The seemingly conflicting reports may be due to environmental and agronomic specificity. Benefits from cover crops can be maximized with the right seed selection and agronomic management that suits the target of producers. Though moisture deficit is the main limiting factor in the Southern Great Plains, the use of cover crops can still be economically justified with proper management and termination timing. Cover crops can also be grazed before the next commercial crops are planted for direct economic return.
Rye has been popular among cover crop growers because of its weed-smothering capacity and high carbon residue CTIC, In this regard, triticale, as a rye progeny, can be a viable alternative as a cover crop. Triticale is among the best overwintering species that help reduce soil erosion and capture residual nitrogen, which in turn increases annual forage yield and quality Ketterings et al. A separate crop ideotype that suits the edaphic and atmospheric scenarios of target environments will help optimize gains from cover crops. High allelopathy is one of the potential traits to look for in triticale and rye breeding for better weed control Cheng and Cheng, To date, none of the triticale varieties used as cover crops was purposely selected for cover crop use.
Vigorous and high-yielding triticale varieties that are required for high winter forage may not be good options as cover crops in the Southern Great Plains, as they may cause water and nutrient depletion for the next crop. Therefore, it will be reasonable to have selection traits tailored for cover crop purpose only. Triticale is a self-pollinating species with a low degree of out-crossing; as a result, it is amenable for pure line selection.
Pure line selection involves the hybridization of two or more parents and segregation of lines until they attain homozygosity before any selection is practiced except for some qualitative traits that are easy to score Lelley, ; Randhawa et al. However, in the case of triticale, selection in segregating lines needs to be delayed until the wheat—rye genome composition attains stability Lelley, Though the first wheat—rye hybrid was developed about years ago Stace, , formal triticale breeding and selection was started in the mids Larter et al.
The main challenges at the start of these breeding programs were excessive plant height and lodging, flower sterility, delayed maturity, and shriveled grains Mergoum et al. Through coordinated breeding efforts, improved varieties better adapted to various environments have been released since then Supplementary Table 1.
The accidental identification of a triticale naturally out-crossed with a semi-dwarf bread wheat resulted in the first major breakthrough in triticale breeding Hede, Grain yield was improved from 2. Triticale breeding was focused mainly on improving grain yield for human consumption Randhawa et al.
However, the initial grain cultivars were spring-type, which contributed to their inconsistent performance and lower popularity in the market Randhawa et al. The competitive market prices of other crops that were covered under crop insurance were also one of the reasons why triticale was not as popular among growers as first expected Blount et al. Triticale is gaining popularity as a winter forage crop. Research to develop high-yielding and winter-hardy triticale cultivars is a priority at the Noble Research Institute to fill the forage gap during winter-fall seasons Newell and Butler, Though forage and grain yield are the main agronomic traits of selection in triticale breeding, other component traits also need to be improved to achieve market adoption.
Lodging, pre-harvest sprouting, disease susceptibility and inferior grain end-use quality are still unsolved challenges in triticale breeding Sodkiewicz, ; Tyrka and Chelkowski, ; Randhawa et al. Contrary to obvious expectations, triticale is not as winter-hardy as rye because of genetic inhibition of the rye freezing-tolerance gene by the wheat genome Blum, Therefore, breeding for winter hardiness needs to incorporate more freezing-tolerance genes from wheat until the genetic barrier inhibiting rye genes is resolved through research.
Improving the end-use quality of triticale will mark the beginning of a new era in the evolution of the crop. Future breeding research for forage and cover crop use needs to focus on generating genotypes that are well-adapted to cold winters and have high biomass yield between successive cuttings.
