The application of grazing management to increase sustainable livestock production : the McClymont lecture

Abstract

Animal Production in Australia 1998 Vol. 22 THE McCLYMONT LECTURE THE APPLICATION OF GRAZING MANAGEMENT TO INCREASE SUSTAINABLE LIVESTOCK PRODUCTION B.E. NORTON Dept of Rangeland Resources, Utah State University, Logan, Utah, 84322-5230, USA SUMMARY Graziers and researchers have reached an impasse regarding the merits of rotational grazing systems that involve a large number of paddocks and short grazing periods. Most research trials have concluded that continuous grazing is either better or no worse than rotational grazing in terms of livestock production. Three reasons are offered to explain these results: (1) The paradigm underlying studies of rotational grazing, namely, that rotational grazing can control frequency of defoliation, is flawed. (2) Continuous grazing in large paddocks causes patch grazing and localized pasture degradation, but this aspect of continuous grazing has not been addressed in trials comparing grazing systems. (3) Continuous grazing in large paddocks creates very uneven distribution of livestock, but research trials have usually assumed spatial homogeneity in forage availability and utilization. The potential for significantly higher livestock production under a cell grazing system can be justified from scientific arguments using existing research data. The key to sustainability of cell grazing is very high stock density to reduce selectivity, and moderate utilization during grazing to maintain forage productivity. More even animal distribution is automatically achieved by such a system, and the benefit of this to livestock production is already evident from research studies involving small paddocks. Keywords: rotational grazing, livestock production, distribution INTRODUCTION The divergence between theory and practice in the use of rotational grazing to enhance livestock production has widened in the past 20 years. The rotational grazing strategies which many producers claim are responsible for higher profits and better enterprise sustainability are the same kind which the research community has concluded can offer only marginal benefits to production and probably are not cost-effective. Researchers have struggled for decades to test the value of rotational grazing systems against continuous grazing. The conceptual model underlying this research relies on the assumption that continuous grazing fails to protect desirable species from heavy, selective and destructive utilization, and that rotational grazing will solve that problem (Wheeler 1962), with the expectation that more forage would be available and livestock production enhanced, compared to pastures continuously grazed. But the results of grazing trials have been counter-intuitive. Just about every comprehensive review of rotational grazing research has found that the majority of studies conclude that continuous grazing is no worse than rotational grazing, or may even be preferable from a livestock production point of view (Wheeler 1960; Driscoll 1967; Herbel 1974; Gammon 1978a; Morley 1981; Valentine 1990; Heitschmidt and Taylor 1991). Whenever an economic assessment of rotational grazing has been attempted, continuous grazing was usually the recommended choice (eg Wilson et al. 1987; Hart et al. 1988; Heitschmidt et al. 1990b). Yet a significant number of graziers in several countries continue to claim that rotational grazing has been one of the keys to successful, profitable livestock production, the other keys being better record-keeping, better planning, and better business management in general, including the courage to be innovative and the wisdom to be adaptive. No one argues that improved managerial skill will not create a more profitable enterprise, but we do debate whether we can substantially increase livestock production by changing the way we manage the relationship between forage resources and the livestock which exploit them. Thus far the research community, in the United States as well as Australia, has nothing to recommend except continuous grazing and conservative stocking. Graziers are dissatisfied with that, and are looking elsewhere for advice. This paper is an attempt to resolve this impasse. I will propose a theoretical argument which validates the practice of certain forms of rotational grazing. But in view of the dichotomy between research results and practical experience (McCosker 1994) it is not sufficient merely to show that substantially higher production from rotational grazing has a solid, science-based rationale; it is also necessary to explain why hundreds of grazing studies have arrived at 15 Animal Production in Australia 1998 Vol. 22 a contrary conclusion. I do not believe that the research was poorly conducted, but I do suggest that the terms of reference were limited, leading to erroneous extrapolation of conclusions to a commercial livestock operation, and I think that the paradigm upon which the research enquiries were based may be flawed. Some introductory comments are necessary to clarify concepts and terminology. Rotational grazing occurs in several different forms that were classified neatly by McCosker (1993) according to the number of paddocks involved and the