3.1. Effectiveness of Phenotypic Selection from Spaced Plant Nursery
The observed seasonal changes of herbage yield and quality are consistent with previous reports [19
]. The herbage yield of hybrid progeny in a sward setting was not significantly associated with parental mean herbage yield in a spaced plant trial. Current practices of phenotypic recurrent selection also involve selection from a spaced plant nursery, followed by testing progenies in sward conditions [20
]. This raises the issue of whether parental genotypic performances in a spaced plant trial may be trusted to be correlated with the performance of the progeny in a sward setting. The results of the present study suggest that selection of high-yielding individual Italian ryegrass genotypes from a spaced plant nursery cannot guarantee high-yielding progeny populations in a competitive sward condition. Spaced plant evaluation was also found to be unpredictive of sward yield in tall fescue [21
]. Nonetheless, the observed correlation of progeny yield with mean parental plant height indicates that indirect selection for yield may be achievable by direct selection for plant height, which displays a higher heritability value in Italian ryegrass. This correlation effect may be more relevant for Italian ryegrass than perennial ryegrass, as the former typically shows a more erect growth habit in which the vertical length of tillers will more accurately reflect total biomass. However, plant height was also found to be negatively correlated with persistence, such that any increases in yield would probably be achievable in the first growing season. Selection heavily and solely dependent on plant height would consequently not be ideal in the long term. Genotypic recurrent selection using full-sib or half-sib families [22
] coupled with genomic selection [23
] may hence be a necessary part of the breeding system if forage yield is the target.
The observed significant correlation between parental mean WSC content in a spaced plant trial and the WSC content of progeny under sward condition indicated that direct selection based on the former would be effective. The significant regression coefficient may arise because WSC content has never been the target of selection in the development histories of the particular varieties used in the present study. In such cases, there would be presumably a higher degree of genetic variability for the unselected character than for those traits (such as herbage yield) that have been subject to strong directional selection. This would ensure a broad range of WSC content in the regression analysis. In contrast, if the genetic variability was small and environmental effects were prominent, the regression analysis would probably not obtain significant results. Another possible explanation is that WSC content may be less sensitive to the inhibitory effects of plant neighbour interactions than a morphophysiological trait such as yield.
Based on these observations, phenotypic selection from spaced plant trials would be more effective for breeding of quality parameters than for yield. Since the quality parameters were affected by plant maturity, it would be important to measure at comparable stages of vegetative growth. In perennial grasses, selection is typically conducted on a single-plant basis and usually displays a realised heritability for digestibility of about 0.2 to 0.3 [24
], in agreement with the results of the present study. Of the quality parameters, CP displayed lower heritability than for WSC content and NDF, consistent with reported sensitivity to environmental factors such as fertiliser applications [25
3.2. Heterosis and Inbreeding Depression
In the present study, F1
families were generated from pair crosses between two unique genotypes. As a consequence, mean values for a parental population in a field setting could not be obtained, except by vegetative propagation to produce multiple ramets from the selected genotype, which is logistically unfeasible, so the performance of F1
families cannot be compared to those of parents. However, comparisons can be made between F1
progeny sets derived from a particular pair-wise genotypic combination in order to deduce the presence of heterosis in the F1
generation and erosion by inbreeding depression in subsequent generations. High levels of inbreeding depression in parental lines has been shown to be associated with heterosis in hybrids of other plant species [26
]. The magnitude of the observed inbreeding depression in the present study was on average 22.1% for total fresh herbage yield, with a range from negative values to 56.1% for different family-harvest combinations. In other studies of ryegrass species, yield in Italian ryegrass declined by 6% from Syn1 to Syn2, and remained constant from Syn2 and Syn3 [27
], while that of perennial ryegrass declined by 11% from Syn1 to Syn3 of five-parent synthetic [28
]. Dry matter yield in tall fescue declined by 6% from the Syn1 to Syn2 generations of four-parent synthetic [29
]. In smooth bromegrass (Bromus inermis
L.), the average level of heterosis for herbage yield in diallele crosses between seven parents was 14% with a range of −4 to 39%, and the level of inbreeding depression based comparison of selfed progenies with parents was 18%–33% [30
]. The difference between species may be attributable to the number of genotypes used for crossing and different mechanisms of genetic control.
