In any grazed pasture, plant species respond differently to grazing pressure. Understanding these responses lets you use species composition as a long-term trend indicator.
Decreasers, increasers, and invaders
Rangeland ecology classifies pasture plants into three functional groups based on their response to grazing:
- Decreasers — preferred by cattle; reduce in abundance under heavy grazing (e.g. Queensland bluegrass, black speargrass in good condition). A high proportion of decreasers indicates good pasture condition.
- Increasers — less preferred; increase under heavy grazing (e.g. wiregrasses, annual native grasses). A shift toward increasers signals moderate grazing pressure.
- Invaders/weeds — enter from outside when pasture condition is degraded (e.g. Parthenium weed, sicklepod, African lovegrass). Presence of invaders indicates degraded condition.
Pasture condition scoring
DAF Queensland publishes condition score guides for each major pasture type across the state. Scores typically run from 1 (very poor — bare soil, dominated by weeds) to 5 (excellent — full complement of decreasers, high ground cover). Most productive properties aim to maintain a condition score of 3–4.
Conducting a composition assessment
Walk a consistent transect through the paddock. Every 2 metres, stop and record the dominant plant species at the centre of a 30 cm diameter ring. Record 50 observations. Calculate the frequency of each species as a percentage of all observations. Compare to your regional decreaser species list to determine what proportion is decreasers vs. increasers vs. invaders.
Repeat the same transect at the same time each year and photograph the start and end of each transect for a permanent record.