speed of the rotation. Grazing strategies that require from two to seven paddocks demand grazing periods that last on the scale of a year, months or weeks, sometimes with heavy utilization towards the end of the period, and the rationale emphasizes the importance of rest periods to allow the vegetation to recover from grazing. Grazing strategies that involve 10 or more paddocks permit grazing periods as short as a few days, and the rationale stresses the avoidance of excessive defoliation combined with adequate rest. As the number of paddocks in the rotation increases, and the area occupied by the grazing herd decreases commensurately, the justification for such strategies includes reference to the value of high stock density impacts to the functional integrity of the resource. The particular class of rotational grazing practices that require at least 20, 30 or preferably more paddocks, and are characterised by quite short grazing periods, is the main subject of the current debate between scientists and graziers cited above, and thus the focus of this paper. In the popular literature this class is recognized by various names, including time control grazing, cell grazing, short duration grazing, mob stocking and block grazing, and is often placed within the managerial approach known as holistic resource management. As a matter of convenience, in this paper I shall use the term cell grazing (after Earl and Jones 1996), to cover the entire genre, and shall apply the term grazing system to the management of grazing animals according to a set of principles or ideas implemented in an organized, rational fashion. One cannot proceed very far on this topic without invoking the names of Savory and Parsons (1980) who, together with their students and colleagues, have successfully advocated the use of cell grazing among commercial producers as part of a management package which appears to have consistently increased livestock production and often reduced costs of operation in the United States, Australia and elsewhere. They have brought the issue of rotational grazing into the spotlight as graziers look for ways to remain in business in a worsening economic environment and ever-capricious climate. We should also acknowledge a debt to Voisin (1959) and Acocks (1966) who inspired people such as Savory (see Goodloe 1969) to explore the merits of rotational grazing in low-rainfall rangeland ecosystems using grazing periods that were unconventionally short, hence Savorys initial designation of short duration grazing (Savory 1978). The analysis given in this paper would be superfluous if scientific attempts to test cell grazing had not generated results so contradictory to the experience of many commercial graziers, or if Savory, Parsons, and their colleagues had been able to provide a satisfactory theoretical basis for the grazing practices they advocate. Unfortunately, the collection of principles articulated thus far by proponents of cell grazing either fails to provide a cohesive body of testable theory (eg McCosker 1993), or else the evidence offered in support of the principles is anecdotal and data-free (eg Savory 1988) rather than objective and data-based. Before addressing cell grazing per se, I will consider the conundrum of why the benefit of rotational grazing could be so intuitively obvious and yet so difficult to demonstrate in research trials. My discussion of these issues is in the context of rangeland ecosystems with relatively low rainfall (about 750 mm or less) that have strong seasonality and are liable to droughts, rather than more temperate pastures with higher, fairly reliable rainfall more evenly distributed through the year. In the context of eastern Australia, this would include the tablelands and land to the west. A FLAWED PARADIGM Both graziers and scientists have observed that paddocks which are continuously grazed tend to deteriorate. They concluded that continuous grazing at conservative stocking rates gives livestock maximum selectivity, which is expressed in heavy utilization of preferred, palatable species. Heavy grazing pressure on desirable species gives a competitive advantage to less desirable species or exotic weeds, which increase in the pasture at the expense of more palatable plants. Numerous clipping studies in the first half of the 20th century (reviewed by Jameson 1963) showed that repeated, frequent defoliation reduces forage yield. Experimenters presumed with confidence that their results demonstrated the problem with continuous grazing, and paid little attention to the fact that clipping and mowing are imprecise representations of defoliation by livestock. [Papers reporting the classic studies at Cambridge by Woodman and his team (Woodman et al. 1929, 1931; and Woodman and Norman 1932), are entitled The influence of the intensity of grazing on the 16 Animal Production in Australia 1998 Vol. 22 yield, composition and nutritive value of pasture herbage even though the small subplots were mowed, not grazed.] Thus the body of experimental evidence from clipped plants or plots justified the argument that controlled grazing was needed to protect palatable plants from continuous exposure to herbivory. During an imposed rest period in the absence of livestock, the argument continues, the