Tiller dynamics and leaf growth processes of the perennial ryegrass cultivars "Ellett" and "Grasslands Ruanui" as influenced by environmental factors : a thesis in partial fulfilment of the requirements for the degree of Doctor of Philosophy (Ph. D.), Institute of Natural Resources, College of Sciences, Massey University, Palmerston North, New Zealand
In New Zealand, perennial ryegrass (Lolium perenne) is considered the most important and cheapest feed source for dairy cows. The profitability of dairy farming is reliant on the quality, persistence and productivity of perennial ryegrass dominant pastures. However, research shows some 'modern' perennial ryegrass cultivars derived from the Mangere ecotype do not persist under intensive dairy cow grazing. Other research suggested that 'Ellett', a representative of 'modern' ryegrass cultivars, had a different perennation strategy from the older cultivar 'Grasslands Ruanui', used previously. The objectives of this study were (i) to compare the sward productivity, tiller dynamics and flowering behaviour of the perennial ryegrass cultivars 'Ellett' and 'Grasslands Ruanui' under different nitrogen and irrigation treatments and (ii) to determine the morphological and tillering responses of 'Ellett' and 'Grasslands Ruanui' ryegrasses when grown in different light environments. A field experiment at the No 5 Dairy of the Dairying Research Corporation (Hamilton, New Zealand) was conducted from April 1996 to April 1998 to assess the sward productivity of 'Ellett' and 'Grasslands Ruanui' ryegrasses, and their tiller dynamics and flowering behaviours under two levels of nitrogen and irrigation. This field experiment was rotationally grazed by dairy cows stocked at 3.2 cows per hectare. Results showed 'Ellett' had a greater tiller weight and lower tiller density than 'Grasslands Ruanui', which was reflected in higher average yields in spring (3220 vs 2788 kg DM/ha) and summer (2125 vs 1844 kg DM/ha) for 'Ellett' and 'Grasslands Ruanui', respectively. Because of different sward structures 'Ellett' tended to 'pull' more than did 'Grasslands Ruanui' during grazings in summer and autumn, but this did not appear to have greatly influenced the yield comparisons between cultivars. No significant differences in the proportions of white clover, weeds and other grass species were observed between 'Ellett' and 'Grasslands Ruanui' swards. 'Ellett' and 'Grasslands Ruanui' ryegrasses had similar herbage accumulation responses to nitrogen and irrigation; pre-grazing herbage mass, tiller weight and botanical composition were generally affected by these treatments. Overall, nitrogen fertiliser increased herbage accumulation and tiller density, but tiller weight responses were restricted to the second winter and autumn. Irrigation increased herbage accumulation and tiller density in summer and autumn, but did not significantly affect tiller weight. Seasonal tillering patterns for both cultivars were similar and were characterised by high tiller populations in summer followed by a rapid decline in early autumn and then a recovery or tillering 'flush'. This pattern was more pronounced in the second year than the first. 'Grasslands Ruanui' had a higher total tiller number than 'Ellett', regardless of nitrogen and irrigation treatments. From a sward stability diagram developed in Chapter 5, a population index (Pt/Po) was defined as reflecting population changes over time via changes in tiller birth and survival rates. 'Ellett' had a higher population index (Pt/Po) in winter (June 1997) than did 'Grasslands Ruanui' as 'Ellett' had a higher birth rate than did 'Grasslands Ruanui'. This suggests 'Grasslands Ruanui' was more dormant in winter than was 'Ellett'. The ryegrass cultivars when treated with nitrogen had different tiller turnovers in autumn (March 1997) and late spring/early summer (December 1997) giving rise to a significant cultivar x nitrogen interaction. The autumn (March 1997) population index of 'Grasslands Ruanui' (0.704 without nitrogen) decreased when treated with nitrogen (0.627) whereas the 'Ellett' population index (0.788 with nitrogen) increased, compared to the no nitrogen treatment (0.677). The opposite trend was observed between cultivars during the post-flowering period (December 1997) with a higher population index for 'Grasslands Ruanui' (1.129) compared with a lower population index for 'Ellett' (0.996) when both cultivars were treated with nitrogen. Spring differences in tiller population index between cultivars were linked to their flowering behaviour (see Chapter 5 and 6). 