Pasture and grazing management made simple 5: Keeping track of pasture supply

Keeping track of pasture supply using average pasture cover

The main factors contributing to the variability of pasture supply are weather conditions, the growth stage of the plant (leafy vs reproductive) and sward composition (especially the proportion of legumes). Pasture management aids are therefore required to maintain an inventory of the feed supply and to estimate how this might change over the next month or so. Calculating average pasture cover is probably the simplest technique available to farmers for performing these tasks.

Variability in pasture growth is the most serious challenge in successfully using pastures. Under drylot management the forage for the next weeks or months feeding is visible and both the quantity and the quality of the feed can be measured. It is possible, under these conditions, to keep a uniform supply of high quality forage available to the cows throughout the year. This is not the case under intensive grazing, where the amount and nutritive value of pasture available for grazing changes through the season. Not only is the supply of feed continually changing but next month’s herd requirements are not visible (i.e. it is in the process of growing). Clearly there is more risk with the pasture than the drylot management system.

The main factors contributing to the variability of pasture supply are weather conditions, the growth stage of the plant (leafy vs reproductive) and sward composition (especially the proportion of legumes). Pasture management aids are therefore required to maintain an inventory of the feed supply and to estimate how this might change over the next month or so. Calculating average pasture cover is probably the simplest technique available to farmers for performing these tasks.

The first step in determining average pasture cover is to complete an assessment of pasture mass in each paddock. This should preferably be done once weekly using one of the methods described earlier. The effective grazing area of each paddock should be recorded by excluding areas that cannot be grazed (e.g. woods). The average pasture cover can then be calculated by summing the amount of herbage in each paddock and dividing this value by the effective grazing area, as outlined in Table 3. Average pasture cover represents the amount of pasture growth remaining on the paddocks after animal consumption, removal of herbage for hay or silage, and losses through decay.

Table 3 Procedure for calculating average herbage cover for a farm with five paddocks and an effective grazing area of 90 acres

  Ares (acres) 
Herbage mass Total herbage
Paddock Total Effective (Ib DM/a) (Ib DM)
1 20 20 2200 44000
2 30 30 3000 90000
3 10 10 1500 15000
4 25 22 2000 44000
5 10 8 2500 20000
  100 90   213000
      Total herbage cover = 2130000/90
        = 2367 Ib DM/a

If the previous pasture assessment 14 days earlier on August 1 had been 2100 lb DM/a the supply of herbage has increased by an average of 267 lb DM/a or 24030 lb DM over the effective grazing area. Another way of expressing this is to say that pasture growth exceeded the amount of pasture eaten by livestock and losses of pasture through decay by 19 lb DM/day (267 lb DMa/14 days).

In addition to showing how the quantity of feed is changing through time, average pasture cover can be used to evaluate the current grazing program against target levels of feed supply at particular times of the year. By feed budgeting (see Article 4) a series of monthly pasture cover targets can be estimated. Actual pasture covers as the season progresses can be compared against the respectively monthly targets and also against the records of previous years. Figure 1 shows an example for a grazing season. This shows that potential shortfalls in feed supply could have been predicted by mid-May, before the dry conditions became serious during June. Although the management options for pastures in a drought may be limited, it is important to know the magnitude of the shortfall to compensate for this. The management response to pasture cover information will depend upon relative feed prices and expected weather conditions. The critical factor is that the farmer who routinely measures pasture cover creates more time to put a corrective grazing (or other) plan in place to minimise feed costs.

Figure 1: Monthly average pasture cover targets and actual levels of pasture cover recorded by pasture assessment during the previous and current year.

Increasing the length of the planning period is one of the most effective management strategies for reducing the risk associated with variable pasture growth rates. In practice, however, many farmers abandon planning when there are high levels of variability because ‘it is impossible to estimate the outcome with any degree of accuracy’. This suggests that planning only works for these farms where there is no or low variability – which is obviously not true! By regularly updating grazing plans with new average pasture cover information obtained each week, or fortnightly, and by continuing to assess the impact that these levels of pasture cover could have in 2-3 months time, you are in a much better position to make ‘good’ pasture management decisions than the neighbour who has no record of the pasture feed supply. We can conclude that the greater risk associated with feed supply under grazing compared to drylot feeding will not be eliminated but it can be managed.

Author

Warren Parker

Warren Parker
8 articles

Chief Executive of Manaaki Whenua/Landcare Research, NZ

Former Chief Operating Officer—Science at AgResearch, NZ

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