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PERSPECTIVES

Reporting Forage Allowance in Grazing Experiments

^a Agronomy Dep., P.O. Box 110500, Univ. of Florida, Gainesville, FL 32611-0500
^b 5920 W. 53rd St., Stillwater, OK 74074
^c Dep. of Plant & Soil Science, Texas Tech Univ., Lubbock, TX 79409-2122
^d Dep. Zootecnia, ESALQ, Universidade de São Paulo, Piracicaba, SP, 13418-900, Brazil. Fla

^* Corresponding author (les{at}ifas.ufl.edu)

	ABSTRACT

TOP NOTES ABSTRACT Importance of Forage Allowance Overview of the Problem Specific Examples of the... An Alternative Approach REFERENCES

 
Stocking rate has a major effect on animal performance, but comparable stocking rates may result in a wide range in performance across environments because of differences in forage mass or sward canopy characteristics. Forage allowance is a function of both forage mass and stocking rate and can be a powerful tool for explaining differences in animal performance. Some methods used to express forage allowance in the literature do not allow useful comparisons across grazing methods or among management strategies within a method. In addition, many include a unit of time which violates the definition of forage allowance as a point-in-time measure. A meaningful method of reporting forage allowance is needed that applies across a wide range of pasture management treatments. This paper suggests a method that does not include a unit of time, has application across continuous and rotational stocking methods, and within the rotational stocking method applies to any size or number of pasture subunits.

	Importance of Forage Allowance

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Stocking rate is generally thought to have a greater impact on performance of grazing livestock than any other grazing management decision. When stocking rate is imposed across a relatively wide range, it has a profound effect on the forage, especially forage mass and subsequent animal performance (Burns et al., 1989). Stocking rate, however, is only indirectly related to animal performance because by definition it does not describe the quantity or quality of the sward canopy (Burns et al., 1989). As a result, similar stocking rates may result in widely different animal performance depending on forage species, forage mass, and other sward canopy characteristics. Thus, when trying to explain or predict animal performance, there may be merit in a variable that incorporates both stocking rate and a sward characteristic.

Forage or herbage allowance is such an expression, and it has been defined as "the weight of herbage (dry or ash-free) per unit of animal live weight at a point in time" (Hodgson, 1979). Forage allowance has more recently been defined as "the relationship between weight of forage dry matter per unit area and the number of animal units or forage intake units at any one point in time" (The Forage and Grazing Terminology Committee, 1992). According to these definitions, forage allowance is an instantaneous measure without a unit of time. For the purposes of this paper and because of its simplicity, animal live weight will be used in the denominator of the relationship, but it is understood that this number can be converted to animal units or forage intake units if preferred.

Forage allowance can be a very useful tool in explaining animal performance on pasture if allowance occurs across a relatively wide range, such as in fixed stocking rate experiments with multiple levels of the treatment factor. Typically, the relationship between daily gain and forage allowance is linear up to some relatively high allowance, after which gain levels off (McCartor and Rouquette, 1977; Sollenberger and Moore, 1997). The interaction of forage allowance and supplementation rate on milk production has been an important recent area of research in temperate, pasture-based dairy systems (Wales et al., 1999).

	Overview of the Problem

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In contrast to the definition, most references to forage allowance in the literature express it per unit of time, usually per day (Sheaffer et al., 1992; Maraschin et al., 1993; Dalley et al., 1999). This pattern probably originated from the expression of intake, often used in pen studies, as forage consumed per unit of body weight per day. For example, intake required to achieve a specific performance target may be 25 g of forage kg^–1 of body weight d^–1 (i.e., 2.5% of body weight).

