J. Anim. Sci. 2005. 83:2235-2242
© 2005 American Society of Animal Science
Effects of stocking and nitrogen fertilization rates on steers grazing dallisgrass-dominated pasture1
S. A. Gunter2,
P. A. Beck,
S. Hutchison and
J. M. Phillips
Southwest Research and Extension Center, Division of Agriculture, University of Arkansas, Hope 71801
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Abstract
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To compare the performance of steer calves managed under different stocking rates (SR; 3.7, 6.2, 8.6, and 11.1 steers/ha for 140 d; 
I1) and N fertilization rates (112, 224, and 336 kg of N/ha; I2) in May 1996, 1997, and 1998, 72 steer calves (BW = 231 ± 2.5 kg) were assigned randomly to one of 12 0.81-ha dallisgrass (51%)/common bermudagrass (32%) pastures. One-third of the fertilizer was applied in the form of ammonium nitrate in May, June, and August to achieve the prescribed totals. Treatments were separated using a polynomial regression equation: 
i = ß0 + ß1I1 + ß2I2 + ß112I1 + ß122I2 + ß12i1i2 + 
I, with years as replicates. Within the range of the data, ADG and BW gain per steer were greatest at a stocking rate of 3.7 steers/ha and 336 kg/ha of N. Body weight gain per hectare peaked at 701 kg when cattle were stocked at 8.9 steers/ha and the pasture was fertilized with 336 kg/ha of N. The least cost of production was at a stocking rate of 3.7 steers/ha, with 112 kg/ha of fertilizer N applied, and the greatest cost of production was at a stocking rate of 11.1 steers/ha with 336 kg/ha of fertilizer N applied. Fertilization at 336 kg/ha of N produced the most profitable stocking rate at 7.3 steers/ha and returned $355.64. The optimal stocking rate for net return was 79, 81, and 82% of that for maximum BW gain per hectare for 112, 224, and 336 kg/ha of N, respectively. Under the assumptions made in the financial analysis, these data show that the economically optimal carrying capacity of similar pastures can be increased with N fertilizer up to at least 336 kg/ha annually.
Key Words: Cattle • Nitrogen Fertilizer • Paspalum dilatatum • Stocking Rate
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Introduction
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Stocking rate is a fundamental variable for management that affects vegetation, livestock, and economic responses (Bernardo and McCollum, 1987
; Gillen and McCollum, 1992; McCollum et al., 1999). Effects of light to heavy stocking rates are often clear because of differences in residual herbage; however, effects on livestock performance and economic returns are not easily observed (Gillen and McCollum, 1992; McCollum et al., 1999). It was established that stocking rate affects ADG (Guerrero et al., 1984; Bransby et al., 1988; Gillen et al., 1992). At light stocking rates, ADG is maximized, but a heavier stocking rate maximizes BW gain per hectare. Maximum net return per hectare usually occurs between 55 to 60% of the stocking rate that produces maximal BW gain per hectare (Hart et al., 1988).
Dallisgrass (Paspalum dilatatum Poir.) is one of the important forage grasses in the humid southeast. It has a higher CP concentration and in vivo DM digestibility and produced greater BW gains for stocker cattle than bermudagrass (Cynodon dactylon [L.] Pers.; Gil-Beroes et al., 1971; White and Hembry, 1985). However, responses of dallisgrass to N fertilization are not well understood (Watson and Burson, 1985). Stringer et al. (1994) showed that increasing the N fertilization rate from 0 to 448 kg/ha of N annually stimulated a 206% increase in forage DM production by common bermudagrass.
Bransby et al. (1988) proposed that the function describing the relationship between stocking rate and animal performance is unique for each forage type. Moreover, because of the expected increase in forage DM production from increasing the rate of N fertilization, one assumes that the carrying capacity (the stocking rate giving maximum output) also should be increased. The purpose of our experiment was to examine the effects of increasing levels of N fertilization and stocking rate on BW gain and economic performance of stocker cattle grazing dallisgrass-dominated pastures.
