3.5.1
Do cultivars re-rank?
The dynamic relationships between perennial ryegrass and white clover in a mixed sward are
influenced by environmental and management factors and by intrinsic characteristics of both species (W. Harris, 1990; Schwinning & Parsons, 1996a, 1996b, 1996c). Differences in their seasonal growth rates due to different optimum temperatures for growth (Brougham, 1959; W. Harris & Hoglund, 1977; Mitchell, 1956a; Turkington & Harper, 1979a, 1979b) as well as the ability of white clover to fix atmospheric N2 (Ledgard, 1991) facilitate the development of systems in which both species
compete for ‘different space’ according to the de Wit (1960) definition. Their proportion in the sward as well as their dry matter yields are affected by the level of N fertiliser applied and the cultivars used (Camlin, 1981; Collins & Rhodes, 1989; Frame & Boyd, 1986a, 1986b; Gilliland, 1996; S. L. Harris & Clark, 1996; S. L. Harris, Clark, et al., 1996; S. L. Harris, Thom, et al., 1996; Whitehead, 1970) amongst other factors. However, despite the effects of these sources of variation on sward composition and production, under moderately high levels of N, total yield of mixed pastures sown with different perennial ryegrass cultivars tend to reflect the yield of the grass component (Camlin, 1981).
Therefore, the working hypothesis was that the relative ranking of the perennial ryegrass cultivars for total dry matter yield would not change when sown with white clover under high and low N fertiliser application rates.
The combination of N and clover treatments created markedly different environments for plant growth as shown by the large range in annual yield (Table 3.4). White clover was well established in the swards; its content (expressed as % DM) was always greater in pastures of the Low N treatments than in the High N treatments. The perennial ryegrass cultivars provided contrasting phenotypes for two traits that may influence competition between grass and clover: morphology (dense versus open) and heading date (mid-season versus late-season). Thus, the environments and contrasts created by the different treatments and cultivars provided a fair test of the hypothesis.
During the two years of the experiment, significant interactions between cultivar and N and cultivar and clover were detected in winter 2013, but not during the other seasons nor in the total annual yields. As a consequence, no evidence of re-ranking emerged and therefore the hypothesis was supported by the results. Although the white clover content of the swards, expressed as % DM, was significantly different across the perennial ryegrass cultivars in three of the six sampling seasons, these differences were insufficient to cause re-ranking on a total DM yield basis. Moreover, during summer in both years and autumn in the first year, mid heading cultivars supported greater clover content than late heading cultivars, similar to the findings of Camlin (1981), Gooding, Frame, and Thomas (1996) and Hoen (1970), but this difference was not reflected in changes in relative ranking positions.
Therefore, performance values in the Forage Value Index (Chapman et al., 2016; DairyNZ), which are calculated using DM yield data from cultivar evaluation trials conducted using perennial ryegrass monocultures (Easton et al., 2001), do not need adjustment to account for grass-clover interactions over time and their effects on total pasture DM yield. These results have important implications for the breeding industry and the pastoral sector in New Zealand because they support the notion that improvements achieved by breeding programs should be reflected in increments in DM yield in mixed pastures at a farm scale.
3.5.2
Total DM yield
N effects
N had a significant effect on DM yield throughout the duration of the experiment (Table 3.4); annual total DM yield in the High N treatment was 28 % greater than in the Low N treatment during 2012 - 13 and 31 % greater during 2013 - 14. However, this response was not uniform amongst seasons and clover treatments as shown by the presence of an interaction between N and clover on DM yield in summer of the first year, autumn in both years and the annual total in both years (Table 3.4). As a result, DM yields from the Low N with clover treatment were similar to those from the High N treatments and significantly greater than from the Low N without clover treatment. Variation in the response to N is common (Ball & Field, 1982; Ball et al., 1978; Feyter, O'Connor, & Addison, 1985; S. L. Harris & Clark, 1996; S. L. Harris, Clark, et al., 1996; Hennessy et al., 2012; C. W. Holmes, 1982; Laidlaw, 1980; Moir, Cameron, Di, Roberts, & Kuperus, 2003; Shepherd & Lucci, 2011; Whitehead, 1995) and is related to variation in factors such as soil temperature, N supply by the soil, season, pasture composition, and N application rate.