CSIRO PUBLISHING | Crop and Pasture Science
Improving seedling early vigor will enable faster crop establishment and early ground cover, enhancing soil health and nutrient use efficiency Salmon et al. Current triticale breeding is mainly dependent on developing inbred cultivars. Hybrid breeding can also be easily applied in triticale, as inbred parent development and seed multiplication mechanisms are easier than with other small grains. Triticale is a self-pollinating species, enabling easy inbred parent development with low inbreeding depression. As a result, there has been a growing interest among breeders in developing hybrid triticale cultivars Barker and Varughese, ; Warzecha et al.
Previous studies reported vigorous vegetative growth and high grain yield in triticale hybrids because of non-additive gene actions Barker and Varughese, ; Oettler et al. Average mid-parent heterosis values ranging from 8.
Rye contributes high levels of non-additive genetic variability to the triticale genome Oettler et al. Developing viable progeny from a rye—wheat cross, with rye as a female parent, may also open a new source of genetic variability for stress tolerance in triticale. So far, rye as a female parent has not produced fertile progeny Furman et al. Grain and biomass yields are controlled by high levels of dominant gene actions specific combining ability , while additive gene effects general combining ability control other yield component traits Oettler et al.
Therefore, with careful selection of parental lines that have high general and specific combining abilities, hybrid breeding provides a viable option to exploit heterosis for forage and grain yield. Molecular markers play a major role in the genetic improvement of crop plants by facilitating the identification and tagging of important genes for potential transfer or cloning Semagn et al.
Marker-assisted selection MAS is one of the oldest applications of marker technologies in plant breeding. Preferred markers in MAS are those that are abundant in the genome, polymorphic even within closely related individuals, reproducible and amenable for automation Semagn et al.
Functional markers markers in the gene itself are the best types of markers in MAS, as they are completely linked to the quantitative trait loci QTL or gene zero chance of crossover Varshney et al.
However, these kinds of markers are not as abundant as other marker types, which are mostly from the non-coding regions of the genome. Genotyping by sequencing GBS is becoming the standard genotyping technology in terms of its throughput and ease of developing simple nucleotide polymorphism SNP markers Poland et al. Triticale genomics can benefit from marker developments and genomics tools in both wheat and rye, as large proportions of the two parental genomes are conserved in triticale Ma et al.
However, triticale is not a mere mix of the rye and wheat genomes. As a result, wheat and rye markers may not be fully informative in triticale. Unlike wheat and rye, only a limited number of markers have been directly developed from triticale itself Kuleung et al. Most of the markers used in triticale were developed from either wheat or rye.
However, transferability of the limited number of markers tried so far was low.
According to Kuleung et al. Transferability of markers seemed also to depend on the type of marker technology used. Badea et al. Returns from prior investments in wheat and rye genome sequencing can be exploited through comparative genomics and mapping of triticale with wheat and rye, thereby equipping the genomic toolbox of triticale. Comparative mapping of barley with wheat Close et al.
A fairly dense map one unique locus every 4 cM was reported by Tyrka et al. A consensus map consisting of 2, DArT markers spanning a distance of 2, However, this consensus map did not have uniform marker distribution among the three genomes either.
Marker saturation in triticale depends on the contrast between parental lines and diversity in the mapping population Tyrka et al. Most triticale genetic maps were based on DH populations, which reduced the genetic variability that could have been created through meiotic crossing-over in subsequent generations.
For successful application of markers in plant breeding, tight marker-trait linkage or association is essential. Having informative markers and dense genetic maps alone does not guarantee successful QTL mapping. The appropriate number of mapping individuals and the nature of mapping population structured or unstructured are also important factors to consider for accurate detection of QTLs. The optimum mapping population size is dependent on the nature of segregation of markers and traits in the mapping population Bogdan and Doerge, ; Li et al. Several previous studies in triticale reported identification of QTLs for various traits, such as biomass yield, grain yield, thousand-kernel weight, and plant height Busemeyer et al.
