J. Anim. Sci. 2005. 83:E107-E116
© 2005 American Society of Animal Science
Range management for efficient reproduction1
K. C. Olson2
Department of Animal, Dairy, and Veterinary Sciences, Utah State University, Logan 84322-4815
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Abstract
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The purpose of this article is to discuss the relationship between range management practices and reproductive performance of beef cattle. The primary axis of this relationship is via the influence of range management on the nutritional status of the cow and the concomitant effect of nutritional status on reproductive status. Abundant research on beef cattle in confinement has established the relationship of nutritional status on reproductive status. Research evaluating beef cow nutritional responses to range management is more limited, and evaluations of reproductive responses to range management are even fewer. Grazing management practices that influence beef cow performance include stocking rate, grazing distribution, and grazing systems. Stocking rate has the greatest influence, with heavy stocking rates diminishing a cow’s ability to select and consume a highly nutritious diet. This limitation on nutrient intake decreases the cow’s reproductive performance. Range management to improve grazing distribution effectively increases forage supply by enticing cows to consume underutilized forage, thereby altering the stocking rate relationship in favor of supporting more cows or higher nutrient intake by individual cows. Grazing systems that involve multiple pastures so grazing and rest can be scheduled among the pastures have variable effects on cattle performance, but they decrease performance more often than not compared with continuous grazing. The limitation of the pasture area to which cattle have access in a grazing system effectively decreases the forage supply, and can thereby decrease diet selection and nutrient intake. Alternative forages, especially complementary forages that grow in the early spring, can dramatically improve cow nutritional status during the critical pre-and postpartum periods of the annual beef cow production cycle. Supplementation, particularly the use of protein with low-quality forages, can augment beef cow reproductive status, as long as it is managed in concert with proper grazing management. In conclusion, various range management practices can have positive or negative effects on beef cow nutrition and reproduction. Range management needs to be carefully planned and implemented to ensure that it contributes to efficient reproduction.
Key Words: Beef Cattle • Grazing Management • Range Management • Reproduction
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Introduction
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This article evaluates the relationship between range management and beef cow reproduction as it currently exists based on the scientific literature. The objectives are to 1) establish a relationship between nutrition and reproduction; 2) establish a relationship between grazing management and beef cow nutrition; and 3) discuss the influence of various range management practices on beef cow nutrition and reproduction.
There are several issues that limit the ability to evaluate this relationship based on the current literature. First, much range management research has evaluated growth of steers, with less research having been conducted with cows. This leaves limited opportunity for evaluation of reproductive responses. Second, the number of cows in experiments has often been limited, decreasing the power of statistical analyses. This is particularly problematic for binomial data such as pregnancy status because binomial analyses require relatively large sample sizes to gain adequate power to detect statistical differences. Third, grazing experiments often focus on a particular season of the year, such as growing or dormant season, but events throughout the entire annual beef cow production cycle influence cow nutritional status, reproductive performance, and actual calf crop produced. As a result, events occurring during portions of the annual production cycle that fall outside the experimental period can influence or confound results, rendering interpretation difficult. Fourth, even in studies with cows or heifers, reproductive performance has often not been evaluated and reported. These limitations will be considered in the discussion. For example, nutrient status responses will sometimes be interpreted for their expected influence on reproductive performance without actual reproduction data. This is not always an appropriate assumption, as will be pointed out in examples presented later.
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Relationship Between Nutrition and Reproduction
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A great deal of research has been conducted to evaluate the influence of nutrition on reproductive performance (see companion article from this symposium by Hess et al., 2005
). Essentially all of this work has been done in confinement. To establish the relationship in brief herein, two examples from the literature will be used. Perry et al. (1991) considered the influence of NEm levels that were above and below NRC (1984) requirements during both pre- and postpartum periods in a 2 x 2 factorial design (Table 1). As would be expected, energy-deficient diets led to loss of BCS both pre- and postpartum, whereas feeding energy in excess of maintenance generally caused BCS to increase. The authors concluded that postpartum energy level determined whether the cows would return to estrus, and prepartum nutrition determined how soon those that returned to estrus would do so. Houghton et al. (1990) used a similar design to evaluate beef cow reproductive responses to pre- and postpartum energy intake. They evaluated the BCS of cows at critical junctures, regardless of the energy intake treatment that each cow was assigned to. Postpartum interval to estrus declined as BCS at parturition increased (Table 2). Additionally, fertility was higher in their study for cows maintaining or approaching a BCS of 5 than cows moving away from moderate BCS, including cows getting thinner or fatter (Table 3). It can be concluded from these and other studies that livestock management that influences nutrient status of the cow has the potential to influence her reproductive performance. Because grazing management can affect cow nutrient status, it follows that it is one component that can influence reproductive performance. It can also be concluded that BCS is a reliable indicator of nutritional status that can be used as a tool to manage for reproductive success.