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Materials and Methods
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Research Site
This experiment was conducted at the Southwest Research and Extension Center (lat 33°42'N, long 93°31'W) near Hope, AR, on twelve 0.81-ha pastures over a 3-yr period (1996 through 1998). The soil type of the 12 pastures was Una silty clay loam, which consists of deep, poorly drained, level soils (slopes, 0 to 1%) on a flood plain (Hoelscher and Laurent, 1979). This soil type has a seasonally high water table in the winter and spring, and is predicted to produce approximately 18.5 animal-unit-months/ha annually. The swards in the 12 pastures were primarily dallisgrass (50.5%), but also contained common bermudagrass (31.5%), tall fescue (Festuca arundinacea Schreb.; 5.6%), other grasses (4.5%), white clover (Trifolium repens L.; 3.3%), and other forbs (4.6%) as determined by the dry-weight-rank method (Gillen and Smith, 1985). Precipitation at the Southwest Research and Extension Center weather station, located approximately 3.1 km southwest of the pastures, was 103, 110, and 105 cm from January 1 through September 26, 1996, 1997, and 1998, respectively (Figure 1). The 21-yr average precipitation from January 1 through September 26 was 97 cm (average annual precipitation = 137 cm). Rainfall accumulations during the three 140-d grazing periods were 74, 40, and 52 cm (average rainfall during the grazing period = 50 cm) in 1996, 1997, and 1998, respectively. Average daily temperatures during the grazing period were 25, 25, and 27°C in 1996, 1997, and 1998, respectively (21-yr average daily temperature during grazing period = 24°C).
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Figure 1. Accumulated rainfall (bold lines) and average daily temperature (fine lines) from May 6 through September 27 by year at the Southwest Research and Extension Center in Hope, AR.
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Experimental Procedures
Each year, 72 steer calves (Bos taurus x Bos indicus; maximum 1/8 B. indicus; average BW = 231 ± 2.5 kg) were obtained through a local cattle buyer (F & F Cattle Co., Hope, AR). After a 21-d receiving period, starting at 0630, the steers were stratified by BW and divided randomly into 12 groups. The groups of cattle were assigned to one of the following 12 treatments in a 4 x 3 factorial arrangement: 3.7, 6.2, 8.6, or 11.1 steers/ha and 112, 224, or 336 kg/ha of N (i. e., 336, 672, or 1,008 kg of ammonium nitrate was applied; 12 treatments; one replication per treatment annually). Fertilizer was applied three times (in equal amounts) during the grazing season at 52-d intervals beginning on May 6 to total the amounts prescribed. During the first fertilization event of each year, pastures also were fertilized with soil-test recommendations of P and K (Chapman, 1998). The planned grazing season was 140 d, starting on May 6, 1996, May 7, 1997, and May 7, 1998. At 35-d intervals thereafter, cattle were then weighed unshrunk at 0630. Because of insufficient standing herbage mass (<5.1 cm tall) in pastures as a result of increasing stocking rate, cattle were removed from some pastures earlier than the planned 140 d, at which time BW and removal date were recorded. On the first day of grazing, the steers were implanted with Implus-S (Ivy Laboratories, Inc., Overland Park, KS) and again on d 105. On a weekly basis, cattle were provided 0.76 kg/steer of a mineral/salt mixture (Vigortone 32S; PM Ag Products, Inc., Cedar Rapids, IA; contained [% as-fed basis]: 18.2% salt, 13.6% Ca, 7.0% P, 10 ppm I, 2.6 ppm Se, trace minerals [Co, Cu, Fe, and Zn], 662,000 IU of vitamin A/kg, 66,000 IU of vitamin D3/kg, and 221 IU of vitamin E/kg).
Cost per steer estimates included pasture rent, fertilizer, price slide, interest, minerals, receiving feed, veterinary service and supplies, transportation, and death loss (Table 1). Pasture rent was based on an annual cost of $61.73/ha, and it was assumed that the stocker cattle enterprise used 70% of the lease ($43.21) value in proportion to the available grazing. The remaining 30% of the pasture lease was assumed to be used by backgrounding cattle grazing residue herbage and winter annual grasses. The annual pasture rent charge was divided by the stocking rate, resulting in a decrease in pasture rent per steer as stocking rate increased. Price slide from the original purchase weight of the steers was assumed to be $0.007/kg of gain based on a 10-yr (1991 to 2000) summary of livestock market prices from Arkansas, and also was based on steer calves starting at 215 kg in April, gaining approximately 100 kg, and selling in September (Cheney and Troxel, 2004). When interest was calculated, we assumed that the cattle would be owned for 5 mo and an annual rate of 10.00%. Veterinary services and supplies included treating sick cattle, implants, and vaccines. Death loss was assumed to be 1.0% of the value of the steers. The selling price of the cattle was assumed to be $1.66/kg based on a 10-yr (1991 to 2000) average (Cheney and Troxel, 2004). Fertilizer was assumed to cost $220.06/t for ammonium nitrate based on a 10-yr (1991 to 2000) average calculated with data from NASS (1995, 1999, 2004).
