In Chapter 4, by the end of the seedling phase, the FD10 genotype had a 20% higher shoot and 16% greater root biomass than the other two genotypes. These results highlighted a priority of this genotype to vigorous shoot production immediately after emergence. It remained to be seen if this yield advantage was maintained during future regrowth cycles of the established crops. Therefore, Chapter 5 investigated the yield and quality responses of crops of different FD ratings when they were grown in changing environmental conditions under the different defoliation managements.
In Chapter 5, the use of the three DF regimes was effective in creating lucerne crops of different yield potential among the FD ratings. The greatest total shoot DM yield was from DF84 crops followed by DF42 crops and the lowest was from DF28 crops (Table 5.1). Specifically, the crops defoliated at 28 day intervals over the growing season had a 44-49% lower yield than DF42 and DF84 crops. The reduction in shoot DM production of DF28 crops was mainly because shorter regrowth cycles, they were unable to grow at high linear rates for long periods, particularly during favourable spring-summer conditions when shoot growth rates were highest (Figure 5.3). The seasonal change in shoot growth rate appeared related to mean air temperature which was high during spring-summer and low during autumn-winter (Section 3.2.2). Generally, the rates of growth increased with increasing temperature but potentially this was also modified by photoperiod (Pp). For example, when crops were growing into an increasing Pp, growth rate was consistent at around 8.8 and 7.5 kg DM/ha/°Cd for DF84 and
Pp, growth rate decreased at a rate of 1.2 (DF84) and 0.88 kg DM/ha/°Cd (DF42) for each hour decrease (Figure 5.4 a, b). However, crops defoliated at 28 day intervals did not show a Pp response, and were always below 3.5 kg DM/ha/0Cd regardless of the direction of photoperiod change (Figure 5.4 c). The physiological mechanisms responsible for these changes will be discussed in detail in Sections 8.3. With regards to FD ratings, the FD10 genotype showed the most change in yield production over time and due to the effect of defoliation frequency (Figure 5.1). FD10 produced the highest shoot yield after sowing (Figure 4.2), but the yield avantage was less consistent over the 24 month period of regrowth (Table 5.1). Higher shoot DM production for this genotype also occurred in the autumn-winter period but this advantage contributed only 8% of the total annual shoot DM (Figure 5.2). The greater autumn yield from FD10 came from faster shoot growth rates than the FD2 and FD5 genotypes from March to the middle of July (Figure 5.3). During this time, temperature and radiation levels are low (Section 3.2.2). This indicates that different genotypes had different strategies in response to the available environment resources. Despite the yield advantage and growth rate advantage to FD10 in autumn this did not translate to a higher annual yield because it had lower growth in the main spring production period, particularly under a DF28 regime (Figures 5.1 c and 5.3 c).
The yield results alone suggest the longest DF regime should be used to maximise yield. However, from an animal nutrition perspective not all forage is of equal value. Therefore, a comparison of nutritional characteristics was also undertaken to quantify the total CP and ME readily available for animal consumption. This involved separation of palatable and unpalatable herbage fractions. This showed the FD10 genotype produced higher total CP in autumn but it was lower than FD2 and FD5 genotypes during spring-summer (Figure 5.10). Therefore accumulated CP over the regrowth periods gave a lower total CP for the FD10 genotype (Table 5.2). The total ME showed a similar pattern as CP (Figure 5.11). There was no interaction between FD and DF for total CP and ME (Table 5.2 and 5.3). In this study, crops defoliated at 42 and 84 day intervals gave a higher total CP and ME than a 28 day regrowth crop. The change in total CP and ME was explained by an allometric relationship as DM increased, CP and ME increased in a similar pattern for all treatments throughout growing seasons (Figures 5.12 and 5.13 a - d). Leaf fraction have higher quality than the soft and hard stem fractions (Figures 5.12 and 5.13), because stem fractions (soft and hard stems) contain mainly structural components (Gastal and Lemaire, 2002) which have a low N and ME contents than leaf (Figures 5.14 and 5.15). The change in CP among shoot fractions was explained by a decrease of LSR with increasing DM (Figure 5.7). These results indicate that the relationship between yield and quality of lucerne was independent of
There was no evidence that different FD ratings required different defoliation management. This is important on farm, because a similar grazing management could be used for all genotypes. This thesis also demonstrated the spring-summer growth resulted in greater shoot yield and quality than the autumn-winter growth. Therefore, from a yield and quality perspective, the optimum grazing management should prioritize spring-summer to maximise productivity and let crops recharge below- ground in autumn, particularly for the FD10 genotype. This is consistent with previous recommendations based on “Kaituna” lucerne (Moot et al., 2003). Finally, with regards to FD ratings, the winter-active genotype provided more herbage in the colder months but was less consistent, particularly with the increased defoliation frequency. In lucerne, winter survival and tolerance to defoliation frequency are predominant requirements for genotypes to be successful on farms in temperate climates. In this study, the FD10 genotype showed the most vulnerability to frequent cutting and therefore was less persistent than the other genotypes, particularly compared with the FD2 genotype. The following section discusses the physiological mechanisms that explain the agronomic performance among genotypes and between defoliation treatments.