For the entire grazing period (spring to autumn), the average N fertiliser response for the two years was 16.6 kg DM/kg N in the minus clover treatment and 5.9 kg DM/kg N when ryegrass was grown with clover. This considerable difference in response was mainly attributable to the contribution of the white clover to the DM yield in summer and autumn, but other factors such as an increased N supply by the soil in the white clover treatments (not measured) are likely to have also contributed. In agreement with previous work (Feyter et al., 1985; Martin, 1960; Moir et al., 2003; O'Connor & Cumberland, 1973) the yield response to N was greater during spring than in other seasons in both years, with an average across the two springs of 21.8 kg DM/kg N in the minus clover treatments and 15.6 kg DM/kg N in the plus clover treatments. The dominant component of the pastures in the plus clover treatments during spring was perennial ryegrass, comprising approximately 80 and 86 % of the herbage during the first and second spring respectively (average of the High plus clover and Low plus clover treatments). Thus a strong response to N was not surprising given the well-known effect of this nutrient on promoting perennial ryegrass growth (S. L. Harris, Thom, et al., 1996; Whitehead, 1970; Woledge & Pearse, 1985). The contribution of clover to herbage mass (expressed as % DM)
was low during spring due to its requirement for a higher temperature for optimum growth
compared with ryegrass. This intrinsic relative competitive disadvantage of white clover in spring was accentuated when more N was available, especially in the spring of the establishment year, when N2
fixation could have been insufficient to sustain clover growth, and the legume may have been ‘competing’ with grass for the ‘same space’ (same N) according to the de Wit definition (W. Harris, 2001). Meanwhile, the contribution of clover (expressed as % DM) in the Low N treatment during the first spring was greater, probably due to less competition for light (W. Harris, 2001) from the grass which was limited by N availability. In this environment of less N available in the soil and greater clover content, N2 fixation by the legume should have been greater than under higher N availability
(Ledgard et al., 2001), creating the basis for the development of an ‘exploitation’ interaction
(Chapman et al., 1996; Schwinning & Parsons, 1996a) between both species. Average yield responses to N in the absence of clover continued at a high level in summer (17.6 kg DM/kg N; average of two summers), indicating a considerable limitation in the N supply from the soil. In the presence of clover, however, a very low response of 2.5 kg DM/kg N (average of the two years) was observed. During this time of the year higher temperatures gave a relative advantage to white clover and greater contributions to the DM yield were expected from the legume. In the Low N treatment, clover comprised 50.5 % and 36.4 % of the DM during the first and second summer respectively and a consequent increase in N2 fixed (not measured) compared with the High N treatments pastures
(which had 14.6 and 11.7 % DM of clover in the same periods) would be expected. Thus, the
important contribution to the total DM by the clover, as well as the increased N availability for grass growth in the system due to N2 fixation, created a smaller gap in yield between High and Low N
treatments in the presence of clover during summer, and the consequent lower response to
additional N applied as fertiliser. Average response to N during autumn was lower than in spring and it was higher in the absence than in the presence of white clover (11.8 versus 1.7 kg DM/kg N, average of the two years; Table 3.5). This lower response in autumn compared with spring is likely a consequence of less favourable environment conditions for growth as temperature, day length and radiation intensity decrease (Frame & Boyd, 1986a). Variable responses to N at this time of the year have been observed in previous studies (Feyter et al., 1985; O'Connor, 1982), and one of the possible explanations is the difference in soil N mineralisation rates during this season. A high response to N in mixed swards was reported by Moir et al. (2003) in Canterbury during autumn when N was applied in the form of urea to mixed pastures (between 10 and 15 kg DM/kg N). However, pastures in their study contained less clover during this time of the year than the Low N with clover treatment in this experiment.
Clover effects
White clover had a significant effect on the DM yield during every season, with the exception of the first spring of the experiment, when the swards were still establishing. Mean annual total DM yield in the with clover treatments was 23 % greater than in the without clover treatments at the end of the
first year and 28 % greater at the end of the second year. Most of the increase in DM due to white clover occurred in summer and autumn, which together accounted for 99 % and 72 % of the total increase during the first and second year respectively. However, due to the interactions between N and clover treatments mentioned above, the significance of clover inclusion on DM yield differed under High or Low N treatment (Figure 3.9).
Figure 3.9 Growth rate (kg DM/ha/day) of pastures sown plus or minus clover and receiving High or Low N fertiliser annually. High N minus clover (solid blue line), High N plus clover (dashed blue line), Low N minus clover (solid red line), Low N plus clover (dashed red line).
DM yield gains due to the inclusion of clover in pastures have been reported previously (Enriquez‐ Hidalgo et al., 2016; Ledgard et al., 1990; Reid, 1983) and they are a consequence of several factors. The different seasonal growth patterns of grass and clover in New Zealand determined by their temperature requirements for optimal growth (Brougham, 1959; W. Harris & Thomas, 1973; Mitchell, 1956b; Turkington & Harper, 1979a) allow the clover to contribute to yield during warmer times of the year, especially when the ryegrass growth is depressed post-flowering (Anslow, 1965). In this way, the ‘resource space’ (W. Harris, 2001) is used more efficiently in a mixture than in a
monoculture. The ability of clover to fix N2 also contributes to the yield gain through the reduced
competition for the available N in the soil, and through the increase in the pool of this nutrient in the system. Thus, when both grass monoculture and grass/clover mixture are grown under the same N regime the increased yield of the mixtures is possible because amongst other factors, the species are operating in ‘different N spaces’. The more effective use of resources by perennial ryegrass and white clover mixtures than by the corresponding monocultures has also been indicated by Turkington and Jolliffe (1996), using the index relative resource total.
