for the years of use was higher, if the ryegrass was fertilized with nitrogen in comparison with the variant without N fertilization and differences were proven statistically only for a dose of 60 kg N·ha^-1 (Table 2). The effect of the nitrogen doses applied on thousand seed weight in particular years of seed harvesting was not indicated (Fig. 3). Similar results were obtained by Goliński [4] in a study on four cultivars of perennial ryegrass for two years of full use with the doses of spring nitrogen fertilization from 0 to 90 kg N·ha^-1. A small positive effect of increasing the dose up to 160 kg N·ha^-1 on thousand seed weight was indicated by Hampton et al. [5].

The seed yield in the present study carried out on light soil in the variant of the highest N dose did not exceed 515 kg·ha^-1 in the first year of use (Fig. 4). On medium-heavy soils the same cultivar gave over three times higher yield at a similar level of nitrogen fertilization [16]. A negative correlation of seed yield and the length of use (r = -0.33*) was shown, as well as a positive effect of increasing the number of generative tillers (r = 0.57*), seeds per spike (r = 0.63*) and a thousand seed weight (r = 0.24*) on seed yielding. Irrespective of the nitrogen doses applied, the seed yield was the lowest in the fourth year after sowing, which was related to a reduction in the number of generative tillers and – to a less degree – the number of seeds per spike. In the study by Kryzeviciene and Zemaitis [8] on stability of different species of grass growing for seeds, a number of generative tillers in perennial ryegrass in the fourth year amounted to only 25% of the tillers from the first year, which had a direct effect on a seed yield decrease.

The seed yield of perennial ryegrass fertilized with 60 kg N·ha^-1 was significantly lower in the third year of use as compared with the first one, while with a dose of nitrogen increased by 30 kg plant productivity was similar for the first three years (Fig. 4). Nordestgaard [12] indicated that to maintain the high yielding level of perennial ryegrass in the second year of seed harvesting it is necessary to increase the dose of nitrogen by 10-15 kg·ha^-1. Goliński [4] considered 72.5-89.0 kg N·ha^-1 as the optimal doses in the first year and 3.2-5.3 kg N per ha more – in the second year of full use.

In the present study, the yield of ryegrass not fertilized with nitrogen was the poorest in all the years of seed harvesting (Table 2). Natural resources of mineral nitrogen in soil covered plant requirements for producing up to 204 kg of seeds and up to 710 kg of straw from 1 ha (date not presented). In the variant without nitrogen fertilization, as well as in the objects that were fertilized, the lowest yield was obtained in the fourth year of seed harvesting.

In the first and second year of full use an increase in nitrogen dose from 30 to 60 kg·ha^-1 resulted in a significant decrease in seed yield (Fig. 4). In these years similar production effects were obtained using 60 and 90 kg N per ha, which was possible due to the comparable ability to form generative tillers. In the third year, the application of 90 kg N·ha^-1 caused a significant increase in seed yield as compared with the other nitrogen doses. In the fourth year of harvesting, 60 kg N·ha^-1 proved to be more effective than 30 and 90 kg·ha^-1 due to increasing the number of seeds per spike.

The seeds were characterized by a high sowing value. The seed germination energy and germinability on average for the nitrogen doses were the lowest in the third year of full use (Table 2). In particular objects with different level of nitrogen fertilization the effect of the length of use on germination energy and germinability was insignificant (Fig. 5 and 6). The previous studies [17] showed a decrease in germination energy along with the prolongation of plantation use. A poor relationship, or the lack of such relationships shown in the present study may result from a high thousand seed weight, irrespective of plant age and correlated significantly with germination energy (r = 0.28*). The present study did not find an effect of nitrogen fertilization dose and its interaction with the length of ryegrass use on the analyzed features of seed sowing value. Similarly, in the studies by Cookson et al. [1] and Goliński [4] the effect of nitrogen doses on perennial ryegrass seed germinability was insignificant.

CONCLUSIONS

1. The spring nitrogen fertilization at the dose not exceeding 60 kgˇha^-1 was sufficient in the first and second years of full use for perennial ryegrass grown for seeds on light soil.

2. One can prevent a decrease in perennial ryegrass productivity in the third year of seed harvesting by increasing a dose of spring nitrogen fertilization from 60 to 90 kgˇha^-1.

3. Perennial ryegrass seed yield in the fourth year of use is lower in comparison to the first one by 27.1-61.4%, depending on the nitrogen dose applied; a decrease in reproductive power results mainly from the reduction in the number of generative tillers and seeds per spike.

4. Germinability and germination energy of ryegrass seeds were lowest in the third year after sowing; the influence of nitrogen dose and its relationship with the length of use on seed sowing value was insignificant.

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Accepted for print: 21.12.2006

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