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Table 1. Effects of pre- and postpartum energy level on beef cow body condition score and reproduction (adapted from Perry et al., 1991)a
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Table 2. Effect of body condition score at parturition on postpartum interval to estrus (PPI; adapted from Houghton et al., 1990)
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To manage nutritional status so that optimal levels of reproductive performance can be achieved, targets need to be established for cow BCS at critical points in the annual cow production cycle, such as those in Table 4. These can then be used to monitor management practices to ensure that they meet these goals. They will be used in the following discussion to evaluate the influence of various grazing management practices on nutritional status and reproductive performance. This approach allows one the ability to explore the effect of various research results that only cover a portion of the year/annual production cycle.
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Table 4. Appropriate expectations for body condition score at critical points in the annual production cycle of the beef cowa
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Relationship Between Grazing Management and Beef Cow Nutrition and Reproduction
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Some range management textbooks, such as Bell (1973), list four principles of grazing management. Although other range management texts don’t list these per se, their discussion of grazing management still revolves around them as the key elements of managing grazing. The four principles are 1) number of animals (stocking rate), 2) spatial distribution of grazing, 3) temporal distribution of grazing (grazing systems), and 4) kind of animals
Although these principles were originally intended for range managers to use to ensure that the land was properly managed, they are particularly valuable from the viewpoint of ensuring that the livestock are properly managed. This is because they all influence the relationship between nutrients available to be harvested by livestock and the nutrient requirements of the livestock (i.e., they are the controllers of a supply–demand relationship). The first three relationships will be discussed in the following sections. Kind of animals will not be discussed because the focus of the symposium is beef cattle.
Stocking Rate
Of the four principles, stocking rate is the most important because it directly creates the ratio of supply to demand based on the number of animals per unit of forage. The others serve as modifiers of the stocking rate influence by managing where and when the forage supply is provided to the animals. In actuality, a manager usually does not know the amount of forage available in a pasture, whereas he or she probably would know the amount of feed offered in a confinement setting. As a result, area of land available to the animal to graze is used as a proxy for amount of forage on offer. Stocking rate is expressed as units of land (i.e., hectares) per animal unit month (AUM). Animal unit month is defined as the amount of forage needed to support one cow for 1 mo, and it is usually considered to be 340 to 355 kg of DM. The quantity of forage produced per unit of land is site specific. It depends on climate, soils, topography, vegetation type, and other factors. Thus, a generalized recommended stocking rate is not possible, and recommended stocking rates to achieve cow reproductive performance goals can only be provided on a regional basis, but in actuality, stocking rate needs to be set on a pasture-by-pasture basis.