0 10 20 30 40 50 60 70
10-Jul-12 18-Oct-12 26-Jan-13 6-May-13 14-Aug-13 22-Nov-13 2-Mar-14 10-Jun-14 18-Sep-14
kg DM/
h
a/
d
When the grass/clover system is operated under a low N fertiliser rate, the clover uses its ability to fix N2, and becomes independent of N supply from the soil, creating the conditions for the
development of an ‘exploitation’ interaction (W. Harris, 1990; Schwinning & Parsons, 1996a, 1996b). This type of interaction is characterised by the occurrence of cycles in which the increased N in the soil as a consequence of greater N2 fixation will favour grass dominance since it benefits more per
unit increase in mineral N than the legume does (Schwinning & Parsons, 1996c; Thornley et al., 1995). The increased grass growth will diminish the pool of this nutrient in the soil, promoting the development of another cycle of legume dominance. This type of interaction allows the coexistence and self-regulation of both species in the community (Schwinning & Parsons, 1996a, 1996b). In this environment of low N fertiliser, the increased yield due to clover inclusion will be not only the consequence of different seasonal growth patterns (Figure 3.9) and better use of the ’light space’, but importantly due to the increased N supply through N2 fixation and the contribution of the clover
itself. The duration of this study limited the ability to detect the development of exploitation interaction; observations during a longer period of time would have been needed to overcome this limitation.
If the grass/clover system is managed under a high N fertiliser application rate, clover plants may substitute part of their N needs previously met from N2 fixation with mineral N uptake from the soil,
which has lower metabolic cost for each unit of N assimilated compared with biologically-fixed N (Ryle et al., 1979). However N fixation will continue when N is freely available in the soil, albeit at a lower rate (Ledgard, Penno, & Sprosen, 1999) and some benefit from the return of N through breakdown of dead clover material could be expected. Gains due to different seasonal growth patterns are expected in this situation as well, although at a reduced scale (Figure 3.9) due to lower clover content in the sward. The interaction between the components in the mixtures in this high N environment moves more towards competition for the ‘same light space’, a process that is mediated by the fact that both species have different types of leaves (plagiophile leaves for the grass and planophile leaves for the clover).
Cultivar effects
DM yield differed among perennial ryegrass cultivars in spring and autumn in both years, winter 2013 and in the total annual 2013 – 14. Prospect AR37, Bealey NEA2 and One50 AR37 were generally the highest yielding cultivars while Kamo AR37, Abermagic AR1 and Commando AR37 were generally among the lowest yielding cultivars from autumn 2013 onwards (Table 3.4). Differences in the structural characteristics of the swards associated with the different cultivars, such as leaf size, tiller density, as well as in the phenotypic plasticity of the different genotypes, may result in different herbage accumulation rates (Bahmani, 1999; Lee et al., 2012; Lemaire & Chapman, 1996; Sartie, Matthew, Easton, & Faville, 2011; van Loo et al., 1992).
In the experiment, tiller density was the only grass sward characteristic measured that could be used to explain differences in yield among cultivars. However, despite the key role of tillering in the productivity of pastures, the size-density compensation response of grass to environmental and management factors (Matthew, Assuero, Black, & Hamilton, 2000; Yoda, 1963), limits the utility of the tiller density alone as an indicator of productivity. Regression analysis of the relationships between tiller density and autumn yield revealed significant negative associations in the High N – clover treatment in autumn 2013 (P <0.001), autumn 2014 (P = 0.047) and in the Low N + clover treatment in autumn 2014 (P = 0.002). However, these relationships accounted for a low proportion of the variation in the yield (R2 between 0.10 and 0.27). Therefore, the effect of tiller density on DM
yield has to be considered in conjunction with tiller size. For the same eight perennial ryegrass cultivars used in this experiment, lamina width, length and area, pseudo-stem length and diameter, tiller shape index, leaf : non leaf ratio, and tiller dry weight and density were assessed by Griffiths, Matthew, Lee, and Chapman (2016) when grown in monoculture. Significant cultivar differences were observed for all traits, with the exception of the pseudo-stem length. Principal component analysis in their study revealed that tiller morphology and DM yield were independent. Griffiths et al. (2016) also observed a lower slope in the relationship between logarithmic tiller dry weight and tiller density (- 1.0) compared to the theoretical (- 1.5). They concluded that the constant yield
compensatory relationship observed could be the consequence of breeding and selection programmes.
Therefore, in this experiment, those cultivars with greater yields may have combined in a more effective way a collection of attributes that promoted DM accumulation under the management and environmental conditions of the experiment, and this condition should have held under both N treatments and in monocultures or mixtures with white clover.
Phenotypic contrasts were included in this study with the purpose of creating different environments for clover growth and to identify if grass phenotype characteristics, more than cultivar
characteristics, could be linked to herbage yield and to interactions with white clover. Thus, cultivars were selected to provide contrasts for two traits that may influence competition between grass and clover: morphology (dense versus open) and heading date (mid-season versus late-season) (Frame & Boyd, 1986a; M. A. Sanderson & Elwinger, 1999). The morphological contrast did not work as
expected (see Results 3.4.2 and section 3.6 Limitations of this study) and lacks the internal