vigour of grazed plants will be restored and their ability to produce forage will be sustained. A critical corollary of this argument is that each grazing period should not be long enough to allow the herbivore to defoliate the regrowth of a tiller which had been grazed earlier in the same period. When translated into practice, these ideas become the foundation for rotational grazing, which, in addition to preserving the forage value of the grazed vegetation, presents livestock with fresh feed on a regular basis. The ideas have been in print, in English, for a long time, at least since Andersons writings in the late 18th century (quoted by Johnstone-Wallace and Kennedy 1944, and by Voisin 1959). The crux of the matter is the proposition that under continuous grazing plants are defoliated repeatedly and severely, but the data in direct support of that notion are relatively sparse. Even Voisins textbook has no data on this issue. Instead, he asserts: Without committing any great error, we can say that very short rest periods of 6 [to] 12 days...correspond more or less with what takes place in the case of continuous grazing with cattle (Voisin 1959, page 19). In the case of high-rainfall temperate pastures suited to dairy farming, Voisins assumptions regarding continuous grazing may not be far off the mark, given high stocking rates and small paddocks. In lower-rainfall rangeland ecosystems, however, the frequency of defoliation under continuous grazing does not appear to be as severe as was assumed. A large fraction of plants or tillers are not touched at all, a small percentage are grazed twice, and relatively few receive three or more defoliations in a long grazing period (Gammon 1978b; Norton and Johnson 1981; Hart et al. 1993b). It follows that if frequent and severe defoliation is not a problem with continuous grazing, then the implementation of rotational grazing should have little effect on defoliation frequency (Gammon 1984; Gammon and Twiddy 1990; Barnes and Denny 1991; Kirkman and Moore 1995). And that indeed appears to be the case. Working at the Matopos Research Station in Zimbabwe, Gammon (1978b) recorded minor differences in number of defoliations per tiller over a six-month period when he compared continuous grazing with a six-paddock rotation using a 12-day grazing period. There was a tendency for a higher percentage of tillers to be grazed twice under the rotation (28.2 vs. 21.8%, averaged over five species); only about 6% of all tillers were grazed three or four times. In two studies in Wyoming seven years apart, Hart et al. (1993b) found no difference between continuous grazing and an eight-paddock rotation in terms of frequency of defoliation for western wheatgrass grazed over a five-month grazing season. In one year of their study (1990) they recorded significantly more defoliations for blue grama continuously grazed, but only 12% of tillers were grazed twice under the heavy grazing treatment. Gammon and Twiddy (1990) in Natal could not find a difference in defoliation pattern between a four-paddock rotation involving 14-day grazing periods and an eight-paddock rotation with seven-day grazing periods. Gillen et al. (1990) obtained an average of only 52% of tillers defoliated per grazing period of three to seven days in an eight-paddock rotation stocked above the recommended level in Oklahoma. In a subsequent test of this rotation (Derner et al. 1994), frequency of tiller defoliation was significantly higher in the continuous-grazing treatment, but at the highest stocking rate 25% of tillers subjected to the rotation were either ungrazed or defoliated just once in 150 days. The difficulty of imposing a particular pattern of tiller defoliation by implementing a rotational grazing system was also demonstrated in Texas by Heitschmidt et al. (1990a). They looked at defoliation frequency in a ten-paddock rotation with two to four days of grazing stocked at twice the rate recommended for the region. In each of four consecutive grazing periods, more than half the tillers were not grazed at all, and only one of the five species being studied experienced a substantial proportion (40%) of tillers grazed three or four times over the four-month experimental period. Studies which include stocking rate comparisons invariably show higher defoliation frequencies at higher stocking rates, but the effect of the rotation per se on defoliation pattern is weak or absent, at least in the context of most experimental designs which employ four to eight paddocks in the rotation. THE CONCLUSION FROM RESEARCH Researchers have been trying to control, through grazing management, the periodicity and intensity of defoliation and, via optimal defoliation regimes, to increase forage production. Such increases have been elusive because, as noted above, rotational grazing has not substantially altered the pattern of defoliation. Not surprisingly, scientists have concluded that rotational grazing per se cannot be expected to increase forage 17 Animal Production in Australia 1998 Vol. 22 production. The emphatic statement by Wilson is representative of the general view that the use of grazing systems for the improvement of short-term animal production is specifically rejected (Wilson 1984, page 222). A window allowing