Much research has been conducted throughout the world on stocking rate (Jones and Sandland, 1974; Hart, 1978). As previously mentioned, much of this work was done using yearling cattle with growth as the measure of animal performance. One of the best evaluations of beef cow-calf performance response to stocking rates was conducted over a 25-yr period from 1932 through 1957 at the Livestock and Range Research Station near Miles City, MT (Houston and Woodward, 1966). Three stocking rates (light, moderate, and heavy) were replicated twice. Cattle grazed yearlong on two sets of pastures. The summer and winter pastures were grazed for 6 mo each. Purebred Hereford cows were permanently assigned to one of the six treatment groups and remained in it for lifetime. Cows that were culled for health or failure to reproduce were replaced with a heifer from the same group. Over the life of this project, two sets of cows were used. The first set was removed from the project in 1936 because of extreme drought. The second set of cows was assigned to treatments in 1948 as yearling heifers and remained in the experiment through 1957. Performance of this set of cows reflected accumulated influence of stocking rate on range health and productivity from initiation of the project in 1932 through the life of the cows. Mean lifetime performance of the second set of cows and their offspring reflected stocking rate (Table 5). Although calf birth weights were similar among stocking rates, calves gained less from birth to weaning under heavy stocking, as reflected in lower weaning weight for heavy stocking. Additionally, reproductive performance was reduced by heavy stocking based on calf crop percentage. Interestingly, of those cows that did produce a calf, similarity of calf birth date indicates that calving interval was not prolonged by heavy stocking. Unfortunately, this experiment predates the practice of scoring body condition as a measure of nutritional status of the cow. However, cow BW was recorded when the cows were moved from summer to winter pastures and vice versa. Because the cows comprised a uniform group, weight differences were probably reflective of differences in body condition. The cows subjected to heavy stocking were 59 kg lighter than the average of the light and moderately stocked cows at the end of summer grazing, indicating that this treatment did not prepare them to enter winter in the highest nutritional status possible, as would be desirable (Table 4
). As a result, these cows were still 31 kg lighter during the postpartum period in the spring, despite receiving more supplemental hay during the winter. The conclusion is that heavy stocking decreased the nutritional status of the cows, contributing to decreased reproductive performance.
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Table 5. Cow-calf performance responses to stocking rates, Miles City, MT (adapted from Houston and Woodward, 1966)
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Not all literature from stocking rate experiments has shown a difference in reproduction among stocking rates. In a 10-yr study on rangeland in western Oklahoma, wherein stocking rate treatments were imposed throughout year-round grazing (Gillen and Sims, 2002), stocking rate did not consistently influence cow BW (P > 0.05), but the heavy stocking rate tended to decrease calf crop percentage (P = 0.11). However, in a 5-yr study on seeded pastures in Louisiana (Bagley et al., 1987), also with year-round imposition of stocking rate treatments, cow BW was decreased throughout the year by the heavy stocking rate (P < 0.01), but pregnancy rate was not influenced by stocking rate (P > 0.05). Results are also variable in studies where the stocking rate treatments were only imposed during the growing season. In a 9-yr study on rangeland in South Dakota with stocking rate treatments imposed from early May to early November, cow BW decreased as stocking rate increased, but calf crop percentage did not vary across stocking rates (Johnson et al., 1951). A 14-yr study was conducted on California rangeland dominated by annual plants (Bentley and Talbot, 1951). The study was a factorial of three stocking rates during the growing season by two supplement treatments (with or without supplement) during the dormant season. Increasing stocking rate reduced cow BW. Pregnancy rate and calf crop percentage responses to stocking rate depended on the supplement treatment. These reproductive variables did not respond to stocking rate when the cows were supplemented during the dormant season, but both reproductive variables were reduced by the high stocking rate if the cows were not supplemented. Apparently, improved dormant season nutrition ameliorated the stocking rate effect; however, unsupplemented cows could not compensate for the negative influence of heavy stocking on reproductive performance. Despite the variation in results among studies, it seems more probable than not that reproductive performance will respond to stocking rate in rangeland settings if other livestock management practices are not imposed to ameliorate the stocking rate effect.