In many cases, problems of reporting and interpretation of forage allowance in the literature can be associated with its expression per unit of time. For rotational stocking, calculation of allowance during an entire grazing period is often based on one measure of forage mass taken at initiation of grazing. If no unit of time is imposed, this measure of forage allowance is correct at only the time when grazing begins, not for the entire grazing period. When a unit of time is inserted into the calculation of allowance so as to represent an entire grazing period, the reported quantity does not meet the definition of forage allowance and, in most cases, does not accurately reflect changes in forage mass that occur over a grazing period. For example, this approach assumes that the daily decline in forage mass during a grazing period equals pregraze forage mass divided by number of days in the grazing period (Fig. 1A) . For a pregraze forage mass of 3500 kg ha^–1 and a 7-d grazing period, the amount of forage available for grazing is presumed to be 500 kg ha^–1 d^–1. This presumption does not consider either incomplete utilization or continued accumulation of forage. In reality, forage mass may quite possibly be 2000 kg ha^–1 or more during the entire grazing period (Fig. 1B). The assumption of constant rate of decline in forage mass is nearly always incorrect because canopy characteristics change over time leading to progressively lower intake throughout a grazing period (Blaser et al., 1986). Thus, it is rare, as assumed by the calculation of pregraze forage mass divided by days of grazing, that the mass of forage actually consumed in a grazing period equals the measure of pregraze forage mass.

View larger version (27K): [in this window] [in a new window]   Fig. 1. Two perspectives are illustrated that describe the decline in forage mass during a 7-d grazing period on a rotationally stocked pasture. In 1-A, forage mass is assumed to decline daily by the product of one over the number of days in the grazing period (7 in this example) and pregraze forage mass. This is generally an erroneous assumption that is made in some calculations of forage allowance. In 1-B, forage mass declines at a rate that may more nearly represent that observed in practice as forage intake and digestibility decrease during a grazing period.

Using calculation methods that include a unit of time increases the difficulty in making reasonable comparisons of forage allowance among rotationally stocked pastures with different numbers of paddocks, among different grazing methods (continuous and rotational stocking), and among continuously stocked pastures differing in number of days of grazing. A method of calculating and reporting forage allowance is needed that will allow meaningful expression of this relationship across a wide range of pasture management treatments. The objective of this paper is to propose a method of calculation and reporting that does not include a unit of time, as per the definition (Hodgson, 1979; The Forage and Grazing Terminology Committee, 1992), but does have application across grazing methods and within grazing methods where length of the grazing season (continuous stocking) or grazing period (rotational stocking) varies.

	Specific Examples of the Problem

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On rotationally stocked pastures, the equation often used to calculate forage allowance includes a term for length of the grazing period (in days) and may or may not account for forage accumulation during the grazing period (Eq. [1]).

[1]

We will apply this equation to a situation where a 1-ha pasture is divided into five, 0.2-ha paddocks with a grazing period on a given paddock of 7 d and a rest period of 28 d. For this example, we will assume a pregraze forage mass of 3500 kg ha^–1, a forage accumulation rate of 50 kg ha^–1 d^–1, and a stocking rate of five, 400-kg yearling animals ha^–1. Incorporating these data into Eq. [1] and solving, we get the following:

This answer is pleasing because it is approximately two to three times the expected intake of a grazing animal and it appears reasonable. Unfortunately, this approach has at least two flaws. The first error is that it implies that if the number of pasture subunits in the rotationally stocked pasture increases, then forage allowance also increases. Using Eq. [1] and the example cited above, forage allowance is 4.4, 5.5, and 5.8% when number of paddocks is 2, 5, and 8, respectively (note that in this calculation the rest period for a given paddock remains 28 d regardless of number of paddocks, i.e., what changes is the residency period on each paddock in a grazing cycle, and paddock area in the calculation must be adjusted based on the number of paddocks in the pasture). An increase in forage allowance simply as a result of more paddocks per pasture is not rational because we have not changed the size of the overall grazing unit (it is still 1 ha) nor the stocking rate. The second flaw is that by including a unit of time the assumption is made that forage mass decreases each day by the fraction of one over the number of days in the grazing period (Fig. 1A). As mentioned earlier, this is usually not the case, as intake is greater in the first days of a grazing period and declines as forage nutritive value and mass decline (Blaser et al., 1986).

