LECTURE NOTES 1
By Dr Deric Charlton
In the late 1950s I was one of those privileged to study pasture agronomy under Professor Martin Jones when he held the Chair of Agricultural Botany at Kings College, the Newcastle arm of Durham University. He inspired me to follow pasture agronomy in my career and I kept the notes I scribbled down in his lectures. Clive Dalton has asked me to transcribe them into a relevant form, so I hope you learn from them as I did.
How plants grow
All our energy needs come from green plants through oxidation of carbohydrates and proteins, with plants taking in nitrates, phosphates and other nutrients, together with more energy from the sun.
Most farmed plants are used by the farm livestock before humans obtain this energy. The animal uses three-quarters of the plant energy in digestion, the remainder being stored as meat. Meat is more suitable for Man as it supplies more energy in a short time. Besides we cannot digest the fibrous plant material whereas they can, and they return the energy they use up to the soil in manorial form. When it is oxidised by bacteria it can then be re-used by the plant.
When a seed germinates, only oxygen is needed to release energy for synthesis (building reserves). Carbon dioxide is then needed for building sugars as the seedling develops green leaves. The longer the plant remains growing in the leafy stage, the more energy it stores.
When a plant enters its reproductive stage it uses more energy than it stores. Cereals, for example, are very rapid developers and can be harvested within a year, so are very widely grown in tropical areas where adverse conditions are more prevalent. In the temperate zones however, more energy can be obtained by keeping plants in the leafy or vegetative stage, as they are always building up energy stores and not breaking them down.
When plants are establishing from seeds much energy is used, so bigger seeds grow stronger seedlings – “well begun is half done”.
When pasture plants are in the leafy (vegetative or tillering) stage energy is being collected and stored. Tillers (branches) are produced by a grass plant when conditions prevent stem production – usually in early spring. So tillering is a means of storing energy in a pasture.
When pasture plants enter the reproductive stage (during mid-late spring) tillering stops and energy is used to develop flowering stems.
So for effective pasture management, the farmer must maintain leafy pastures and prevent them entering the reproductive stage. At the same time it is important to prevent over-grazing.
All plants can be regarded in two forms:
- Trees, shrubs and herbs.
- Annuals, biennials and perennials.
A plant’s growth stages are controlled from within the plant, but this control responds to external factors:
- Food supply from soil and air.
- Water and air supply.
- Freedom from acidity.
Manurial ingredients must be in a soluble form to enable the plant take them in. Carbon dioxide is needed from the air for photosynthesis (food manufacture in plants). Water is needed for food transport through the plant but soil drainage is essential so that air can reach the roots.
Excessive acidity slows down food intake, and much lime is needed to prevent soil acid build-up (far more than is need for food).
Light is the source of energy for the plant to manufacture food. The intensity and duration of light. Intensity is more important during winter whereas duration of light in summer months allows most of the energy to be made and stored.
The critical day temperature is 5ºC as below this level the plant ceases food storage. In summer, temperatures above 40ºC are too high for food manufacture in plants, but around 25-35ºC is just right, although higher temperatures make plants age quicker.
Plant scientists use Accumulative Temperature to measure Day-degrees, the temperature quota needed for a certain growth stage. This is usually measured as the number of degrees over 20º C, so on a day that the temperature reaches 30º the AT is 10, and on a day that the temperature reaches 40º the AT is 20. Plants have different AT requirements, so if Plant A ripens before Plant B, the first plant will have reached its AT requirement before the second. For example, oats need 800 day-degrees from brading (breaking through the soil surface) to ripening, and this is the same in a hot season as it is in a cold one.
Day-degrees therefore determines whether a ryegrass is early or late-flowering. It regulates the suitability of a particular crop to a region. In Britain for example, oats grow best in the cooler regions of Scotland, whereas wheat grows better in England’s southern regions.
Temperature and pasture growth
Soil temperatures (in the top 10cm) affect pasture plant growth very much. When a farmer sows a new pasture the seeds germinate much faster in warmer soils, although perennial ryegrass and white clover will still emerge much quicker than other species, which is probably a major reason for their popularity with farmers – they are easier to manage. (NB. More drought-tolerant grasses like tall fescue and cocksfoot emerge much faster in warmer soils so are best sown earlier in autumn or later in spring than ryegrass -DC)
Leafy Growth in Grasses
The tillering (branching) process is very important in grasses. Every year a grass plant develops new tillers, so a plant may be 20 years old but is physically only a year old.
Pasture plant breeders have revolutionised their variety development by taking advantage of this feature, greatly diminishing the annual/biennial/perennial classification. For example Italian ryegrass is now available in forms that grow high yields of digestible foliage for several months, up to forms that persist several years but spread their production over these years.