We conducted an experiment during two summer grazing seasons to evaluate the nutritional response of lactating beef cows to stocking rates on irrigated pastures at the Utah Agricultural Experiment Station located near Panguitch in south-central Utah (Eddington, 2001). Diet samples were collected in the evacuated rumen (Lesperance et al., 1960; Olson, 1991) of ruminally fistulated cows to evaluate diet nutritional quality across stocking rates. Fecal output was estimated using chromic oxide as an external marker so forage intake could be compared among stocking rates. Nutritional response variables were regressed on stocking rate (expressed as AUM/ha). In early July, when forage was plentiful in all pastures, digestibility did not respond to stocking rate, as displayed in a nonsignificant (P = 0.58) slope for the regression of diet digestibility on stocking rate (Figure 1). However, in late August, it was visually apparent that higher stocking rates had substantially reduced forage availability. This decreased the cows’ ability to select their diets. As a result, diet digestibility was decreased (P = 0.01) by 0.56% for each additional AUM/ha (Figure 1). In turn, this was reflected in the change in BCS of the cows over the grazing season (Figure 2). During the first half of the grazing season (designated July in Figure 2), all cows maintained BCS, with no difference among stocking rates (P = 0.47). During the last half of the grazing season (designated August on Figure 2), cows at the lightest stocking rate maintained BCS, but cows progressively lost more body condition as stocking rate increased (P = 0.0001). Again, the conclusion is that heavier stocking rates mean that cows enter winter in poorer nutritional status (Table 4). Because of the small number of cows used in this study and intensive management during the remainder of the year, we did not evaluate reproductive performance.
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Figure 1. Influence of stocking rate on diet digestibility during the first (July) and second (August) halves of the grazing season. Actual = actual data points; predicted = the regression line through the actual data points. Adapted from Eddington (2001). AUM = animal unit month.
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Figure 2. Influence of stocking rate on change in BCS during the first (July) and second (August) halves of the grazing season. Actual = actual data points; predicted = the regression line through the actual data points. Adapted from Eddington (2001). AUM = animal unit month.
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Grazing Distribution
Animals do not distribute themselves evenly across the landscape. A large variety of abiotic and biotic influences either attract or distract animals from utilizing forage in particular areas (Bailey et al., 1996). However, management practices such as fencing, herding, water developments, supplement placement, burning, and fertilizing can be used to improve the distribution of livestock (Bailey, 2004). Several recent studies have evaluated the effect of some of these management practices on grazing distribution (Ganskopp, 2001; Porath et al., 2002; Bailey et al., 2004). If management that improves grazing distribution causes animals to consume forage in areas that had previously been underutilized, it essentially increases the forage supply, thereby modifying the supply–demand relationship. This could potentially allow either an increase in stocking rate or an increase in performance at a given stocking rate if a given stocking rate were high enough to limit diet selection. For example, Porath et al. (2002) evaluated development of water sources in upland areas as a method to decrease overutilization of riparian zones during summer grazing. The cattle with upland water sources altered their utilization of the landscape with less use in the riparian zone and more utilization of the upland vegetation types compared with cattle that had access only to water in the stream. Cows with access to upland water developments gained more weight (29.2 kg) over the summer grazing season than did cows with water in the stream only (17.7 kg; P < 0.01). Additionally, calf ADG was 1.01 or 0.87 kg with or without offstream water, respectively (P < 0.01). It can be concluded that performance was enhanced by spreading forage utilization across the landscape so that availability of forage was increased for each cow; however, cow BCS did not respond (P > 0.05) to improved grazing distribution in this study. Change in BCS over the 42-d grazing period was 0.18 vs. 0.09 units for cows with or without offstream water, respectively. One must wonder whether a statistical difference in BCS would have been detected if the grazing period had lasted longer. Assuming that the weight difference is a function of a difference in nutritional status, improved grazing distribution during the summer grazing season could contribute to higher BCS in the fall (Table 4).
Grazing Systems
Grazing systems are defined as planned strategies of providing periods of rest or grazing. The intent is for the periods of rest to allow the vegetation to recover from the periods of grazing. They are generally implemented by providing multiple pastures so that herds of livestock can be moved among pastures to attempt the above controls. Many grazing systems have been developed based on combinations of numbers of pastures, numbers of herds, length of grazing and rest periods, size of pastures, and other variables. The potential number of combinations is essentially infinite. For the purpose of this discussion, grazing systems are classified into two broad categories. The first category includes traditional systems. These are systems that were conceived longer ago and therefore have a longer history of use, thus the designation as a "traditional" system. They involve few pastures (generally less than six) and infrequent rotation among pastures (rotation on a seasonal basis). The other category includes intensive systems. These systems were conceived and have become more popular recently than traditional systems. They involve more pastures (generally at least eight, but recommendations can be for 60 or more; conceptually, more is better) and frequent rotation among pastures (hourly to a few days). They are considered intensive because they require a greater commitment to manage the resources and frequent movement of livestock.