On continuously stocked swards, the issue of time is also troublesome. Using the same forage mass and animal numbers as in the previous example, dividing 3500 kg ha^–1 by the animal live weight of 2000 kg gives 1.75 kg of forage kg^–1 of live weight. Thinking in terms of forage intakes in the range of 0.02 kg of forage kg^–1 of live weight d^–1 (i.e., 2% of live weight d^–1), this number seems unreasonable. An alternative approach is to divide this number by some unit of time to make it more nearly equal to expected intake. What unit of time should be used? If the experiment were 80 d long, then it may seem reasonable to divide by 80 and get an answer of 0.022 kg of forage kg^–1 of live weight d^–1. Another option is to divide by the length of time between forage sampling dates. If sampling were done every 28 d, then forage allowance would be 0.063 kg of forage kg^–1 of live weight d^–1. These numbers fit with the concept of supplying forage at several times the expected rate of intake, but is it rational that frequency of sampling by the researcher affects forage allowance or that forage allowance would automatically be four times greater if length of the grazing season were 20 d than if it were 80 d? The latter could be true if intake was the only factor affecting forage mass, but growth and tissue turnover are ongoing as well. Clearly, these methods of incorporating a unit of time into the calculation of forage allowance on continuously stocked pastures ignore the biology of grazing systems and are incorrect.

	An Alternative Approach

TOP NOTES ABSTRACT Importance of Forage Allowance Overview of the Problem Specific Examples of the... An Alternative Approach REFERENCES

 
Scientists often are interested in comparing livestock responses on continuously and rotationally stocked pastures (Mathews et al., 1994) as well as on rotationally stocked pastures with different numbers of pasture subunits (Mislevy and Dunavin, 1993). A standard method of reporting forage allowance, one that is applicable across grazing methods and management practices within methods, is desirable. To this end, we propose an approach that does not include a unit of time, that allows rational comparison of forage allowance across a range of grazing methods and management strategies, and that expresses forage mass and animal live weight on the basis of the entire grazing unit, not one paddock of a rotational system.

This approach uses the example of the continuously stocked pasture as its basic frame work and then builds from there to accommodate different rotational stocking strategies. The continuously stocked pasture is useful as a starting point because of the simplicity of the calculation. The entire grazing unit is accessible to all animals, so problems of pasture subunits (paddocks) are not encountered.

In this example, consider a continuously stocked pasture, 1 ha in area, and with a forage mass at sampling of 3000 kg ha^–1. At sampling, the pasture is stocked with five, 400-kg animals.

To calculate forage allowance we use Eq. [2].

[2]

Thus, forage allowance on the continuously stocked pasture is 1.5 kg of forage kg^–1 of animal live weight. Note that this is a value quite different from intake for intake must be expressed per unit of time (e.g., day, month, grazing season) and cannot be expressed as a point in time. This approach to calculation of forage allowance on continuously stocked pastures is not new; it was used in the paper of McCartor and Rouquette (1977)( Fig. 2) .

View larger version (19K): [in this window] [in a new window]   Fig. 2. Relationship of average daily gain and forage allowance (kg of forage per kg of animal live weight) for continuously stocked pearl millet [Pennisetum glaucum (L.) R. Br.] pastures. Adapted from McCartor and Rouquette (1977).

Extrapolating this method to rotationally stocked pastures is conceptually more difficult. Part of the difficulty may be the greater similarity of rotational than continuous stocking to pen feeding, where a certain allocation is provided to the animal for a given unit of time. Another issue is that forage mass generally changes much more rapidly under rotational than continuous stocking and there may be value in accounting for these changes in reporting forage allowance. Calculating forage allowance, however, does not change for rotational stocking (Eq. [2]). In reporting forage allowance for a particular grazing period under rotational stocking, it is recommended that an average forage allowance be calculated from two or more point-in-time measures during the grazing period.

As an example, consider a rotationally stocked pasture, 1 ha in area, divided into five, 0.2-ha subunits, with a pregraze forage mass of 4000 kg ha^–1 and a postgraze forage mass of 2000 kg ha^–1. The pasture is stocked with five, 400-kg animals. It is standard procedure in rotational stocking to determine pasture forage mass both before and after grazing. This facilitates the calculation of forage allowance at the beginning and end of the grazing period. In the example, animal live weight remains the same for both calculations, but the end-of-period weight could be used if the data are available. The weight change that occurs during a grazing period of typical duration (i.e., 1 d to 2 wk) is not sufficient to have a major impact on the magnitude of forage allowance.