Of the 10 different families obtained in the present study, 8 were derived from pair-crosses of genotypes from different cultivars. If they were assumed to be unrelated, the inbreeding coefficient would be 0 and 0.25 for F1
progeny sets, respectively. For the other two families (1857 and 1826), the two parental genotypes were obtained from the same cultivar (“Tabu” for 1857 and “Warrior” for 1826), and the two parents within each family, due to an origin from a single polycross, must by definition be to some extent related. “Tabu” was bred from a mass selection made from an 18-month old field trial of variety “Flanker” [31
], while “Warrior” was bred from breeding pools obtained from various sources [32
]. It is therefore likely that the two parental genotypes of family 1857 were more closely related than those of family 1826; consequently, the increase of inbreeding coefficient from the F1
generation for these two families will be less than 0.25, especially for 1857. A lesser degree of inbreeding depression would hence be expected in these two families. Results were consistent with this expectation, as the decline in total herbage yield for 1857 was 13.5%, the least among the 10 families. On average, heterosis increases as the genetic disparity of the parental stocks increases and interspecific crosses show greater heterosis than intraspecific crosses [33
]. However, the extent of correlations between molecular marker-based genetic distance and heterosis are not conclusive. There are several prerequisites for positive correlations, such as high trait heritability and close marker linkage [34
]. It is not a surprise that the present study revealed no correlation between yield in the F1
generation and genetic distance between the parents, since yield showed low heritability and random low-density distribution of markers used for diversity analysis. Cultivars of the interspecific hybrid between perennial and Italian ryegrass species (L. x boucheanum
), which combine advantages from each species, are available in the commercial marketplace. However, they are not F1
hybrids, and any heterosis in the initial crosses would have been eroded through seed multiplication during the variety development process and so would not be expected to exploit the full potential of heterosis.
A 30.5% reduction in survival rate was seen in the F2
B generations as compared to F1
generations across the various families, consistent with heterotic effects. Survival rate is a measure of persistence in forage grasses. It may be regarded as a fitness-related trait, which generally exhibit lower heritability values than morphological traits [35
]. Nonetheless, heterosis for fitness-related traits is commonly observed [27
]. Hybrid vigour for persistency in Italian ryegrass would be beneficial to farmers to allow the growth of more productive pastures for longer periods, with reducing re-sowing and establishment costs. Although inbreeding depression was observed for yield and persistence, no such effects were seen for quality parameters. Similarly, there is little evidence of heterosis for WSC content in perennial ryegrass [36
] or for total soluble solid and crude protein content in sorghum [26
]. Population hybrids (generated through controlled pair-crossing between varieties) of orchardgrass (Dactylis glomerata
L.) exhibited higher mean phenotypic values than source cultivars for some trait-harvest combinations (such as increased CP, WSC, and decreased NDF); however, the magnitudes of difference were very small [37
]. Based on these results, the feasibility of exploiting hybrids for improvement of quality traits in ryegrasses appears limited.
To date, very little information has been generated on the extent and prevalence of heterosis in Italian ryegrass. As the genetic base of Italian ryegrass is known to be narrow, it may be assumed that lower levels of differentiation between different germplasm pools are present, compared with the closely related species, perennial ryegrass. If so, a lesser extent of heterosis might be anticipated. However, the results of the present study suggest that the magnitude of heterosis in Italian ryegrass, for comparable traits, is no less than for perennial ryegrass. The significant heterosis observed in herbage yield and survival rate provided foundation data for the implementation of the F1 hybrid breeding strategy in Italian ryegrass. However, controlled pair-crossing between specific genotypes is not a feasible approach for commercial seed production, especially as any advance for seed multiplication by reproduction will cause inbreeding depression. As a consequence, the development and maintenance of divergent and compatible parental population pools will be critical for F1 hybrid breeding. Nonetheless, if an average of 20% yield advantages from F1 hybrids could be achieved in commercial production, the gain would be enormous compared with the less than 1% of annual genetic gain in yield that has been achieved via conventional breeding programs.