Two previous literature reviews (Driscoll, 1969; Pieper, 1980) were completed on livestock response to traditional grazing systems. Both reviewed literature that compared a traditional grazing system to continuous grazing. Continuous grazing was based on one herd of livestock in one pasture so there was no rotation among pastures to allow periods of grazing and rest. Both reviews indicated that the vast majority of research was conducted with young, growing animals, and very little has been done with cows to allow for the opportunity to evaluate reproductive responses. Driscoll (1969) reported that of 29 studies that he reviewed in which livestock weight gain was reported, 12 studies indicated better gain under continuous grazing than the rotation system, nine studies displayed no difference in gain between grazing systems, and only eight studies reported greater gain under the rotation than continuous grazing. Pieper (1980) found similar results in 24 studies of gain in young cattle that he reviewed: 14 studies showed greater gains under continuous grazing, four studies showed no difference, and six studies showed improved gains under the rotational systems. In summary of these two literature reviews, half exhibited an advantage to season-long or continuous grazing, one-quarter showed no difference, and the remaining one-quarter of the studies showed an advantage to the grazing system. If one assumes that these weight gain data would be reflective of nutritional status in beef cows, one would expect that traditional grazing systems lower cow BCS, with a possible concomitant reduction in reproductive performance. Wood (2003) conducted a comparison of continuous grazing with two traditional grazing systems, deferred rotation and rest rotation, during summer grazing on high-elevation, forested rangeland. Deferred-rotation was based on two pastures and one herd that was rotated at the midpoint of the grazing season. The order in which the pastures were grazed and rested was switched each year. Rest rotation was based on three pastures and one herd. One pasture was rested for the entire grazing season, and the other two were grazed for half the season in a manner similar to deferred-rotation. Over a 3-yr period, each pasture in rest-rotation grazing was subjected to each grazing/rest period. The experiment was conducted for 6 yr. Cow ADG and final weight at the end of the grazing season were both lower (P < 0.05) under deferred- and rest-rotation compared with continuous grazing (Table 6). Over the grazing season, cows increased BCS by a similar amount (P = 0.34) in all grazing treatments (Table 6). Like Porath et al. (2002), one must wonder whether a statistical difference in BCS would have been detected if the grazing period had lasted longer or if the system of scoring body condition were more sensitive to small changes in BCS. Results of this study agree with the majority of studies in the reviews by Driscoll (1969) and Pieper (1980) that grazing systems decrease cattle performance. This would be counter to the goal in Table 4 if one assumes that the difference in BW reflects a difference in nutritional status, with the result being lower nutritional status at the end of the summer grazing season. Traditional grazing systems were designed to improve rangeland health with little consideration for their influence on livestock performance (Pieper, 1980). Restricting the animals to only a portion of the land and leaving them there for lengthy periods of time effectively increases the stocking rate for the period they are in each pasture. This changes the supply–demand relationship and decreases the animals’ ability to select a diet. Additionally, forage matures and loses nutritional value in pastures that are ungrazed for lengthy periods of time, further decreasing the ability to select highly nutritious forage when livestock are rotated into those pastures.
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Table 6. Cow performance response to continuous, deferred-rotation, and rest-rotation grazing on high elevation, forested rangeland (adapted from Wood, 2003)
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Theoretically, animal performance should improve under intensive grazing systems for at least two interrelated reasons. First, rapid rotation through the pastures is expected to keep the vegetation cropped regularly so that it remains in an immature state wherein it will have high nutrient content. The high stocking density caused by having all animals in one small pasture is intended to ensure that this effect is uniform across all plants. The expected result is that the nutritional value of the entire crop of herbage is improved. This high and uniform nutritional quality theoretically overrides the potential decrease in diet selectivity associated with the high stocking density. Second, rapid rotation is also expected to improve diet selectivity because the animals are being regularly rotated to a new pasture before the pasture they are in has been heavily grazed.