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Table 1. Assumed cost required to graze stocker steers stocked at different rates on dallisgrass pasture fertilized at three different rates during the summer
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Statistical Analyses
First, the effects of stocking rate (steers per hectare) and N fertilization (kg/ha of N) on grazing days, ADG, total BW gain per steer, and BW gain per hectare were analyzed by ANOVA using the PROC GLM procedures of SAS (SAS Inst., Inc., Cary, NC) to more fully describe the data before analysis by regression. Pasture was used as the random experimental unit with replication in time. Least squares means were separated using linear and quadratic contrasts.
Second, the effects of stocking rate and N fertilization rate on grazing days, ADG, total BW gain per steer, BW gain per hectare, total cost per hectare, gross return per hectare, and net return per hectare were separated by using polynomial regression as described by Bransby et al. (1988). These data were fitted to the following model for two independent variables: i = ß0 + ß1I1 + ß2I2 + ß112I1 + ß122I2 + ß12i1i2 + I, for which I1 equals stocking rate and i2 equals N fertilization rate using year as replicates (random effect) with the PROC REG procedure of SAS. The maximum dependent variable was estimated by setting the first derivatives equal to zero for the constructed equations, after which the maximum value of the dependent variable was used to solve for the optimal stocking rates.
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Results and Discussion
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Cattle Performance
As analyzed by ANOVA, the effects of stocking rate did not interact (P > 0.15) with N fertilization rate for days of grazing, ADG, total BW gain, or BW gain per hectare (Table 2). Increasing stocking rate linearly decreased (P < 0.01) the number of grazing days; however, increasing N fertilization rate linearly increased (P < 0.02) the number of grazing days. Average daily gain was decreased (P < 0.01) by increasing the stocking rate, but increasing N fertilization rate did not increase (P = 0.08) ADG. Increasing stocking rate resulted in quadratic decreases (P < 0.01) in the total BW gain per steer, whereas increasing N fertilization rate linearly increased (P < 0.01) the total BW gain per steer. Increasing stocking rate quadratically decreased (P < 0.01) the BW gain per hectare, and increasing N fertilization rate linearly increased (P < 0.01) the BW gain per hectare.
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Table 2. Least squares means of grazing days and performance by stocker steers stocked at different rates grazing dallisgrass pasture fertilized with different rates of nitrogen during the summer
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Using regression analysis, the equation developed for grazing days explained 61% of the variation (P < 0.01; Table 3). In addition, stocking rate interacted (P = 0.03) with N fertilization rate. It was noted that increasing the stocking rate from 3.7 to 11.1 steers/ha decreased the predicted number of days that cattle could be grazed from 138 to 86, or by 38.2%, when 112 kg of N/ha was applied (Table 4). Conversely, at the N fertilizer rate of 224 kg/ha, increasing the stocking rate from 3.7 to 11.1 steers/ha only decreased the predicted number of days that cattle could be grazed from 144 to 111, or by 22.9%. At the N fertilizer rate of 336 kg/ha, increasing the stocking rate from 3.7 to 11.1 steers/ha decreased the predicted number of days from 137 to 121, or by only 11.7%. The application of N fertilizer increased the number of grazing days on a pasture and decreased the probability that cattle might need to be removed early during the grazing season. Analysis of research from northwest Oklahoma with steers grazing old-world bluestem (Bothriochloa ischaemum [L.] Keng.) has shown an increase in the number of grazing days by 0.66 for each kg of N applied as fertilizer (r2 = 0.71; Berg and Sims, 1995).
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Table 3. Predicted grazing days and physical and financial performance by stocker steers stocked at different rates grazing dallisgrass pasture fertilized with different rates of nitrogen during the summer
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Table 4. Predicted grazing days and performance by stocker steers stocked at different rates grazing dallisgrass pasture fertilized with different rates of nitrogen during the summer
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The equation developed for ADG explained 51% of the variation noted in animal performance (P < 0.01; Table 3
). Stocking rate did not interact (P = 0.56) with N fertilization rate. Average daily gain decreased as stocking rate increased (Table 4). For each unit increase in the stocking rate, ADG decreased by 0.034 kg (r2 = 0.86). Research trials with similar experimental designs have shown responses between ADG and stocking rate that are comparable to that noted in the present study (Adjei et al., 1980; Guerrero et al., 1984; Bransby et al., 1988). Adjei et al. (1980) reported that with cattle grazing at three different stocking rates averaged over three stargrass (Cynodon spp.) varieties, ADG decreased 0.04 kg/d (r2 = 0.94) for each unit increase in stocking rate. A study examining five varieties of bermudagrass (Guerrero et al., 1984) showed a similar decrease in ADG as stocking rate increased (R2 = 0.67).
