Animal management: grazing
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Well-managed grazing practices stimulate improved plant growth, increased soil carbon deposits, and overall pasture and grazing land productivity while greatly increasing soil fertility, insect and plant biodiversity, and soil carbon sequestration.
These practices not only improve ecological health, but also the health of the animal and human consumer through improved micro-nutrients availability and better dietary omega balances.
Feedlots and confined animal feeding systems contribute dramatically to:
- unhealthy monoculture production systems
- low nutrient density forage
- increased water pollution
- antibiotic usage and resistance CO2 and methane emissions
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All of these factors together yield broken and ecosystem-degrading food-production systems.
Basic principles of grazing performance
The optimal resting point of pasture is the optimal time for grazing, as it combines the needs of the plants and the needs of the livestock. At this point, the plant has passed the stage of maximum growth, has recovered root reserves, consumes water more efficiently, and its nutritional value is balanced. If cattle graze before this point there is a degradation of the grass, and if they graze after this point there is a loss of production.
Understanding the functioning of plant regrowth ecophysiology is essential to ensure good grazing and the associated environmental and economic benefits. The functioning of regrowth can be summarised in the post-grazing growth curves of the above-ground and below-ground parts (Figure 1).
Figure 1: Changes in organic matter in the above-ground and below-ground part of a grass plant after grazing. Source: Polyfarming manual, CREAF
The changes that occur over time are of three types:
- Growth pattern (quantitative changes). The grass plant, after being grazed, starts to regrow the aerial part using the reserves accumulated in the roots. The growth of the aerial part is maintained at the expense of the roots by consuming the accumulated reserves. This continues until the photosynthetic capacity of the new leaves is sufficient to generate a surplus that can be stored again as a reserve in the roots.
From this point onwards, there is very rapid growth of the aerial part and recovery of the reserves accumulated in the roots. Growth during this period can be more than 10 times higher than growth during the first few days of regrowth. Shortly before the plant reaches maturity, the reserves in the roots have already recovered and the growth of the aerial part slows down rapidly to a standstill. This coincides with the appearance of flower primordia and plant reproduction.
- Changes in the nutritional value of the plant (qualitative changes). Grass at the beginning of regrowth (2-3) is low in fibre and rich in soluble nitrogenous compounds that can cause diarrhoea in cows. Before reaching maturity, the composition of the grass is more balanced, with a better proportion of fibre and nitrogen in the form of amino acids, more suitable for animal nutrition.
From this point onwards, the protein content decreases and the plant begins to lignify, resulting in a loss of nutritional value. This loss is more accentuated in C4 plants than in C3 plants*. From a nutritional point of view, while the plant is in a growth phase (before maturity), there are important differences in nutritional value within the same plant. The upper third of the plant, where growth takes place, has the lowest cell wall content and a high protein content (around 14-18% regardless of the species). As a consequence, this upper third of the plant has a higher nutritional value than the rest.
- Variations in water consumption. From the onset of plant maturity, growth is rapidly reduced to zero. However, water consumption by the plant is maintained, so that the water expenditure per unit of dry matter produced is greatly increased. If the plant is re-grazed (or harvested) before this time, water consumption per unit of production will be much lower. This explains why grass grazed at the optimum point can extend the growing season when the dry season arrives.
*C3, C4 and CAM plants: C3, C4 and CAM are the three different processes that plants use to fix carbon during the process of photosynthesis. Fixing carbon is the way plants remove the carbon from atmospheric carbon dioxide and turn it into organic molecules like carbohydrates. C3 photosynthesis produces a three-carbon compound via the Calvin cycle while C4 photosynthesis makes an intermediate four-carbon compound that splits into a three-carbon compound for the Calvin cycle. Plants that use CAM photosynthesis gather sunlight during the day and fix carbon dioxide molecules at night.
Nutritional behaviour of animals on pasture
Pasture management requires combining the characteristics of pasture performance with the needs and behaviour of the animals. The aspects to consider for pasture management are:
- the way the animals eat;
- the amount they eat;
- the nutritional needs they have; and
- the impact they have on the field.
These aspects vary considerably whether free grazing is carried out on the whole grazing area, or whether intensive controlled grazing is used, where the pasture is divided into plots and the herd enters only one of them each day.
In free grazing, the animals roam the grazing area and choose the plants they are most interested in, selectively grazing them. This means that on the first day the animals eat the plants they like the most. On successive days, as the best plants are exhausted, they start to eat the plants that are least attractive to them, and so on until the last few days when the least beneficial plants are eaten.
