Silvopasture BMPs

Silvopasture, like all agroforestry opportunities, has tremendous potential to leverage production, environmental, economic and social benefits through integration. However, like any system with many parts and complex interactions, if they are not structured and managed properly they may be doomed to failure and you can be far worse off than if you had keep things separate.

Beneficial management practices (BMPs) in silvopastures revolve around structuring and managing the system to promote positive (facilitative) interactions and minimize negative (competitive) processes between the three major components: livestock, forages and crop trees.

It is also equally important to understand that the nature of the interactions will vary as the silvopasture matures. We can separate silvopasture development into three distinctive phases based on the growth and development of the crop trees:

1. In the Herbaceous Phase, tree seedlings are subject to strong, direct competition with the forage crop, and most are susceptible to direct livestock impacts;

2. Once at the Intermediate Phase, trees have extended their canopy above the forage layer and competition between trees and forages is generally limited to soil-based resources (water and nutrients). Most potential livestock impacts on trees are also diminished, although this varies greatly with livestock species and their preference for feeding on the crop trees relative to the other forage and browse options available; and,

3. In the Arboreal Phase, mature trees become the dominant elements in the system in terms of both size and resource use, and they control the availability of most resources and the understory microclimate. Livestock have limited impact on trees at this stage.

Agroforesters need to carefully manage three critical interactions over these phases to ensure synergies and production gains are developed:

  1. Tree-forage interactions in the herbaceous phase
  2. Livestock-tree interactions in the herbaceous phase
  3. Tree-forage interactions in the arboreal phase

For managing the tree-forage interactions in the herbaceous phase, species selection is the first and best tool to avoid competitive interference that can set back tree growth. Use large, vigourous tree stock that will grow rapidly above the herb layer. And choose forages that are slower to establish, are short-statured, shallow rooted, or grow at a different time of year than the crop tree, thus separating competitive resource use. In circumstances where you are growing tree species that are poor competitors, or where soil moisture deficits are predictable, it may be necessary to keep a 1- to 2-m vegetation-free buffer around the crop trees. This will inevitably drive up costs whether using mulches, chemical, manual or mechanical weeding. Tightly controlled grazing can also be used to alleviate competition, but be vigilant because tree seedlings are also most vulnerable to browsing and trampling damage at this stage.

Indeed, all trees are susceptible to trampling until they are at least 0.5-m tall. It is advisable to exclude or minimize use of the silvopasture until the mean tree height exceeds this threshold. Similarly, some minor browsing is to be expected (generally less than 2% of the plantation) until the terminal branches on the trees are inaccessible to livestock. Severe damage can occur when no other forage or browse is available, or if tree is a preferred species, so it is important to use livestock with higher preference for the forage species present than crop trees. Rotational grazing may also restrict time trees are exposed to grazing, resulting in lower trampling and browsing damage. But in cases where the livestock density or preferences make damage unavoidable, you many need to employ individual tree barriers (e.g. plastic tubes or mesh), or temporary fencing to protect tree clusters or rows. Chemical deterrents sprayed onto the trees may provide effective protection, but also need to be reapplied after heavy precipitation events.

For managing the tree-forage interactions in the arboreal phase, it is necessary to manipulate tree form or density to maintain understory growth. Forage production can taper off after the overstory exceeds 30 to 35% canopy cover, and trees should be thinned in advance of this threshold or, alternatively, they could be planted a low starting density. The major drawback to thinning / low density plantations is that for many tree species when grown in the open (i.e. without other tree neighbors) tend to have greater stem taper and more branches. Both of these characteristics are undesirable for log quality and value. Therefore if the trees are being grown for clear lumber or veneer products they should be pruned after thinning. Pruning will reduce branches, maintain tree form and also increase light and precipitation input to the understory forage crop and can ease livestock movement through the silvopasture.

In all other respects, use BMPs for conventional livestock, forage and timber management and you can reap the benefits of integrated production.