Intensive grazing systems were heavily investigated by many research institutions in the western U.S. and elsewhere in the 1980s. Most of the research is difficult to interpret because two variables were confounded in the experimental design. This is because some of the proponents of intensive grazing systems in that era recommended that stocking rate had to be dramatically increased when the rotation was implemented. Many researchers increased stocking rate per this recommendation when they implemented the rotation grazing treatments, but left the stocking rate at a moderate level in the continuous treatment. Thus, it is impossible to determine whether livestock responses were due to the effect of stocking rate, rotational grazing, or a combination of both. An example wherein stocking rate was the same in both the continuous and rotation treatments was Olson and Malechek (1988; Table 7). Yearling replacement heifers grazed crested wheatgrass (Agropyron desertorum) seeded rangeland in central Utah in one of two treatments, continuous grazing or a 10-pasture system wherein they were rotated about every third day. The study was repeated for 3 yr. The heifers were grazing in the systems during the breeding season. Esophageally fistulated heifers were used to collect samples of the diet selected. Heifer performance, diet quality, and forage intake were similar between the two grazing systems (Table 7). The level of performance and plane of nutrient intake suggested that both sets of heifers were in a positive nutritional status and should have been reproductively sound; however, fall pregnancy rate based on rectal palpation was lower for the heifers that had been in the rotational grazing treatment. Table 7 presents the 3-yr means, but the responses were similar within each of the years. This is an example wherein positive nutritional status does not necessarily equate into improved reproductive performance. The reason for the decreased pregnancy rate was not clear. However, there are two possible explanations. First, we evaluated daily changes in diet quality over the 3 d that heifers were in each pasture in the rotation system (Olson et al., 1989; Table 8). Diet quality significantly decreased over the 3-d period. It is unknown what influence this cyclic effect on nutrient intake could have on metabolic variables. Second, an animal behaviorist evaluated the social behavior of the heifers in both systems (D. F. Balph, Utah State University, Logan, UT, unpublished data). He concluded that the heifers in the rotation grazing treatment displayed evidence of stress associated with crowding because the density of heifers was 10 times greater in the rotational pastures than in the continuous pasture. He also noted undue aggressive behavior among the bulls in the rotational grazing treatment, and attributed this to increased bull-to-bull contact and fighting near heifers in estrus under the more crowded conditions. It is unknown whether these stressors were great enough to decrease fertility in the heifers or impact the ability of the bulls to mate with the heifers.
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Table 7. Heifer performance and nutritional responses to continuous or intensive, rotational grazing systems on crested wheatgrass range in central Utah (adapted from Olson and Malechek, 1988)
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Table 8. Daily change in nutrition within pastures in an intensive, rotational grazing system on crested wheatgrass range in central Utah (adapted from Olson et al., 1989)
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Another grazing system example is a study that was conducted on seeded pasture in Tennessee (Chestnut et al., 1992). Intensive rotational grazing using seven pastures was compared with continuous grazing at equal stocking rates during the growing season for 4 yr. All cow responses did not differ (all P > 0.05) among treatments, including pregnancy rate.
Research conducted at Throckmorton, TX, allowed for the simultaneous evaluation of grazing systems and stocking rate (Table 9). Continuous grazing was evaluated at both moderate and heavy stocking rates. Deferred-rotation was evaluated at a moderate stocking rate, and intensive rotational grazing was evaluated at a heavy stocking rate. Although grazing systems differed between the two stocking rates, one can still compare two systems (with or without rotation) at two rates. Diet quality (McKown et al., 1991) and forage intake (Walker et al., 1989) were only measured in the moderate, continuous, and intensive rotation treatments. Nutrient intake was lower under intensive rotation, but one cannot conclude whether that was because of the rotation or the heavy stocking rate. However, fall pregnancy rate was similar in both moderately stocked treatments and in both heavily stocked treatments, but was higher in the moderately stocked treatments than in the heavily stocked treatments (Heitschmidt et al., 1990). The conclusion is that stocking rate provides the primary influence on cow nutritional status and reproductive performance, and grazing system provides a minor role.