Free grazing means that every day the animals change feed, which has a negative effect on production. All animals have the same feed, regardless of whether their nutritional needs are different (nutritional needs depend on the age, sex and reproductive status of each individual). The animals remain in the field for several days, so that, for favourable growing seasons, regrowth of the first plants eaten (the most qualitative for animals is initiated while the animals are still in the field. The animals mainly select these regrowths, so that the needs of the plant are not respected and overgrazing occurs. In addition, the animals trample the ground during their stay (several days in many cases of free grazing) which affects soil compaction.
In intensive controlled grazing, the animals only have the necessary amount of feed available to them each day (one tenth of the paddock) (Figure 2).
Figure 2. Example of intensive controlled grazing. Source: Campo galego
Under these conditions the grazing behaviour of the animals changes and they graze in a more aggressive way, where they know that they have to eat everything in the field.
In a herd, when the animals are at high densities, the presence of the other animals in a confined space causes the cows to acquire this aggressive grazing pattern. The result of this feeding behaviour is that the animals eat the same quality of grass (mixture) every day and there is no change in feeding, unlike in free grazing.
The farmer can divide the herd into two batches, one with the highest nutritional needs, and one with the individuals with the lowest nutritional needs. The batch with the highest nutritional requirements can enter the plots first and will take advantage of the tops of the plants, which have the highest nutritional value. The second batch, with lower nutritional needs, will then replace the first batch of cattle and eat the middle and lower parts of the plants. The cows are only in each plot for one day and therefore do not have time to eat the shoots of the first plants they ate.
In addition, the effect of compaction by trampling disappears because the cows are only in the plot for one day, they move less and the plot has a very long recovery time (rest days).
The PRV (Voisin Rational Grazing) is a grazing technique designed by André Voisin in the first half of the 20th century. Luiz Carlos Pinheiro Machado has incorporated many technological advances over the last 50 years. PRV is not simply intensive grazing. It is a complete agro-ecological technology that is characterised by: being totally environmentally friendly: regenerating soils, without inputs or poisons.
Figure 3. PRV design. Source: Roger Rabés
The day-to-day management of a PRV becomes simple when the basics are understood. Its cornerstone is to use grass at the Optimum Resting Point (OSP), which is when the plant has managed to complete its reserves thus achieving a subsequent vigorous regrowth. This is the time of maximum harvest of organic matter per unit of soil per year.The plant has the nutrients in balance to feed livestock correctly.
As a result, the carrying capacity of the space is increased, reaching maximum animal production per unit of soil. The PRV is based on 4 fundamental laws:
- Law of rest. For a grass cut by the animal’s teeth to give its maximum productivity, it is necessary that, between two successive cuts made by the animal, sufficient time has passed to allow the grass (i) to store in its roots the necessary reserves for vigorous regrowth and (ii) to have reached its optimum production per day and per hectare;
- The time of occupancy of the land. The overall occupancy time of a field must be short enough so that grass that has been cut to tooth on the first day of occupancy is not cut again before the animals leave the field;
- Maximum Yield Law. Aims to help animals with higher feed requirements to harvest as much and as high quality grass as possible; and
- Law of regular yields. In order for a cow to give a stable yield, it is necessary that she does not stay more than three days in the same field. Maximum yields will be obtained if the cow does not stay more than one day in the same field.
When we practice correct PRV management, we maximise the forage supply, which allows us to increase the stocking rate. In this way, more organic matter will be added to the soil and, consequently, we will see an increase in soil fertility.
Earlier this week, you heard from several farmers who have adopted regenerative agriculture techniques to manage their land. We invite you to discover (or rediscover) the implementation of PRV techniques through the story of Kepa Agirregoikoa here.
Grazing provides regulating ecosystem services, one of the main ones being its contribution to combating climate change: by stimulating the growth of plants that improve the soil’s filtering capacity, the land on which it is practised achieves a neutral carbon balance. And by encouraging infiltration, it improves water regulation and the maintenance of our aquifers.
It is therefore common to see an increase in plant diversity on pasture land, which also helps to control pests and stimulate pollination, and contributes to the prevention and mitigation of erosion and fires. For this, it is important to know how the grass works in order to take advantage of the growth cycles in the most efficient and durable way possible.
Regularly moving cattle to different plots is labour intensive.
- As labour shortages are one of the main problems in the primary sector, do you think these practices are a viable solution?
- How do you think periods of drought could affect these grazing techniques?
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