The amount plant science has contributed to improvements in concentrates has been underappreciated and undervalued in literature, however, the role these crops have on livestock production has been reviewed previously Erb et al. Forage crops can be grown in mixed species cultivation to provide nutritional and environmental benefits. By offering livestock mixed grazing pastures or blending feeds, nutritional quality can be enhanced. For example, alfalfa is the highest-yielding perennial forage legume and produces more protein per unit area than other forage legumes and so can be grown alone or in combination with a range of different grass species.
The nutritional status of a forage crop depends upon the concentration and ratios of carbohydrates, proteins, and lipids. Such calculations are becoming increasingly prevalent when growers are deciding which crop to grow based and particularly dependent on if the animal is non-ruminant or ruminant.
The primary types of carbohydrate are the insoluble structural saccharides cellulose and hemicellulose, or the storage forms such as starch and water-soluble polymers e. These are degraded into simple sugars through cleavage of glycosidic bonds, either by the animal itself non-ruminant and ruminants or via microbial digestion and subsequent animal absorption ruminants only. Different ratios of carbohydrates within the forage crop will have altered downstream digestibility for the animal, especially if the cell-wall structure constrains digestion by the microbial population or limits plant cell wall penetration Weimer, Although lignin, a polyphenolic compound within forage, is not a carbohydrate, it has a dramatic impact on the digestibility of cellulose hemicellulose; lignin binds with structural carbohydrates and cell wall proteins and reduces nutrient availability.
For forages increased lignin concentration in the growing crop will increase the percentage of indigestible DM. Of the major forage crops grown globally grasses, particularly Lolium perenne , have high digestibility due to high soluble sugar content alongside low lignin content Ruckle et al.
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Animal digestion of simple carbohydrates produces monosaccharides which can be readily metabolized. In ruminants, only microbial digestion of structural carbohydrates produces simple sugars which are subsequently metabolized to pyruvate. Pyruvate is absorbed by the animal and is metabolized further into volatile fatty acids VFAs which are a major energy source, Bergman, Ruminants absorb VFAs in their rumen, and the rate of this is dependent on the concentration of individual VFAs, rumen pH and the absorptive area in the ruminal lining.
Nitrogen N availability to animals is predominantly from forage proteins and are estimated using crude total protein Kjeldahl measurements. This is especially true when comparing content in grasses with herbaceous legumes, with red clover Trifolium pratense , white clover Trifolium repens and lucerne Medicago sativa grown widely due to their high protein value Ruckle et al. Again, lignin will severely affect the digestibility of protein. Some micronutrients like proanthocyanidins or condensed tannins also change the digestibility of protein, but they inhibit protein degradation through binding.
Frontiers | Improving the Yield and Nutritional Quality of Forage Crops | Plant Science
This can be advantageous as rapid protein degradation is causative of bloat, however, too high a tannin content will mean protein passing through the digestive track is unabsorbed and therefore a loss in nutrition value Lees, ; Piluzza et al. This means there is a balance between reduced bloat and animal productivity Mueller-Harvey, Other N-containing compounds can be found in forage such as nucleic acids, nitrate and ammonia Wallace et al.
Lipids in forage crops are mostly found as polyunsaturated fatty acids PUFAs in the range of 10 — 30 g kg -1 Hatfield et al. These dietary lipids are important in final animal product quality; forage diets with lower PUFA levels than cereal diets can produce leaner meat Wood et al.
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Moreover, fresh forage has been shown through numerous studies to produce milk with lowered PUFA content and increased trans -fatty acids Elgersma et al. Striking differences in PUFA content can be seen within species through profiling cultivars, and moreover the harvest period and its environment Elgersma et al. For example Lolium perenne , Festuca pratensis meadow fescue , and Festulolium hybrids of the two have been shown to vary not only between species at the beginning of their growth season, but more prominently between individual cutting regimes Dewhurst et al.
Minerals and trace elements from forages are important for maintaining livestock health.
As there is a move toward using fewer antibiotics in animal production the nutritional balance of feed takes on additional importance. Zinc is particularly important for the immune system and supplements can be added to animal feed, but addition of too much results in wasteful excretion, reviewed in Brugger and Windisch Contrastingly, avoiding accumulation of toxic minerals can also be important for forage crops. Getting the balance right is crucial as low levels of selenium can be beneficial for livestock, but high concentrations are toxic Zhu et al.
Some elements accumulated in plants can make them unpalatable for livestock, but the ability of forage crops to grow fast and quickly recover from cutting makes them ideal crops for phytoremediation [e. Probably the most important trait of any forage crop is rapid biomass production, as crops are either cut or grazed directly, and nutritional quality depends on the rate of biomass production. Intensive production with faster growth often decreases this nutritional, but this depends on the species grown and some cultivars have better recovery from defoliation.
Plant height correlates well with biomass for most crops e. Many plant species can be grown for forage production, but the ability of the shoot meristem to respond with increased growth after cutting is essential. In some forage species, aboveground grazing or cutting has been correlated with increased root exudation Paterson and Sim, This flush of carbon release by roots can stimulate rhizosphere microbes that in turn help to mobilize soil nutrients to sustain aboveground regrowth.