'Ellett' had a higher percentage of reproductive tillers than did 'Grasslands Ruanui', regardless of environmental factors, and these differences were stronger in the second year (1997/98), despite there being no clear differences in tiller turnover between 'Ellett' and 'Grasslands Ruanui' during the pre- and post-flowering periods. Moreover, 'Ellett' had a tendency to produce more reproductive tillers in response to nitrogen whereas 'Grasslands Ruanui' favoured vegetative tillering, especially after the establishment year. Due to an acceleration of leaf area expansion, as tiller number increased in response to nitrogen, the half-life of ryegrass tillers was reduced for the tillers tagged in September 1996 and June 1997. Irrigation increased total tiller number for both cultivars in January 1997 and February 1998 but tiller populations declined in March 1998 with 'Grasslands Ruanui' losing more tillers than 'Ellett' (see Chapter 5). Reproductive tiller number averaged across cultivars increased with irrigation in November and December 1996 but decreased in October 1997. Both ryegrass cultivars had similar responses to irrigation with respect to reproductive and vegetative tillering, except in December 1996 and January 1997, when 'Grasslands Ruanui' had more vegetative tillers than did 'Ellett' (see Chapter 6). To characterise the morphological traits affecting vegetative tillering of 'Ellett' and 'Grasslands Ruanui' ryegrasses, an experiment during vegetative growth with different light environments (control and shade treatments, see Chapter 4), was carried out at INRA, Lusignan, France from February to May 1998. To study the relationship between leaf size and tillering, the morphogenesis of 'Ellett' and 'Grasslands Ruanui' were compared with short- and long-leaved perennial ryegrasses ('SL' and 'LL', respectively) resulting from divergent selection in France based on lamina length. Results from this experiment, after imposing thirty three days of light treatment, showed that ryegrass genotypes had different leaf morphology and tillering. 'Ellett' had longer leaves, higher tiller weight and lower tiller number than 'Grasslands Ruanui' regardless of light environments. Differences between genotypes in leaf length were attributable mainly to higher leaf elongation rate in the two long-leaved genotypes ('Ellett' and 'LL') compared with short-leaved types ('Grasslands Ruanui' and 'SL'), as leaf elongation duration (LED) did not differ significantly between genotypes. 'Grasslands Ruanui' maintained greater site filling and a similar leaf appearance rate to 'Ellett' in both light environments, offering an explanation for the higher tiller number per plant for 'Grasslands Ruanui' than for 'Ellett'. However, 'LL' had a higher tiller number per plant than 'SL', because of a higher leaf appearance rate, although this difference decreased during the experiment under the control treatment as 'SL' tended to have a higher site filling ratio than 'LL'. When shaded, differences in tiller number between 'LL' and 'SL', were more mediated by the leaf appearance rate than site filling. With regard to the relationship between leaf growth characteristics and tillering, these results show genotypes with high LER and long lamina length, even though associated with reduced site filling in both light environments, do not necessarily have low tiller number per plant. To describe post-flowering tillering in a dense canopy of 'Ellett' and 'Grasslands Ruanui' ryegrasses, before and after removal of the seed-head, a second controlled light experiment was carried out at INRA, Lusignan, France (see Chapter 6), from October 1997 to July 1998. Results of this experiment after imposing the different light treatments (near full light and shading) for forty five days showed that 'Ellett' had a higher proportion of reproductive tillers than did 'Grasslands Ruanui' during the summer of the establishment year, regardless of light environment (34 % vs 25 %, respectively). These results were in agreement with previous field observations (Chapter 6). Shading reduced the proportion of reproductive tillers produced by both cultivars compared with the full light treatment. Vegetative tiller weight of both cultivars was similar under both light treatments whereas shading reduced reproductive tiller weight by 40 % for 'Ellett' and 43 % for 'Grasslands Ruanui' compared with full light. From the results obtained in field and controlled light experiments, it was concluded that an explanation of the tiller dynamics pattern for 'Ellett' and 'Grasslands Ruanui' could be related to their morphological characteristics (see Chapter 7 and 8). Thus, to assist with the understanding of the seasonal tillering patterns of 'Ellett' and 'Grasslands Ruanui' ryegrasses in different environments, a conceptual approach was proposed, using the morphological ceiling leaf area (MCLA) as an agronomic indicator regulating tiller density.
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Bahmani I., Thom E. R., & Matthew C. (1997). Effects of nitrogen and irrigation on productivity of different ryegrass ecotypes when grazed by dairy cows. Proceedings of the New Zealand Grassland Association, 59 : 117-123