It is when Eq. [2] is applied to rotational stocking that there are several differences between this proposed approach and that often used in the literature (Eq. [1]). In the numerator of Eq. [2] there is no accounting for days of grazing or the size of the pasture subunit. The calculation, like that for continuously stocked pastures, must be based on the entire grazing unit. In practice, a researcher may wish to sample pregraze and postgraze forage mass in more than one paddock per grazing cycle and use the average pregraze forage mass and average postgraze forage mass in the calculation of forage allowance. The data are entered into Eq. [2] as follows:

Average forage allowance can be calculated on the basis of these two instantaneous measures and equals 1.5. Thus, for an average forage mass of 3000 kg ha^–1 and the same stocking rate, the same average forage allowance is given for continuously and rotationally stocked pastures. Likewise, the answer would be the same regardless of the number of pasture subunits in the rotational method. Thus, this approach meets our condition of applicability across grazing methods and across management strategies within methods.

Two questions arise. First, does this calculation meet the point-in-time criterion for forage allowance? In fact, it is simply the average of two point-in-time measures, one at initiation and one at cessation of grazing. We routinely average point-in-time measures (e.g., forage mass) in pasture research to describe longer periods of time. Second, one might question whether this expression of average forage allowance accounts for changes in forage mass that occur during a grazing period and are associated with defoliation, growth, and senescence. We argue that it does so through inclusion of the postgraze forage mass term. Postgraze forage mass is a cumulative function of all of these processes that occur during grazing, and it has the added benefit of being a routine measurement in experiments evaluating rotationally stocked pastures.

Other issues may arise relative to use of these data. In many cases, it may be useful to present average forage allowance across a grazing season, but this may seem to be a violation of the point-in-time condition of the forage allowance definition (Hodgson, 1979; The Forage and Grazing Terminology Committee, 1992). We argue that the very same situation occurs with forage mass, another instantaneous measure (Hodgson, 1979; The Forage and Grazing Terminology Committee, 1992). Forage mass is routinely averaged across sampling dates to meet the needs of the researcher (Burns et al., 1989), but likewise it can be presented as a function of time, which is strongly recommended by others (Aiken, 1996). Forage allowance should be no different. Sampling at multiple times throughout a grazing season provides opportunity to use forage allowance to explain seasonal changes in animal performance. In other circumstances, averaging forage allowance across sampling dates may better accommodate the constraints of data presentation.

It is appropriate to note that even if the calculation method is standardized, differences in sampling methodology, especially stubble height to which forage mass samples are clipped, can result in widely different reported forage allowances for the same pasture. It would greatly aid those studying relationships of forage allowance or forage mass with animal performance if more consistent sampling methodology was used for determining forage mass. Indeed, as defined by The Forage and Grazing Terminology Committee (1992), forage mass is total dry weight of forage at a defined reference level.

The recent literature considering forage allowance comes mainly from intensively managed temperate pasture systems for dairy cows (O'Brien et al., 1997; Wales et al., 1999). In these papers, daily forage allowance is expressed as kilogram of forage per cow per day and cows were allocated a new pasture subunit daily. Treatment levels are often in the range of 20 to 70 kg cow^–1 d^–1. In the numerator of these calculations, forage mass is multiplied by paddock size to determine the allocation of forage per animal, much like one might do in an indoor feeding trial. This approach, although not truly an expression of forage allowance, results in useful relative comparisons among treatments where cattle are allocated a new pasture subunit each day. Unlike the approach that we have proposed, however, it would not permit rational comparisons across grazing methods or when number of paddocks varies within a method.

In conclusion, forage allowance can be useful in explaining and perhaps predicting animal performance on pasture. Because it integrates forage mass and stocking rate, the relationship between forage allowance and animal performance for a particular forage has potential application across a broader range of environments and situations than relationships of animal performance vs. either forage mass or stocking rate. Some methods used to calculate forage allowance are erroneous and limit the effectiveness and breadth of its use. We have proposed a simple approach that overcomes these limitations and allows rational comparison of forage allowance across a range of grazing methods and management strategies. Further, we suggest that the value of forage allowance data would be enhanced by greater standardization of forage mass sampling protocols, thus removing a major source of variation in calculations of allowance.