qualified endorsement of rotational grazing is left open with the suggestion that it may favour changes in the botanical composition of the vegetation which, in turn, could generate a pasture inherently more productive than its predecessor, or than a comparable pasture remaining under continuous grazing (Gammon 1978a). Similarly, rotational grazing has been proposed as a remedy for range deterioration (Kirkman and Moore 1995). Researchers have claimed that it will permit sustained stocking rates that would otherwise be deleterious in the long term (Wilson 1984), but the magnitude of the increase in forage production or stocking rate has been judged in the neighbourhood of 15 to 30% (McMeekan and Walshe 1963; Morley 1968; Tainton et al. 1977; Gammon 1984; Bryant et al. 1989), scarcely worth the effort of implementation. And whether a certain rotational grazing system is likely to achieve a desirable change in vegetation will be site-specific, depending on the species composition of the grazed vegetation, the relative tolerances of species to the combined stresses of defoliation and competition, and the number and kind(s) of livestock being manipulated. And that somewhat equivocal conclusion comprises the sum total of what rotational grazing research can tell us. Solving problems arising from poor animal distribution is, however, not part of this research portfolio. THE MISREPRESENTATION OF CONTINUOUS GRAZING Research studies of rotational grazing systems are carried out using small paddocks, usually less than 25 ha and often less than 5 ha each. Researchers generally use such a paddock for the control treatment, continuous grazing, to which the rotational grazing system will be compared. The intent is to mimic a large paddock on a commercial property, but in translation to the research context a critical aspect of continuous grazing is lost, namely, uneven utilization over the landscape. What the researchers are actually representing under experimental conditions is a landscape which has been divided up into many small paddocks, all of which are continuously grazed at similar stocking rate. Yet the conclusions from the research tend to be extrapolated to all pastoral situations, regardless of paddock size. The usual conclusion is that rotational grazing is no better than continuous grazing, and within the terms of reference of the studies, that is perfectly valid. But if the spatial dimension is taken into account, a different interpretation may emerge, as discussed in the following section. When researchers use small paddocks which receive relatively uniform grazing impact, they eliminate from their studies the most harmful consequence of continuous grazing, namely, patch grazing. Livestock entering a virgin field will establish an initial pattern of use which becomes reinforced as the season progresses (Daines 1980; Ring et al. 1985). Animals are attracted to areas previously grazed (Willms et al. 1988; Fuls 1992b), enlarging them and creating new ones nearby. Patches grazed heavily one year are more likely to receive heavy utilization in subsequent years, and areas neglected by livestock one year are likely to receive little use again. Willms et al. (1988) found that the tendency for neglected areas to be perpetuated is stronger than the perpetuation of heavily grazed patches, and that the reinforcement of these patterns is more pronounced under lighter grazing pressures. The phenomenon of semi-permanent grazing mosaics means that the stocking rate on heavily grazed patches is de facto much higher than the intended stocking rate for the paddock as a whole (Suckling 1965; Kellner and Bosch 1992). Intensity of defoliation, especially in terms of frequency, increases as stocking rate increases (Briske and Stuth 1982; Hart et al. 1993b), and the deleterious effects of high stocking rate are manifest in the patches so affected, leading to localised changes in vegetation and soil which are not easily reversed. Overgrazed patches in Alberta lost 28% of the soil A horizon, the normally dominant perennial grasses were replaced and forage production was depressed by 35% (Willms et al. 1988). Grazed patches in South Africa had lost most of the A horizon, exhibited lower soil water content, less vegetative cover and a higher proportion of undesirable species, yet they continued to receive much heavier utilization by sheep than adjacent patches in better condition (Fuls 1992b). Grazed patches in Zimbabwe had higher annual variability in plant production than less-degraded sites, with almost no forage produced in drought years (MacDonald 1978). Resting such areas during a drought may not facilitate their recovery if the patch has deteriorated beyond a threshold condition; on the contrary, the patch size may expand even without further grazing (Fuls and Bosch 1991). The expression of very uneven distribution of grazing, and the consequent development of relatively stable, degraded patches which receive far higher impacts than would be determined from average stocking rate calculations, is a serious detriment to the practice of continuous grazing. But this dimension to grazing 18 Animal Production in Australia 1998 Vol. 22 management has been almost ignored when continuous grazing and rotational grazing are compared in