Stocking rate, or the number of animals per unit area of pasture (usually expressed as cows per acre) has a large impact on the amount of pasture available per animal. It is important to distinguish between the average stocking rate for the entire grazing season and the total pasture acreage, and the stocking rate at an individual grazing of a pasture. The season average stocking rate for 100 cows on 50 acres is 2.0 cows per acre, but at an individual grazing of a 5 acre pasture the 100 cows will be stocked at 20 cows/acre. The reason average reflects the farm policy (e.g. herd size and the area of pasture) while the paddock stocking rate is a within-year adjustment (i.e. this value will change frequently during the season to ensure that herd production targets are met). In some respects an indoor feeding system limits the options that a dairyman has with respect to stocking rate because barn capacity rather than the area of pasture available is the main determinant of herd size.
The effect of stocking rate on the amount of pasture available per cow can be quickly seen in Table 5.
Table 5: Effect of stocking rate on pasture available per cow.
||Pasture production cows(lb DM/acre)
||Pasture per cow(lb DM/cow)
The numbers in Table 5 simplifies what happens in practice because stocking rate also affects the type of pasture and the utilisation of pasture grown. Usually, for example, a mainly orchard grass sward will exist at low to medium stocking rates, but under high stock rates (and repeated heavy grazings), a bluegrass dominant sward will develop. Similarly under high stocking rates more of the herbage grown is consumed by the cows (i.e. the utilization of herbage is greater) than under a low stocking rate. This will probably result in better average pasture quality for the higher stocked pastures. If stocking rates are too high cows will be underfed, while excessively low stocking rates will lead to pasture wastage or under-utilisation (as shown in Table 5, where 50% of the pasture gown would not be eaten at 0.5 cows/acre). Both extremes are unlikely to realise the full economic benefits of grazing. This can be seen in the schematic diagram (Figure 2) showing the effects of stocking rate on cows fed a mainly pasture diet, in terms of milk production. Maximum milk production is achieved at a ‘medium’ stocking rate.
Figure 2: Illustration of the effect of stocking rate on milk yield per cow under an all pasture feeding regimen.
How can you determine whether the stocking rate on your farm is at the right level? This is difficult to answer because of the big differences that may occur in pasture production between and within years, but the following may help:
i) a weed infested very open pasture may be indicative of a stocking rate which is too low.
ii) The development of a bluegrass pasture where orchard grass was previously dominant may indicate that stocking rate is on the high side. A complete transition to a bluegrass sward may be undesirable because of reduced mid-summer production compared with orchard grass.
iii) A consistently low level of milk production when cows are grazing may reflect underfeeding because the stocking rate is too high. Usually this will be associated with heavy grazing pressures (i.e. low post-grazing pasture heights).
iv) A formal evaluation of herbage growth and herd feed requirements using a feed budget will provide a good indication of the ‘ideal’ stocking rate. The results of the feed budget should be interpreted in relation to the information obtained for (i) to (iii). The use of feed budgets for strategic planning will be discussed in the next article.
In New Zealand, where grazed pasture is almost the sole diet of lactating cows, the choice of stocking rate is one of the most important strategic decisions that a dairyman makes. Elsewhere, where pastures are being used, stocking rate is also of critical importance and needs to be determined with care.