	NOTES

TOP NOTES ABSTRACT Importance of Forage Allowance Overview of the Problem Specific Examples of the... An Alternative Approach REFERENCES

 
Agric. Exp. Stn. Journal Series no. R-09782.

Received for publication April 7, 2004.

	REFERENCES

TOP NOTES ABSTRACT Importance of Forage Allowance Overview of the Problem Specific Examples of the... An Alternative Approach REFERENCES

 

• Aiken, G.E. 1996. Relationship between livestock output and time on pasture. p. 154. In Agronomy abstracts. ASA, Madison, WI.
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• Burns, J.C., H. Lippke, and D.S. Fisher. 1989. The relationship of herbage mass and characteristics to animal responses in grazing experiments. p. 7–19. In G.C. Marten (ed.) Grazing research: Design, methodology, and analysis. CSSA Spec. Publ. 16. CSSA, ASA, Madison, WI.
• Dalley, D.E., J.R. Roche, C. Grainger, and P.J. Moate. 1999. Dry matter intake, nutrient selection and milk production of dairy cows grazing rainfed perennial pastures at different herbage allowances in spring. Aust. J. Exp. Agric. 39:923–931.[CrossRef]
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• Maraschin, G.E., A. de Morales, L.F.A. da Silva, and J. Riboldi. 1993. Cultivated pasture, forage on offer and animal response. p. 2014–2015. In M.J. Baker et al (ed.) Proc. Int. Grassl. Congress, 17th, Palmerston North, New Zealand. 8–21 Feb. 1993. Keeling and Mundy, Ltd., Palmerston North, NZ.
• Mathews, B.W., L.E. Sollenberger, and C.R. Staples. 1994. Dairy heifer and bermudagrass pasture responses to rotational and continuous stocking. J. Dairy Sci. 77:244–252.[Abstract]
• McCartor, M.M., and F.M. Rouquette, Jr. 1977. Grazing pressures and animal performance from pearl millet. Agron. J. 69:983–987.[Abstract/Free Full Text]
• Mislevy, P., and L.S. Dunavin. 1993. Management and utilization of bermudagrass and bahiagrass in south Florida. p. 84–95. In Proc. Beef Cattle Short Course, 42nd, Gainesville, FL. 5–7 May 1993. Univ. of Florida, Gainesville.
• O'Brien, B., J.J. Murphy, J.F. Connolly, R. Mehra, T.P. Guine, and G. Stakelum. 1997. Effect of altering daily herbage allowance in mid lactation on the composition and processing characteristics of bovine milk. J. Dairy Res. 64:621–626.[Medline]
• Sheaffer, C.C., G.C. Martin, R.M. Jordan, and E.A. Ristau. 1992. Forage potential of kura clover and birdsfoot trefoil when grazed by sheep. Agron. J. 84:176–180.[Abstract/Free Full Text]
• Sollenberger, L.E., and J.E. Moore. 1997. Assessing forage allowance-animal performance relationships on grazed pasture. p. 140–141. In Agronomy abstracts. ASA, Madison, WI.
• The Forage and Grazing Terminology Committee. 1992. Terminology for grazing lands and grazing animals. J. Prod. Agric. 5:191–201.
• Wales, W.J., P.T. Doyle, C.R. Stockdale, and D.W. Dellow. 1999. Effects of variations in herbage mass, allowance, and level of supplement on nutrient intake and milk production of dairy cows in spring and summer. Aust. J. Exp. Agric. 39:119–130.[CrossRef]

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Related articles in Crop Science Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (8) Citing Articles via Google Scholar Google Scholar Articles by Sollenberger, L. E. Articles by Pedreira, C. G. S. Search for Related Content PubMed Articles by Sollenberger, L. E. Articles by Pedreira, C. G. S. Agricola Articles by Sollenberger, L. E. Articles by Pedreira, C. G. S. Related Collections Forage Management Grazing Management