experimental studies, which tend to be designed on the assumption of spatial homogeneity in forage availability and utilization. The reason may lie in the difficulty of accommodating spatial variability within the small size of research paddocks, but it is also a function of the paradigm which directed the research process, namely, the perception that rotational grazing should be used to control the level of defoliation experienced by individual plants, not the nature of grazing distribution across paddocks. The neglect of spatial variability when large paddocks are continuously grazed can also slant a theoretical comparison of grazing systems towards unrealistic conclusions. A concept often employed for such comparisons is grazing pressure - the ratio of livestock demand for forage relative to the amount of forage available. Scientists usually assume that all the forage within a paddock fence is available to the grazing animals it contains, no matter how large the paddock may be. When Heitschmidt and Taylor (1991) compared various grazing systems using grazing pressure as the principal criterion, continuous grazing was judged superior to all rotational systems because the latter required paddock subdivision, which automatically created higher grazing pressure as a function of smaller paddocks having less total forage available. However, in order to preserve the superiority of continuous grazing in their analysis they had to make the assumption that paddock subdivision was not a distinguishing feature of rotational grazing! In the absence of any impediment to the free movement of livestock around the paddock, it may seem logical to define grazing pressure without spatial consideration, but that is nevertheless unrealistic for large paddocks. The amount of forage available to grazing animals is not only a function of the size of the paddock, but also of the ability of livestock to explore the landscape and to search parts of it at close quarters. In other words, the behavioural parameters of walking distance, locational preferences and time spent grazing determine the amount of forage to which an animal or herd has reasonable access, and which might therefore be considered available forage on a daily basis. INCORPORATING THE SPATIAL DIMENSION The small herd of animals in a continuously grazed paddock on a research station has no problem exploring the entire paddock at least once a day or more often. The estimation of available forage in the paddock can be expressed without spatial qualification because all of it is accessible, and the problem of uneven access to the forage resource, as exhibited by animals grazing a large paddock, is not an issue. Rotational grazing trials have been spatially neutral, with few exceptions (eg Walker and Heitschmidt 1986; Hacker et al. 1988; Walker et al. 1988; Hart et al. 1993a). Most research has wrestled with temporal variability in defoliation, but for a commercial enterprise spatial variability is equally important. Uneven distribution of grazing in the form of predictable patterns of use is well documented for large paddocks (eg Hodder and Low 1978; Low et al. 1980; Orr 1980; Senft et al. 1985; Owens et al. 1991). A New Zealand study of sheep grazing a 850 ha paddock is a particularly dramatic example (Scott and Sutherland 1981). Ash et al. (1996) considered this phenomenon in terms of the difficulty of extrapolating stocking rate experiments from small-paddock studies to the large paddocks of a commercial operation. I would like to address the scale issue in terms of the extrapolation of rotational grazing studies conducted with small paddocks. Livestock grazing a large paddock exhibit spatial patterns of repetitive use; by inference, as well as observation, they repeatedly neglect or lightly use some areas of the paddock. The larger the paddock, and the lower the stocking density, the greater the proportion of paddock area neglected. At a finer scale, heavily grazed patches occur within the preferred communities. Graziers need management strategies to minimise patch overgrazing and improve the utilization of undergrazed areas (Fuls 1992a). I believe that space management of livestock grazing is the principal key to increasing sustainable livestock production. If there is a grazing management system which will exploit forage that is otherwise neglected by livestock, without detrimental consequences to the pasture or rangeland resource, such a system will, by definition, increase the carrying capacity of the land. As paddocks become smaller, the opportunity to improve the spatial efficiency of forage utilization increases, although uneven distribution of grazing may never be eliminated (Taylor et al. 1985). Grazing trials on research stations are usually carried out with relatively small paddocks, and frequently are conducted at, or include among their treatments, stocking rates which are higher than those recommended for the district. Unfortunately, most reports of rotational grazing studies fail to express their stocking rates in relation to levels recommended for commercial enterprises in the neighbourhood. Whenever the experimental stocking rate is given such a perspective, however, one often finds that much 19 Animal Production in Australia 1998 Vol. 22 higher