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Table 9. Cow responses to combinations of stocking rates and grazing systems in north Texas (adapted from Walker et al., 1989; Heitschmidt et al., 1990; McKown et al., 1991)
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The implication of the results of these studies is that cattle response to grazing systems is highly variable, with the potential for negative effects on reproductive performance; however, this does not negate the value of grazing systems for other purposes. They are important to management of rangeland health, wildlife habitat, watershed characteristics, and other uses and values of rangeland. The conclusion is that the choice and management of a grazing system must provide for those rangeland needs while simultaneously minimizing negative effects on the livestock.
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Alternative Forages
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Another means of improving the nutritional status of beef cows at critical periods in the annual production cycle is to provide an alternative forage resource that is nutritionally superior to native range. This is based on the concept of seasonal suitability, which is defined as grazing vegetation types during the season when they provide highest nutrition and are most tolerant of grazing. A common way of providing this is by using a complementary forage, which can be defined as any crop that will grow at a different time of year than native range. Examples of complementary forages include perennial, cool-season grasses that green up earlier in the spring than native range such as crested wheatgrass and Russian wildrye (Psathrostachys juncea). By greening up earlier, they provide nutritious forage when native range is still dormant and low in quality. This often occurs during the pre- and postpartum periods that are nutritionally critical for the beef cow. One example of beef cow responses to spring complementary forages is an experiment that was conducted at the Livestock and Range Research Station near Miles City, MT (Houston and Urick, 1972). Three treatments were evaluated: 1) cattle that spent the entire year on native range; 2) cattle that grazed spring pasture comprising a crested wheatgrass–alfalfa (Medicago sativa) mixture or 3) cattle that grazed spring pasture comprising a Russian wildrye–alfalfa mixture for 6 wk (late April to early June), then spent the rest of the year on native range. Like the previous example from Miles City, the research predates the practice of scoring cow body condition as it is currently done. Also like the previous example, this was a uniform group of purebred Hereford cows, so differences in BW probably reflected differences in nutritional status. Interestingly, not only did the seeded pastures increase cow BW gain over native range while the cows grazed the spring pastures, but the cows that had been in both complementary forage treatments continued to gain at a more rapid pace after they had returned to native range (Table 10). The reason for this is unclear, but perhaps the improved nutrition on the spring pastures allowed them to reach their nadir in body condition sooner and they were capable of utilizing nutrients from the summer native range more efficiently because they had been in a positive nutrient balance for longer in the spring. Pregnancy rate was 5% higher for the complementary forages than for native range (statistical significance was not indicated). Calf crop percent tended (P = 0.06) to be higher for complementary forages than for native range (90.8 vs. 81.7%, respectively).
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Table 10. Cow performance responses to spring complementary forages, Miles City, MT (adapted from Houston and Urick, 1972)
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The response to crested wheatgrass and Russian wildrye as spring complementary forages in other studies has not always agreed with the previous study. In a study that was also conducted at Miles City, MT, Adams et al. (1989) found that complementary forage treatments did not influence pregnancy rate, cows in estrus before the breeding season, or date of calving (P > 0.05) compared with native range only. This occurred despite a greater increase in BCS by cows grazing complementary forages than by cows grazing native range. In another study, Hart et al. (1983) found that cow BW and pregnancy rate were similar between crested wheatgrass as a complementary forage and native range. The authors of these two publications suggested that complementary forages are still valuable for increasing carrying capacity, even though they did not affect animal performance in their studies. It is unclear why responses were different among these three studies. Differences in stocking rates or a variety of other variables could have influenced the results.
We evaluated nutrient intake responses to spring complementary forages using two varieties each of crested wheatgrass and Russian wildrye compared with native range (K. C. Olson, Utah State University, Logan, UT, unpublished data). The study was conducted during 2 yr on rangeland in central Utah. Diets were collected using esophageally fistulated cows to estimate diet digestibility, and chromic oxide was used as an external marker to estimate fecal output so forage intake could be calculated. All complementary forages increased (P < 0.05) diet digestibility and forage intake compared with native range (Table 11), and the Russian wildrye varieties supported greater (P < 0.05) digestibility and intake than the crested wheatgrass varieties. Reproduction response was not evaluated in this study because the number of cows was too small to provide adequate statistical power.
