Genotype by environment interaction effects on yield and persistence

2.4.1. Natural environment

The natural environment encompasses factors beyond management control and includes the climatic and edaphic conditions to which the plant is exposed. The only economically viable method available to address limitations presented by the natural environment is to exploit repeatable genotype by environment interactions either through the selection of cultivars adapted to a particular environment or the

45 Evidence of a genotype by environment interaction on growth can be found in the differences between shoot growth rates of winter-active cultivars compared to winter-dormant cultivars in North American climates (Perry and Larson 1974; Stout and Hall 1989) and in some locations and years in Mediterranean climate regions of Australia (Leach 1970a; Humphries and Hughes 2006), but not in the subtropical regions of Australia (Rogers 1981; Lloyd et al. 1985; Lodge 1985; Lowe et al. 1985; Lodge 1986; Gramshaw et al. 1993). The cool temperate regions of Australia are intermediate climate types, with less extreme winters than those experienced in North America, and cooler summers than those experienced in Mediterranean environments. Shoot growth of lucerne genotypes with differing levels of winter activity remains to be defined for this environment.

An interaction between genotype and photoperiod affects the process of cold acclimation, and hence winter survival in environments that expose the plant to subzero temperatures over winter. This genotype by photoperiod interaction effect on cold acclimation and winter survival has been observed by Hodgson (1964), Bula

et al. (1965), Klebesadel (1971) and Heinrichs (1973) when comparing the performance of freezing tolerant cultivars in subarctic Alaska and Canada to their performance in the continental USA. When reviewing these experiments,

Castonguay et al. (2006) concluded that cultivars to which ssp. sativa was the major contributor of parent material, with minimal contribution from ssp. falcata, were insensitive to the rapid decrease in photoperiod experienced during autumn in the northern latitudes. Consequently they did not initiate the process of cold acclimation as they would typically do when grown in continental USA. The southern dairy regions of Australia are situated at an equivalent latitude to continental USA and have similar changes in photoperiod. Photoperiod changes should be able to be sensed by cultivars comprised of ssp. sativa, with no need to rely on ssp. falcata.

The genotype by temperature interaction is highlighted by the distribution of genotypes across the North American continent. The freezing temperatures

experienced in the northern regions prevent the use of winter-active cultivars (Melton

et al. 1988) as they do not possess the capacity to adapt to freezing winter

46 The range of winter activity levels that are successfully grown increases as winters become milder. In areas with minimal risk of experiencing subzero temperatures during winter, winter-active cultivars are favoured (Melton et al. 1988).

The cool temperate dairy regions of Australia have considerably milder winters compared to areas of equivalent latitudes in the northern hemisphere, with daily average minimum temperatures remaining above freezing (Figure 2.1). There should be no limitations to the use of cultivars with any level of winter activity, as there is no need for adaption to freezing conditions. However, anecdotal reports suggests that winter-dormant cultivars are the best adapted genotypes in these regions (Knox et al. 2006). It is possible that winter stress may not be the limiting environmental factor preventing the successful use of winter-active material in cool temperate environments. This has been proposed by Berg et al. (2007), who identified that, in an environment that exposed plants to freezing winters, the majority of plant death occurs over the summer period. Clearly there is a need to scientifically evaluate the yield and persistence of a range of winter-activity

genotypes in the cool temperate regions of Australia to determine their suitability for use in these environments.

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Figure 2.1. Average monthly maximum (solid lines) and minimum temperatures (broken lines) and total monthly rainfall (bars) for four locations in the temperate dairy regions of Australia (Bushy Park, Tasmania; Elliott, Tasmania; Ellinbank, Victoria; Mt Gambier, South Australia). Data sourced from the Australian Bureau of Meteorology (2009).

2.4.2. Management environment

Management encompasses factors that can be controlled or influenced (e.g.

defoliation, irrigation and soil fertility). Cultivar recommendations from subtropical regions of Australia suggest that winter-dormant cultivars are best adapted to

extensive pastoral systems with low levels of management inputs, while winter- active cultivars are suitable for intensive irrigated hay production and grazing

enterprises (Lloyd et al. 2002). This recommendation highlights that, when selecting

Bushy Park - TAS

Temp er atu re ( oC) 0 5 10 15 20 25 30 Elliott - TAS Ra infa ll (mm) 0 20 40 60 80 100 120 140 160 180 Ellinbank - VIC

Jul _{Aug Sep Oct Nov Dec Jan Feb Mar Apr} May Jun Temp er atu re ( oC) 0 5 10 15 20 25 30 Mt Gambier - SA

Jul _{Aug Sep Oct Nov Dec Jan Feb Mar Apr} May Jun Ra infa ll (mm) 0 20 40 60 80 100 120 140 160 180

48 a lucerne cultivar, consideration of genotype by management interactions is just as important as consideration of the genotype by natural environment interactions.