stocking rates can be sustained on the research station. I suggest that a large part of that higher carrying capacity is due to the smaller paddocks employed on the station. The examples of this phenomenon presented in Table 1 emphasize data from the continuous grazing treatment, if available, and ignore any differences between the grazing systems, if evident. A strong tendency to adhere to the flawed paradigm of rotational grazing discussed earlier, and to neglect spatial considerations, is evident in published reports which show clearly that carrying capacity was substantially increased under the conditions of the experiment but omit any reference to this result in the conclusions (eg Smoliak 1960). Morley (1987) noted the discrepancy between performance in grazing studies on a research station and performance on graziers properties and suggested that it may be due to the inherently better soils and greater uniformity of pastures found on research stations. His assertion was not accompanied by specific examples, however, and although site quality may be a confounding factor, the case is yet to be presented convincingly. A SCIENTIFIC ARGUMENT FOR CELL GRAZING The critical component of cell grazing is the concentration of grazing animals at high stock densities. The critical caveat is that utilization of forage is moderate, even though grazing pressure is high. If a grazier manages for these two factors, it follows that he must use small paddocks to achieve high stock density and employ relatively short grazing periods to prevent heavy utilization. If his overall stocking rate remains the same or rises modestly, the only way to implement these precepts in practice is by moving animals around a large number of paddocks, at least 30 and preferably more, that together form the cell of one management unit. Because stock density is high in the paddock being grazed, utilization is spatially more even than would otherwise occur, and once the animals have moved through the entire cell, every part of the landscape is explored by livestock and access to forage is maximised. This argument hinges on the importance of grazing at high stock densities, in which the vegetation experiences high grazing pressure for short periods. The advantage of high stock density is that it can eliminate, or at least substantially reduce, selectivity by grazing livestock. The goal is to spread defoliation across as many plants as possible. When this happens, species ceases to be the primary criterion for diet selection. There are a number of graziers who have observed that livestock in a grazing cell will consume species which they would not normally touch at lighter grazing pressures. People report that problem weeds have been reduced in this manner, and I have observed cattle grazing young brigalow. Can such anecdotes be backed up by research? Few research studies have attempted to include a treatment representing a cell of 30 or 50 paddocks; most research trials employed four to eight, perhaps up to 16 paddocks. However, a study in South Africa is particularly relevant. Fifty head of cattle were grazed for one to two days on less than 1 ha, with approximately one month between grazing events, at two sites of mixed grassland vegetation. From two years of data collection, OConnor (1992) found that the likelihood that a plant would be grazed was principally a function of plant size and previous grazing. Species identity was not important; grazing selectivity was not primarily a matter of species selection. Kirby et al. (1986) observed that cattle in an eightpaddock short-duration grazing cell consumed a greater variety of species, at utilization above 10%, than cattle under continuous grazing. The unpalatable wiregrass (Aristida ramosa) can be depressed by shortterm heavy grazing on native pastures in northern New South Wales (Lodge and Whalley 1985). Earl and Jones (1996) recorded less wiregrass in cell-grazed pastures compared to continuously grazed areas nearby. In the western US, Pierson and Scarnecchia (1987) observed more uniform tiller defoliation when 124 cowcalf pairs grazed 24 ha for 12 days, five times the normal stocking rate. Within six days 90% of all tillers had been grazed at least once, and by the end of 12 days 80% had been grazed at least twice and mean tiller height had been reduced by 60%. On the other hand, improved uniformity of utilization as stock density increased was rejected by Walker et al. (1989) in Texas, although they did not observe grazing impacts at a plant or tiller level. The second major element articulated above is that utilization of a grazed paddock must be moderate, implying short grazing periods. In the cell management unit, the rest periods are automatically long (80 to 150 days) as a function of the number of small paddocks required to achieve high stock density. The grazing regime thus described, ie low defoliation frequency at moderate intensity, matches the one identified by clipping studies (Jameson 1963) as being most likely to maximize forage production. Twenty years ago Denny and Barnes (1977) published the results of a rotational grazing study in which the length of the grazing period varied from five to 10 or 20 days. At both high and low stocking rates, livestock production increased 20