In conclusion, provision of complementary forages, particularly during the pre- and postpartum periods in the annual beef cow production cycle, can be a powerful tool to improve the nutritional status and reproductive performance of the cow.
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Supplementation
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Another tool to improve the nutritional status of the cow so it will be reproductively sound is supplementation. Supplementation can be thought of in two broad categories. The first includes classic strategies wherein a deficient nutrient, such as protein or energy, is provided. The second category involves the use of specific classes of nutrients, such as undegradable intake protein or specific fatty acids as metabolic triggers to induce desired responses. The discussion herein will focus on the first category only because that is related more to range management than the second.
When forage quality is low, such as grazing dormant rangeland in the winter, protein is the recommended nutrient to supplement. When crude protein is deficient in the forage (generally less than 7%), supplemental protein causes a positive associative effect, meaning not only does it overcome the protein deficiency, but it also stimulates increased utilization of nutrients from the basal forage. This is because the supply of protein N to the ruminal microorganisms stimulates microbial growth and fermentation of the structural carbohydrates in the low-quality forage. In effect, both the protein and energy status of the host ruminant are elevated by protein supplementation. In a recent review of research on protein supplementation of low-quality forages, DelCurto and Olson (2000) reported that protein supplementation increased digestion and intake of forage in 11 studies, had no effect in one study, and decreased forage intake in one study. In terms of performance of beef cows consuming low-quality forage, they reported that protein supplementation reduced BCS loss in mature cows (five studies) and increased growth in heifers (two studies). They found three studies that reported reproductive responses to protein supplementation. One study with multiparous cows reported no response, whereas another indicated a shortened calving interval. Another study with heifers indicated greater conception rate.
Conversely, when fed with low-CP diets, energy supplementation without correction of the ruminal protein deficiency tends to cause a negative associative effect (decreased forage intake and digestibility), particularly when the energy source is nonstructural carbohydrate-based, such as cereal grains (Bowman and Sanson, 2000). Energy sources that are structural carbohydrate based, including milling by-products such as soy hulls, wheat middlings, or sugar beet pulp, tend to have a neutral to slightly positive associative effect. In the case of both nonstructural and structural carbohydrate sources, the type and level of associative effect expressed is dependent on supplement intake and forage quality (Bowman and Sanson, 2000). In general, higher levels of supplement and higher quality of forage increases the probability that a negative associative effect will occur.
Grazing management plays an important role for effective supplementation. Stocking rate is important because an adequate amount of forage must be allowed for a positive associative effect to be expressed. In other words, a cow cannot increase forage intake when there is little or no forage available to consume. An appropriate stocking rate should minimize substitution of supplement for forage.
Management of grazing distribution and supplementation interact with each other. Supplement placement can improve distribution (Bailey and Welling, 1999). Alternatively, care must be used to not limit supplement availability and utilization by placing the supplement in areas that are so inaccessible or underutilized that cattle never go to the supplement.
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Implications
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Range management affects the nutritional status of grazing beef cattle, which can in turn influence reproductive performance. Management practices need to be designed and monitored to ensure that body condition score targets are met at key times in the annual production cycle of the beef cow to ensure return to estrus and fertility at the beginning of each breeding season. Research opportunities abound to improve our understanding of the axis from range management to nutritional status to reproductive capability. Future grazing research needs to be conducted in integrated systems to evaluate the effect of seasonal range management practices on the annual production cycle of the beef cow. Besides evaluating the nutritional and reproductive responses of the cattle, the forage resource also needs to be characterized, such that all components of this axis are evaluated. Only then will the mechanistic relationships among all components be elucidated. The minimum characterization of the vegetation should be measurement of the available herbage for the cow to select from.
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Footnotes
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1 Presented at the Western Section of the American Society of Animal Science meeting as part of the Beef Cow Symposium "Reproductive Management for Extensive Environments," June 16, 2004. 
2 Correspondence—phone: 435-797-3788; fax: 435-797-2118; e-mail: ken.olson{at}usu.edu.
Received for publication July 16, 2004.
Accepted for publication December 2, 2004.
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Abstract
Introduction