Maintaining an optimal nutrient and water supply is very important for forage biomass production. For example, the importance of N supply for re-growth after cutting grass has been demonstrated Dawson et al. Furthermore, the previous N status of alfalfa influences its regrowth ability Meuriot et al. Due to the relatively recent cultivation of forage crops compared to other agricultural plant species, there were few improvements before Recently, agricultural trends and the global economic importance of forages, mean new cultivars have been bred.
These improvements are helped by many closely related wild populations which can be used in development of new lines Boller and Green, The most desirable improvements are increasing dry matter yield DMY , crop durability and resistance to diseases particularly by pathogenic fungus and pests particularly nematodes, digestibility of DM, and nutritional content of this tissue.
Arguably the greatest improvements have been made in breeding of Medicago spp. Similar tropical grass breeding programs include the Brachiaria partnership between the International Centre for Tropical Agriculture based in Colombia C. Breeding programs for forage crops are fraught with difficulties. Individual plants have high genotypic and phenotypic heterogeneity with many species being polyploid, a problem which is exacerbated by in-breeding across many grasses, and few agronomic traits being linked to distinct genes Poehlman, ; Vogel and Pedersen, Studies have focussed on this problem in specific legumes Jahufer et al.
Regardless of these problems there have been some major developments in breeding lines for forages, especially in Medicago and Lolium. Figure 2 shows a brief historical timeline of Lolium cultivation, and includes the current breeding regimes for grasses; future breeding possibilities are also included and discussed in later sections. One of the most interesting breeding developments is the exploitation of closely related species of Lolium and Festuca Thomas and Humphreys, ; Humphreys et al.
Backcrossing of Festulolium have generated novel hybrids with more stable protein content when compared to parental lines Humphreys et al. Advances in phenotyping are making it easier to include the quantification of characteristics in the field; such as high level imaging of growing crops to accurately determine later traits like biomass Walter et al. Cultivation timeline of Lolium perenne perennial ryegrass , including current typical breeding regime and future possibilities for breeding programs. The historical timeline is described in brown boxes Haber, ; Hirata, , and general trends in green areas, including modern grass breeding regime as used across Europe Bruins, New cultivars are being helped by advances in sequencing methods that can provide more transcriptomic data Barrett et al.
Draft genomes for such crops and cultivars are becoming increasingly common Byrne et al. Such research is providing clues to candidate genes which could be used for nutritional enhancement. Identification of potential candidate genes is usually through quantitative trait loci QTL analysis or marker-assisted selection MAS provided from the above completed genomes. Those identified are studied in relation to biomass and growth traits; in M. In Lolium , transcriptomics showing differentially expressed genes between wild-type and a dwarf mutant enabled identification of three key genes associated with dwarfism Li W.
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Markers are used to infer both phenotypic traits and to track inheritance to aid breeding. For instance chloroplast SSRs have been investigated in Lolium through a similar technique as above Diekmann et al. A thorough re-annotation of the model forage M. Iterative mapping software such as BioMercator Sosnowski et al. Moreover, BioMercator used to decipher flowering time and height in M. Such potential ease for transferring model plant knowledge to forage crop research is further discussed below.
Despite the need to ensure optimal nutritional content especially in the end-product feed, rapid vegetative biomass accumulation is the most desirable trait of a good forage crop, especially those which undergo extensive cutting throughout the growing season. Due to this phenomenon, candidate genes for improving the crops are associated with either photosynthesis or nitrogen use efficiency NUE. More generally for resistance to biotic and abiotic stresses there is also a huge opportunity for improving traits in forage crops using our genetic knowledge from model plants see Figure 3.
Figure 3 summarizes some of the traits that can be considered for all forage crops. Such improvements in traits could be aided by achievements in transformation and genetic marker techniques.
Reproducible and high efficiency transformation has been developed for temperate cultivars of Festuca Wang and Ge, ; Zhang et al. Some examples of gene editing forage crops to confer stress tolerances have been successful in aiding both biomass increases but also nutritional quality. Transformation of M. Importantly these increases were shown both in the laboratory but also in field conditions. This study also identified the increased expression of many interesting genes, including M. Forage NUE is a target for breeding, particularly as protein content of crops is so valuable.
Protein accumulation is linked to N status and when the supply is supra-optimal greater storage occurs. When compared with grain crops that have been bred for high seed starch, forage crops often require N in greater amounts due to their increased growth, storage capacity and higher fiber content Parsons et al. Principally NUE for forage crops can be based on N utilization efficiency NUtE as we are interested in the highest achievable biomass of the shoot which will form the content to be dried for feed production Xu et al.
Not only does this include biomass, but also the relative N levels in this tissue; it is not enough to only have a high yield of biomass in the shoots, it must also yield optimal amounts of N. Moreover, when looking at the effect of fertilizer use we are also interested in how both the biomass and N status change on application and thus also N uptake efficiency NUpE. Forage crops offer challenges for NUE as there is a requirement for optimal yield of shoot biomass with a high N content NUtE while also optimizing N fertilizer acquisition NUpE throughout the growth season.