The livestock farmer’s aim with pastures should be to maintain as much as possible out-of-season leafy growth, and to obtain leaf production in late spring/summer when the pastures are impelled into stem growth.
Managing the pastures using the livestock, and sometimes silage-making equipment, is the way to influence the form of pasture growth.
A hard-grazed pasture (to remove the primordia) will just go to head if it is rested. A pasture should therefore be rested while the grasses are still in the leaf-producing stage (early spring rather than late spring). This has a bearing on what pasture species should be sown and grown.
The earliest spring-growing grass is tall fescue, closely followed by Italian ryegrass. Perennial ryegrass then starts growing as the spring develops, followed by timothy and cocksfoot, and in Britain, meadow fescue is the latest to grow, along with browntop (Agrostis).
Late-flowering perennial ryegrass types are good summer growers as they remain leafy longer, but avoid resting for a silage cut as the aftermath can become stemmy. The early-flowering perennial ryegrass types are best for aftermath growth following a silage cut, as the primordia are removed in the cut. In late summer grasses will stay leafy as conditions prevent seedhead production then (but watch out for dry/dead pasture as it can harbour fungal spores that cause animal disorders - DC).
The earliest developing grasses tend to grow later into autumn/winter. In Britain late-flowering perennial ryegrasses tend to remain more palatable to stock than the early-flowering perennial ryegrass types, because the latter tend to brown off earlier. At the Cockle Park Research Station in Northumberland grazing animals were put on two pastures – one comprising early-flowering perennial ryegrass and the other late-flowering ryegrass, and they all eventually settled on the early-flowering ryegrass sward. However, with this treatment the late-flowering ryegrass pasture would persist longer (it would be less stressed when the environment is also putting pressure on it – DC).
Exploiting the Grasses
The grazing animals on a farm must always come first – but pastures then take the second place. Remember that labour and storage charges for silage and hay treble the cost of livestock feed from pasture.
When grass growth begins in early spring, new shoot growth saps the root system, but they soon start to manufacture food for the roots, replenishing that lost. In autumn the old roots act as storage organs.
Italian ryegrass is the first main grass to wake up, in early spring as the country begins to warm up again after winter. Perennial ryegrass flows about a month later. In these grasses there is a top-to-root balance year-round, one system replenishing the other once its growth has begun. The other permanent grasses, like browntop survive for centuries because they begin their growth later in spring when grazing stock have plenty to eat, and their nutrient stocks can be replenished easily.
The earlier a grass starts growing in a grazed pasture, the more it is exploited, and the more exploitation it has, then shorter its lifespan tends to be. Early-growing grasses like Italian ryegrass should therefore be rested before late spring to let them recover for later use. A one-month rest while it is still in the leafy growth stage will enable it to rebuild its energy bank. A livestock farmer does this by managing his animals, not often realising what he is doing is benefitting his pasture as well.
However, compensating the grass exploited early in spring will only work when no later-growing grasses are present in the pasture mixture. If a farmer wants perennial ryegrass to persist, then avoid including cocksfoot in the seeds mixture. In a Cockle Park seeds mixture (ryegrass, timothy and cocksfoot with red and white clovers) the ryegrass tends to be grazed off during the first growing season, the cocksfoot is grazed off the next year and the timothy goes in the third year when meadow-grasses begin invading. Simpler seeds mixtures with one ryegrass and one clover variety, will probably resolve this sequence.
On a typical upland farm with 800ha of hills and maybe 15ha of lowland pasture a farmer will carry around 400 ewes on the hills from late spring to autumn, and then bring them down to overwinter on the lowland block. At the same time he will graze cattle on the lowland pastures to prevent seedhead development, at the same time returning droppings to the land. The block would be rested from late summer until late autumn, allowing the pasture to age a bit so that the returning sheep won’t develop bloat and replenishing root nutrient storage. Then in early spring the new tiller growth will help to feed the ewes and their lambs. This illustrates good pasture exploitation.
Two British pastures sown to cocksfoot and growing into their third year, were managed as follows:
Pasture A was grazed from March to mid-May, rested for six weeks and then cut.
Pasture B was rested until mid-May, cut and then rested for six weeks.
In Pasture A the flowering buds (primordia) that form seedheads were below the grazing level so they grew during the rest period and produced stemmy poor-quality hay.
However with Pasture B the primordia were above cutting height and were therefore removed when cut for silage. New tillers were developed in the rest period following cutting, so the “aftermath” growth was all leafy – it takes at least six weeks for a grass tiller to develop another primordium.
Deric Charlton's original lecture notes
and Parker pen