QTLs that were reported using bi-parental populations DH and RIL might not be valid on genetic backgrounds other than the mapping populations themselves because such populations hardly represent the available diversity in the germplasm. Grain and biomass yield, biotic and abiotic stress tolerance, and grain and forage quality traits are controlled by many QTLs. This makes QTL mapping difficult especially with the small number of genotypes. Information generated using bi-parental mapping needs to be validated before any further investment to use it in MAS.
Many major-effect QTLs were identified and validated for root architecture under water stress in wheat Ayalew et al.
Selecting Legumes as Green Manure/Cover Crops
Quantitative trait loci can also be identified and mapped using transcript variations of mapping lines as substitutes for phenotype data to find the marker-transcript association termed expression QTLs eQTLs Schadt et al. This technique could be especially important in triticale to understand the biological and genetic processes happening in the development of the hybrid by revealing the gene regulatory network of the whole genome.
Expression QTLs were reported in several species, including barley, soybean, and rice Chen et al. Bi-parental QTL mapping approach was instrumental in understanding the genetic mechanisms of different traits. However, applications of research outputs QTLs and linked markers in MAS were far less than satisfactory because of several limitations in the approach Heffner et al.
Cover crops in the northern region
Low accuracy in QTL size and location; low representation of genomic allele distribution; and, as a result, the need for validation of markers and QTLs for MAS hampered translation of genetic gains into practical plant breeding Jannink et al. In addition to inflated QTL effects and imprecise genomic locations of conventionally mapped QTLs, results generated from bi-parental mapping populations hardly represent the allelic distribution of a trait in a species Heffner et al.
Markers are in LD when they are consistently co-inherited without being chromosomally linked Slatkin, This approach helps avoid the need to develop structured mapping populations, and it utilizes LD instead of genetic linkage to dissect the genetics of complex traits in breeding populations Jannink et al. As the marker-trait association is rather genome-wide, it is called genome-wide association study. In triticale, though most genotypes originate from a limited number of crosses Ammar et al.
Selecting Legumes as Green Manure/Cover Crops
Significant LD among the three genomes and population structures has been reported in triticale Alheit et al. Growth habits were identified as the main sources of population structure, with the R genome being the less diverse genome compared with the other two A and B genomes Alheit et al.
Apart from the limitations of conventional MAS in practical crop improvement Heffner et al. The success of any breeding program lies in the creation, acquisition, and proper phenotyping of diverse germplasm for target traits and environments.
There is a tremendous possibility of generating genetic variability in triticale through the various parental combinations and the random chromosome reshuffling during meiosis and genomic changes after allopolyploidization. Triticale breeding especially for forage and cover crop use needs to incorporate high grain and biomass yield and grazing tolerance, quick rejuvenation after successive cuttings or grazing, and high disease resistance.
Unlocking the inhibited rye freezing-tolerance gene in triticale will tremendously improve freezing tolerance in triticale, thereby making it more fit for the winter grazing system in many parts of the United States. The selection of genotypes with high early vigor will enable early ground cover and efficient resource utilization, thereby improving both soil health and forage production. Improving nitrogen use efficiency will help fit triticale in rotation cropping after maize, thereby making use of the excess N leftover from the previous crop cycle.
Improving lodging will also enable intensive farm input application with a high rate of return. High-throughput field phenotyping coupled with fast greenhouse or growth chamber screening methods that corroborate field-grown data need to be developed to feed new germplasm into the breeding pipeline to hasten the breeding process. Pure-line selection will remain the mainstay of breeding in triticale because of both the efficiency of pure-line cultivars and the low market interest for hybrid cultivars.
However, with the incorporation of appropriate hybridization techniques for easy seed production, lower production costs and efforts to tackle some of the existing triticale quality problems, hybrid triticale will also have the potential to exploit heterosis. The best chromosomal arrangement or combination in hybrids is yet to be determined for the various target traits and environments. Though there was promise from the advances in molecular genetics, much effort is needed when it comes to triticale improvement.
The mere identification of QTLs will not take us any further unless the identified QTLs are validated and the flanking markers are used in real-life application.