Average daily gain was increased as more N fertilizer was applied (Table 4). Each unit increase in the N fertilizer rate increased the ADG by 0.0004 kg (r2 = 0.75); however, the ANOVA procedure did not (P = 0.08) detect an interaction between N fertilization and stocking rates (Table 2). We are currently unaware of any studies examining the effects of N fertilizer rate at a set stocking rate; however, one study examined the effects of N fertilizer rate with a variable stocking rate that was designed to equalize forage allowance across treatments on old-world bluestem (Berg and Sims, 1995). These researchers reported no average differences in ADG over a 3-yr period, but stocking was doubled as N fertilization rate was increased from 0 to 170 kg/ha of N annually. This increased grazing pressure applied in the Berg and Sims (1995) study would lessen the expected increase in ADG that would be noted with increased N fertilization rate.
The equation developed for total BW gain explained 74% of the variation (P < 0.01), and stocking rate did not (P = 0.14) interact with N fertilization rate (Figure 2). Total BW gain decreased as stocking rate increased (Table 4). For each unit increase in the stocking rate, total BW gain decreased by 6.8 kg (r2 = 0.91). Research trials with similar experimental designs have shown responses between total BW gain and stocking rate similar to ours (Riewe, 1961; Riewe et al., 1962; Petersen et al., 1964). Riewe et al. (1961) reported in a review of 12 studies that cattle grazing at different stocking rates, total BW gain per steer decreased (r < –0.96) for each unit increase in stocking rate. In a following study with steers grazing annual ryegrass (Lolium multiflorum Lam.) or tall fescue, Riewe et al. (1962) reported a 68 or 41% decrease in total BW gain per steer with a 1 steer/ha increased stocking rate, respectively. These percent reductions that were noted by Riewe et al. (1962) were considerably greater than observed in the present study (7.4%). Petersen et al. (1964) reported a similar trend to Riewe et al. (1962) for increased stocking rate on total BW gain per steer with cattle on pasture. Total BW gain per steer was increased as more N fertilizer was applied (Table 4); each unit increase (kg/ha) in the N fertilizer rate increased the total BW gain per steer by 0.09 kg (r2 = 0.97). As with ADG, we are currently unaware of any studies examining the effects of N fertilizer rate at a set stocking rate.
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Figure 2. Predicted total BW gain by steers grazing dallisgrass-dominated pastures stocked at four rates and fertilized at three N rates during the summer.
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The equation developed for BW gain per hectare explained 71% of the variation (P < 0.01), and stocking rate interacted with N fertilization rate (P = 0.002; Table 3). The increase in the stocking rate from 3.7 to 11.1 steers/ha resulted in the predicted BW gain per hectare producing a quadratic response, with the maximum BW yield (498 kg/ha) at 7.2 steers/ha when 112 kg of N/ha was applied (Figure 3). In contrast, when 224 kg/ha of N was applied, the increase in the stocking rate from 3.7 to 11.1 steers/ha resulted in the predicted BW gain per hectare producing a quadratic response, with the maximum BW yield (574 kg/ha) at 8.0 steers/ha. When stocking rate was increased from 3.7 to 11.1 steers/ha, a quadratic response was detected in BW gain per hectare, with the maximum BW yield (701 kg/ha) at 8.9 steers/ha when 336 kg of N/ha was applied. Increasing the rate of N fertilizer from 112 to 336 increased the optimum stocking rate for BW gain per hectare by 24%. Berg and Sims (1995) also showed a quadratic response of total BW gain per hectare to stocking rate with cattle grazing old-world bluestem, and the maximum BW gain per hectare also was increased by increasing the fertilizer N rate. Adjei et al. (1980) reported that the optimal BW gain per hectare was 10 steers/ha when 220 kg/ha of N was applied annually.
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Figure 3. Predicted BW gain per hectare by steers grazing dallisgrass-dominated pastures stocked at four rates and fertilized at three N rates during the summer.
