This series of articles describes the basics of pasture management and the use of grazing for the dairy herd. To get started we need to understand and use the language of pastures and grazing. This is a prerequisite to learning about and improving pasture management skills. Pasture forage is described as herbage or the above ground plant material of grasses, legumes, weeds and scrubs. The terms herbage and pasture are often interchanged. Herbage mass is the total amount of herbage per unit area measured to ground level. Herbage mass is expressed in terms of pounds of dry matter per acre (lbs DM/a). A pasture sward comprises the population of herbage plants, which usually have a relatively short growth habit and provide a high proportion of ground cover.
Livestock are unable to consume all of the herbage present because the jaw, teeth and tongue actions during grazing physically restrict how closely the animal can graze to ground level. In addition the animal is able to selectively graze within the sward, and less palatable plants or parts of plants may be avoided. Selectivity is dependent on grazing pressure which defines the rate of disappearance of herbage material. Under lax grazing pressure there is a slow decline in herbage mass and animals are able to exercise a high degree of preference (selectivity). If high grazing pressure is applied selectivity of herbage components by the animals is reduced. Both grazing pressure and the residual amount of herbage that remains after grazing (post-grazing herbage mass, or residual herbage mass (lb DM/a)), can be controlled by the farmer. In general, as residual herbage mass (often termed residual dry matter or RDM) is lowered animal, intake of herbage is reduced and in the absence of supplements, animal performance is lower.
Many grazing management guidelines describe the relationship between minimum RDM and livestock productivity (Table 1). Sward surface height (SSH), which is an alternative method for defining grazing management, is the average first contact height of green leaves in the sward canopy (the above ground parts of the sward). As SSH decreases animal intake increases. Successful pasture managers are familiar with these relationships and use this information when making decisions to shift animals between paddocks. The herbage available (lb DM/head/day) for grazing refers to the difference between the total amount of herbage present (i.e. herbage mass) and the amount of herbage left after grazing (i.e. the RDM). Herbage available should not be confused with the term herbage allowance which defines the amount of herbage per animal to ground level on a daily basis (i.e. lb DM/hd/d). Herbage allowance values include the post-grazing residual herbage and are always greater than the herbage available to livestock. Multiplying the herbage available by the area to be grazed yields the total amount of herbage available per paddock; dividing the herbage available by the number of animals provides the amount of herbage available per animal (normally this is in terms of lb DM/hd/d).
Table 1 Suggested grazing management guidelines for Holstein dairy cows receiving concentrates during late pregnancy and lactation in terms of herbage allowance, residual dry matter (RDM) and sward surface height (SSH) for predominantly orchard grass pastures. These preliminary estimates are based on the PennState grazing trial.
| State of lactation |
Forage intake |
Allowance |
RDM |
SSH |
|
(lb DM/hd/d) |
(lb DM/hd/d) |
(lb DM/a) |
(inches) |
Dry
|
25 – 30 |
55 – 65 |
1000 |
2 |
| Late pregnancy |
20 – 25 |
55 – 65 |
1200 – 1400 |
3 |
| Lactation - early |
25 – 30 |
100 – 120 |
2000 – 2400 |
5 – 6 |
|
- late |
20 – 25
|
80 – 100 |
1800 – 2000 |
4 -5
|
There are numerous terms used to describe grazing management or the manner in which a farmer controls the grazing of livestock to achieve specific production goals.
Controlled grazing occurs when grazing is regulated to achieve a desired result. Intensive grazing attempts to maximise the productivity of animals and pastures through increased capital inputs (fencing, fertilizer, water, seed) and management. This contrasts with extensive grazing where a low level of inputs and usually a large area of land per animal is used. The type of grazing adopted can essentially be divided into two categories: continuous and intermittent. Under continuous grazing the animals graze the designated area throughout the time period when grazing is allowed. The pastures receive no rest (leaving an area ungrazed or unharvested for a length of time) from grazing. Under intermittent grazing pastures are grazed for indefinite periods at specified or irregular intervals. Rotational, strip, sequence, mob, short duration and creep grazing are all forms of intermittent grazing. Set stocking, or the practice of allowing a fixed number of animals on an area of land for the entire grazing period, is a form of continuous grazing management. The number of animals on a grazing area for a specified period of time is the stocking rate.
There are a lot more terms related to pastures and their use, but these ones will get you well underway. Becoming familiar with terminology helps with communication to farm advisors and staff. Also, scientific papers, conference presentations, news articles, workshop courses and advice from consultants can be interpreted and applied much more readily if you understand the ‘language’. So now you can tell someone what it means to; ‘intermittently graze a sward to a residual dry matter of 2000 lb DM/acre?’. |