2.4.2.1. Defoliation

Defoliation management and genotype interact to affect yield and persistence of lucerne. There is a difference in the performance of lucerne genotypes under

continuous grazing compared to mechanical harvesting or rotational grazing (Kaehne

et al. 1993; Bouton and Gates 2003). Under optimal conditions, there is little genotype by defoliation interaction on the performance of genotypes when

rotationally grazed compared to mechanically harvested (Lodge 1985; Bouton and Gates 2003). This suggests that genotypes differ in their ability to tolerate stress specific to continuous grazing. Stresses specific to continuous grazing include continual trampling, tugging, waste excretion and repeated defoliation of regrowing shoots (Smith et al. 1989). A consistent finding when genotype performance is evaluated under continuous grazing is that winter-dormant genotypes are better adapted to continuous grazing than winter-active genotypes (Smith et al. 1989; Brummer and Bouton 1991; Kaehne et al. 1993; Smith and Bouton 1993; Bouton and Gates 2003; Humphries et al. 2006b). This adaptation is attributed to the broader and deeper set crowns of winter-dormant genotypes compared to winter- active genotypes (Brummer and Bouton 1991; Humphries et al. 2006b). Dairy farms in the temperate regions of Australia employ rotational grazing as a matter of course. As such, the full range of winter activity genotypes should be able to be grown successfully, as no genotype by defoliation method interaction is observed when comparing mechanically harvested and rotationally grazed crops (Lodge 1985; Bouton and Gates 2003). The genotype by defoliation method interaction would need to be considered if the intention was to continuously graze the crop, as winter- dormant genotypes are best suited to this purpose.

The faster regrowth rates of winter-active cultivars compared to winter- dormant cultivars (Leach 1970a; Perry and Larson 1974; Stout and Hall 1989;

49 initiate reproductive growth (Major et al. 1991) and cold acclimation (Bula et al.

1965; Shih et al. 1967; Paquin and Pelletier 1980; Hendrickson et al. 2008), would suggest that the timing of defoliations need to be adjusted with respect to the genotype‟s level of winter activity. However, Gramshaw et al. (1993) showed that the defoliation interval that maximised yield was the same for a range of genotypes, and that defoliations timed to coincide with crown bud elongation were appropriate for a broad range of winter activity ratings under a range of environmental conditions. Timing defoliations to coincide with crown bud elongation ensures that crown buds are readily available for the formation of new shoots and that taproot reserve levels are adequate to support regrowth (Lowe et al. 2002).

Late autumn and winter management is critical for maintaining productive lucerne crops in a wide range of environments (Tomes et al. 1972). Defoliation during cold acclimation interrupts the acclimation process, alters the composition of taproot reserve pools and increases the risk of injury when exposed to freezing temperatures (Belanger et al. 1992; Haagenson et al. 2003a). However, autumn defoliation does not have the same level of impact on freezing injury as does the level of winter activity (Belanger et al. 1992; Haagenson et al. 2003a). When evaluating the impact of autumn defoliation on the abundance of gene transcripts associated with cold acclimation, Haagenson et al. (2003a) found that defoliation had little impact on their expression, or in the case of a winter-active cultivar, the

abundance of transcripts actually increased with defoliation. These counter intuitive results led these authors to question the cause-effect relationship between the

abundance of transcripts for some cold acclimation genes and freezing tolerance (Belanger et al. 1992; Haagenson et al. 2003a). In climates that do not experience freezing winters, late autumn and winter defoliation may be beneficial to lucerne production and persistence, by reducing pest and disease pressure on the plants in this period (Pelton et al. 1988; Wynn-Williams et al. 1991; Bell and Guerrero 1997; Moot et al. 2003; Chocarro et al. 2005). These evaluations have been undertaken across a range of climate types from cool temperate New Zealand to semi-arid California, with genotypes that ranged in winter activity classifications of semi winter-dormant to highly winter-active. This consistency in results suggests that in

50 areas not exposed to freezing winter temperatures, all genotypes can be grazed over winter and that this practice improves lucerne production. In environments where lucerne becomes dormant, winter defoliations should be timed to occur when plant growth is slowest (Collins and Taylor 1980; Douglas 1986). This reduces shoot growth during the remainder of the winter period and ensures that adequate taproot reserves are available to support rapid growth when temperatures increase in spring (Collins and Taylor 1980; Douglas 1986).

2.4.2.2. Irrigation and water deficits

Evaluations of the effects of genotype by water deficit on lucerne performance have either identified no interaction effects on yield (Carter et al. 1982; Hattendorf et al.