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Economic Performance
The total cost per hectare increased (P < 0.01) with increasing stocking rate and the amount of N fertilizer (Table 3). Stocking rate interacted (P < 0.01) with N fertilization rate. The treatment with the least cost of production was a stocking rate of 3.7 steers/ha, with 112 kg/ha of fertilizer N applied, and the treatment with the greatest cost of production was a stocking rate of 11.1 steers/ha, with 336 kg/ha of fertilizer N applied (Table 5). The increases in the cost of production with increased N fertilization rate resulted mostly from increased fertilizer cost. Increases in production cost with increased stocking rate were a result of increased interest, minerals, receiving feed, veterinary supplies, transportation, and death loss expenses (Table 1).
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Table 5. Predicted total cost and gross return ($) of stocker steers stocked at different rates grazing dallisgrass pasture fertilized at three different rates during the summer
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The gross return of stocker cattle grazing the dallis-grass-dominated pastures was simply a function of BW gain per hectare multiplied by the market price. The gross return increased quadratically (P < 0.01) in response to stocking and N fertilization rate (Table 3). The optimal stocking rate for gross return per hectare was 8.8 steers/ha, with 336 kg of N fertilizer, which had a net return of $1,162.20 (Table 5). The greatest gross return predicted by this model would have occurred at a fertilizer rate greater than 336 kg/ha of N, but this point is outside the range of the data, making the prediction invalid.
The net return per hectare (gross returns minus total cost) by stocker cattle depended on the stocking rate and N fertilization rate and their interaction (P < 0.01; Figure 4). With 112 kg of N/ha applied, the most profitable stocking rate was at 5.7 steers/ha, which returned $275.49/ha. At 224 kg of N fertilizer applied/ha, 6.5 steers/ha was the most profitable stocking rate, returning $273.23. Finally, the 336 kg/ha rate of N fertilizer produced the most profitable stocking rate at 7.3 steers/ha and returned $355.64. The increase in net return noted from 224 to 336 kg of N fertilizer applied per hectare was most likely the result the positive quadratic response for ADG to increasing N fertilization rate. Similar to that noted with gross return per hectare, the greatest net return predicted would have occurred at fertilizer rates greater than 336 kg/ha, making the precision of the prediction highly speculative. The optimum stocking rate for net returns occurred at 79, 81, or 82% of the maximum optimal stocking rate for BW gain/ha for 112, 224, or 334 kg/ha of N from fertilizer, respectively. Hart et al. (1988) reported that steers grazing western wheatgrass (Agropyron smithii Rydb.)-dominated rangeland, the optimal stocking rate for net return was between 55 and 60% of the optimal stocking rate for BW gain per hectare, depending on cattle prices. Gillen and McCollum (1992) reported for steers grazing midgrass prairie in southwest Oklahoma that the optimal stocking rate occurred at 0.39 steer/ha, with an annual net return of $19.42. Nonetheless, in that study, optimal stocking for BW gain per hectare could not be estimated because stocking rate was not tested to a point where it began to diminish, so the difference between optimal net return and BW gain hectare could not be estimated.
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Figure 4. Predicted net return per hectare by steers grazing dallisgrass-dominated pastures stocked at four rates and fertilized at three N rates during the summer.
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Based on the data developed in the study, the optimal net return for cattle grazing dallisgrass-dominated pasture occurred between 5.7 and 7.3 steers/ha and increased with increasing N fertilization rate. The point at which optimal net return occurred was approximately 80% of the stocking rate for optimal BW gain per hectare. Under the assumptions made in the financial analysis, these data show that carrying capacity of a pasture can be economically increased with the increased use of N fertilizer up to at least 336 kg of N/ha. Producers grazing stocker cattle on similar pastures will most likely need to choose stocking rates between 5.7 and 7.3 steers/ha, depending on the N fertilization rate selected.
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Footnotes
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1 This project was conducted with funding from the Univ. of Arkansas Agric. Exp. Stn., Hatch Project No. ARK001735 and funding from the Arkansas Fertilizer Tonnage Fees. We also appreciate the support through product donations provided by Ivy Laboratories, Inc. (Overland Park, KS), and we express our appreciation to P. Capps for help in completing this project. 
2 Correspondence: 362 Hwy 174 N. (phone: 870-777-9702, ext. 107; fax: 870-777-8441; e-mail: sgunter{at}uaex.edu).
Received for publication January 3, 2005.
Accepted for publication June 7, 2005.
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Literature Cited
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Abstract
Introduction
