Managing for Pasture Productivity
Pasture production varies considerably between regions, between farms, and even within a farm. There are many reasons for this, and they can be split into three groups:
- The natural environment;
- Modifications to the environment;
- Pasture management.
Most of the time farmers just have to accept the natural environment they farm in. Rainfall can be modified with irrigation, but air temperatures and sunshine hours cannot be controlled (with the exception of trees). Soils can be modified with drainage and sub-soiling, but it is often impossible to remove all limitations on some soils. The difference in climate and soils between regions is responsible for a large part of the variation in pasture production.
Much of the variation in pasture production between neighbouring farms is determined by what each farmer does to modify the environment. The main modification applied is fertiliser and lime, because natural soil fertility levels are low and farmers have applied differing amounts. On many farms, annual pasture productivity can be improved by applying more fertiliser/lime and seasonal growth by applying more nitrogen. The difficulty is deciding where the economic optimum is for spending more money on fertiliser in a system.
As well as driving pasture production, fertiliser also influences pasture persistence. When soils have low fertility, many naturalised species (eg. browntop, sorrel) are better adapted and gradually dominate the high fertility-requiring species (eg. ryegrass) we like to grow in its place.
Weeds will always try to invade pastures, and it is not always possible to prevent this with good fertiliser and grazing management. Once weeds get a foothold in pasture, they often expand, and the increasing coverage of ground reduces pasture growth and quality. It is good practice to monitor and identify weeds regularly, and it is often economic to control them with herbicides.
The way that soil nutrients are processed through animals and deposited again on soil can affect pasture production. On large sheep and beef farms large paddock sizes with variation in aspect, slope, and shelter allow animals to spend more time in parts of the paddock, leading to uneven distribution of dung and urine. This is very inefficient, because parts of the paddock have low soil fertility and parts have wasteful levels. Therefore, fencing can improve pasture productivity.
This leads to grazing management and its influence on pasture productivity. If animals can be grazed in large mobs and small paddocks, so that pasture is utilised quickly (1-3 days), nutrients will be distributed evenly across the pasture, maximising overall pasture production.
Pasture is often described as a solar panel, where its main task is to absorb energy from the sun, and convert that to feed for animals. When pastures are grazed close to the ground (eg. less than 1400 kg DM/ha for cattle and 1000 kgDM/ha for sheep pastures), not all the solar panels are present because some of the sunlight hits bare ground and not green leaf.
The ideal is to remove animals before they expose bare ground. This has a more dramatic effect on pasture production in winter and early-spring, because days are shorter and the sun has less strength. Another reason why pasture growth is restricted by hard grazing is that grass plants store some energy in the base of the stem to enable the initial regrowth of leaves after grazing. These are the reasons behind the old saying, “You need grass to grow grass”.
The conflict however is that in winter and early-spring, sheep and beef farmers generally limit intake by animals so that stocking rate can be optimised. When animals are forced to eat less than they want, they graze the pasture to low levels.
The dilemma is that if you manage for the optimum pasture covers after grazing at this time of the year, you might run out of feed by spring, or have to winter less animals. There are ways to overcome this though, such as grazing animals on greenfeed crops in late-winter, or grazing animals off the farm (a common dairy farm practice).
At some times of the year, farmers actually choose to graze pastures hard, because it can improve subsequent pasture quality. This is true in late-spring, when grass plants start developing reproductive stem. Hard grazing has two effects, it can remove reproductive stem and replace it with leaf (or at least younger stems), and it also reduces pasture production, which can allow animal consumption to more closely match pasture growth.
The Lincoln University Dairy Farm has highlighted the decision between grazing management for maximum pasture productivity, or animal production. They have shown that while lower pre and post-grazing covers tend to reduce pasture production, the improvement in feed quality means that animal production can be increased.
Hard grazing can also reduce pasture growth in the summer without irrigation. In this case it is not so much the solar panel effect, but the removal of energy stored in the base of tillers for regrowth. Pastures that are grazed closely during drought will be slower to recover than those managed to higher residuals. Bare ground also increases soil temperature and exposes white clover stolons to intense radiation, leading to reduced clover growth and content in pastures.
At the other extreme, leaving pastures too long between grazing in growth periods can also reduce pasture productivity. Once pastures reach a certain mass and stage of growth, any new plant growth starts to be matched by death of older plant parts, and so the net gain in green pasture actually stops. For this reason, most farmers graze pasture before it exceeds 3000 kg DM/ha. Pastures grazed at over 3500kgDM/ha are also slower to recovery, and often start to thin out in density.
Of course renewing pasture has a large effect on pasture productivity. Pastures need to be renewed at a regular rate to maintain productivity over the whole farm. When renewing pastures, choose the most productive species/endophyte/cultivar for the environment, and ensure a rapid and strong establishment so that ground cover is maximised and weed content minimised.