1990; Grimes et al. 1992), or, when an interaction existed, it was not repeatable between years and locations (Oloff and Hanson 2008). This is counter intuitive as winter-dormant cultivars are capable of maintaining a greater shoot water potential under drought conditions (Carter et al. 1982; Grimes et al. 1992) and have a greater degree of root branching (Bennett and Doss 1960; McIntosh and Miller 1981), which would suggest winter-dormant genotypes would perform better than winter-active genotypes under conditions of limited water. Evaluations of genotype by irrigation effects on yield have been undertaken under glasshouse conditions (Carter et al.

1982), or in North American environments with exposure to freezing winters (Retta and Hanks 1980; Hattendorf et al. 1990), or in semi-arid climates (Undersander 1987; Grimes et al. 1992). No evaluation has been made in milder cool temperate

environments, which may allow potential differences between genotypes to be more readily and consistently expressed.

Direct evidence of a genotype by irrigation interaction effect on yield components has been identified in glasshouse experiments by Perry and Larson (1974). While winter-active genotypes had the greatest number of shoots per plant when plants were fully watered, when plants were exposed to a water deficit, winter- dormant cultivars maintained a greater number of shoots per plant. When growing under a water deficit, winter-dormant genotypes also produce a greater number of

51 leaves per shoot compared to winter-active genotypes (Perry and Larson 1974). These responses suggest that genotypes with contrasting levels of winter dormancy may utilise different strategies to adapt to water deficits. Identifying how genotypes differ in adaption to water limited conditions will be important in the development of genotype specific best management practices.

While the influence of genotype on taproot carbohydrate and N reserve pools has been documented (Avice et al. 1997b; Kalengamaliro et al. 1997; Cunningham and Volenec 1998; Boschma and Williams 2008) and considerable effects of

irrigation/water deficit on these reserve pools observed (Cohen et al. 1972; Justes et al. 2002; Erice et al. 2007), no evaluation of the effect of irrigation on taproot reserve pools in genotypes of contrasting levels of winter activity has been made. Evidence of possible genotype by water deficit interaction effects on taproot reserves is provided by the contradicting results of Justes et al. (2002) and Erice et al. (2007), where these authors observed contrasting effects of water deficit on VSP abundance. As taproot reserves are important in stress tolerance and support plant recovery after the removal of stress, the extent of genotype influence of carbohydrate and N

reserves during water deficit will need to be considered when developing best management practices for managing lucerne through periods of low water availability.

Irrigation of lucerne in summer and autumn reduces the freezing tolerance attained in winter (Heinrichs 1973). This is attributed to the extra growth achieved during the irrigation period reducing taproot reserve pools to levels that are not adequate to support the process of cold acclimation (Heinrichs 1973). While the studies undertaken by Heinrichs (1973) on cold acclimation and winter survival evaluated a diverse range of winter activity genotypes under irrigated and dryland conditions, experimental design limitations prevented the comparisons of winter survival amongst diverse genotypes. The possible genotype by irrigation interaction on taproot reserve pools may also affect the process of cold acclimation and

52 2.4.2.3. Soil fertility

Adequate levels of mineral nutrition are critical for reserve storage and remobilisation (Collins and Duke 1981; Li et al. 1998; Berg et al. 2009) and genotypes differ in their seasonal partitioning of carbohydrate and N into storage (Avice et al. 1997b; Kalengamaliro et al. 1997; Cunningham and Volenec 1998; Boschma and Williams 2008). However, there appears to be no consistent genotype by soil fertility interaction effect on production, persistence and physiological responses to environmental stimuli (Tindall and Hurst 1988; Gossen et al. 1994; Lloveras et al. 2001). All genotypes respond positively to increasing soil mineral concentrations into the ranges considered adequate to support high levels of production, with improvements in production, persistence and nutritive value

(Tindall and Hurst 1988; Gossen et al. 1994; Lloveras et al. 2001; Turan et al. 2008; Lissbrant et al. 2009).

To ensure maximum plant production with minimal fertiliser wastage, best management practice is to use soil testing and analysis of results to guide decisions about fertiliser applications. The depth of soil testing should reflect the depth of soil in which the majority of the fine root mass is present (i.e. 7.5 to 10cm for temperate pastures and 10 to 30cm for various field crops; Brown 1993). Genotype differences in root system anatomy (Bennett and Doss 1960; Carter et al. 1982; Salter et al. 1984) mean that soil testing for the winter-active genotypes may need to be to a deeper depth than for winter-dormant genotypes. However, as the majority of fine roots for all genotypes are present in the upper 15 cm of the soil profile (Bennett and Doss 1960), soil testing to this depth would ensure that fertiliser decisions are made using information reflecting nutrient availability to the plant. Recent investigations have highlighted possible nutrient interactions between magnesium and potassium and phosphorus and potassium. This demonstrates the importance of considering the overall nutrient status of the soil when making fertiliser decisions (Lissbrant et al.

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2.5. Identifying and managing lucerne genotypes for the dairy industry in the