Spatial Management of Grazing

Article:  Spatial Management of Grazing to Enhance Both Livestock Production and Resource Condition: A Scientific Argument Authors: Norton, Brian E. Journal: Proceedings of the VIIth International Rangelands Congress This article directly addresses the disparity between the scientific theory of grazing management and applications in commercial operations.  Specifically, the author points out that “the majority of rotational grazing studies have failed to demonstrate a significant benefit over continuous grazing in terms of livestock production or condition of the vegetation.”  This is considered “puzzling when matched against reports from ranchers, graziers, and pastoralists in North America, southern Africa and Australia who claim that rotational grazing has caused improvement to species composition and productivity of their land while stocking rate was maintained or even increased.”  How do we account for this “dichotomy between research results and practical experience”? Grazing theory, according to the author, suggests that rotational grazing increases grazing pressure and “is associated with greater competition among grazing livestock for forage on offer” which ultimately results in a poorer diet.   This argument assumes that animals under continous grazing are doing so evenly across the landscape, which may occur in small, research sized paddocks, but is much less likely at larger scales.  As the author points out:

Observers of animal distribution or distribution of use in continuously grazed rangeland paddocks have presented ample evidence to show that livestock grazing is inherently uneven across the landscape.  In practice, livestock use only a fraction of the total area available.

 The author continues:

Behavioral and landscape parameters restrict livestock grazing to preferred, accessible portions of a large continuously grazed rangeland paddock…Thus it is not uncommon for livestock to visit and graze only 50% of the total area of individual paddocks.

This patchwork distribution of animals on the landscape can dramatically increase stocking rates at certain areas, and dramatically reduce it on other areas.  Patchy over-under utilization has resulted in very conservative stocking recommendations.  Yet “landscape subdivision could improve spatial distribution of grazing and enhance overall animal production per area through an increase in land accessible to grazing, and thus an increase in

de facto

available forage.” The above argument is supported by a reexamination of the research results of two studies by Denny and Barnes (1977 & 1991) conducted at the Matopos Research Station near Bulawayo, Zimbabwe.  Two paddock configurations were used in these studies, resulting in six potential configurations of grazing days and rest periods (See Figure below).  Combine these with two different stocking rates (low and high), and twelve treatments were applied to the landscape over a period of 5 years.

 On average, animal performance tended to be poorer on a per animal basis at the higher stocking rates:

Overall the gain in kg/head under the low stocking rate…was 20% higher than the weight gain at twtice that stocking rate, 67.5 compared to 56 kg/hd.  , when expressed on an area basis an in terms of gain per Livestock Unit, the high stocking rate yielded about 40% more animal product, 29.65 versus 17.87 kg/ha.

 These results are consistent with similar results observed in the Charter Estate Grazing Trials.  Although Denny and Barnes concluded that rotational grazing at high stocking rates yields sub-optimal results, Norton puts their results in a new light:

What makes the Denny & Barnes study of particular interest is that although high stocking rate substantially reduced weight gain when the grazing period was 20 days, it failed to have a significant effect when the grazing period was 5 or 10 days.  Expressing this in another way, a doubling of the recommended stocking rate did not substantially reduce animal performance in a rotational grazing system with grazing periods of 5-10 days…There is a suggestion that significant improvements in animal production, including higher carrying capacity, are likely with rotational grazing that employs short grazing periods and long rest periods.

The author supports this contention with information from another study conducted on the Rex Ranch in Nebraska.  As the figures below demonstrate, increases in stocking rate slightly suppress individual animal performance, but can increase production per land unit by at least an order of magnitude.  In summary, rotational grazing can “ameloriate the stocking rate effect on animal production.”  Yet we still must address the issue of range quality: How is this critical factor affected by increases in stocking rate?  This will be addressed in my next post.