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Designing supplements for stocker cattle grazing wheat pasture^1,2

Department of Animal Science, Oklahoma State University, Stillwater 74078

	Abstract

Top Abstract Introduction Mineral Content of Wheat... Protein Supplementation Energy Supplementation Threshold Herbage Allowance for... Development of a Small-Package... Implications Literature Cited

 
Supplementation of cattle grazing wheat pasture is of interest to 1) provide a more balanced nutrient supply and feed additives such as ionophores or bloat preventive compounds, 2) substitute supplement for forage where it is desirable to increase stocking rate in relation to grazing management and/or marketing decisions, and 3) substitute supplement for forage under conditions of low forage standing crops. Two different strategies for providing energy supplements to growing cattle on wheat pasture are presented. One was to develop a "small package" (i.e., target intake of 0.91 to 1.36 kg/d on an as-fed basis), self-limited monensin-containing energy supplement to provide additional degradable OM relative to rumen degradable N and increase nonammonia N supply per unit of ME. The supplement very consistently increased ADG by approximately 0.22 kg and increased profit by $15 to 31/steer depending on supplement cost and profit potential of the cattle. The sorghum grain-based supplement contained (as-fed basis) 4% fine mixing salt, 165 mg/kg of monensin, and 0.75% magnesium oxide. Monensin limited intake, but MgO at concentrations up to 1.75% did not limit intake. Modification of the formula for every-other-day hand-feeding resulted in similar improvements in ADG as achieved with the self-limited supplement. An additional study using small numbers of ruminally cannulated steers in a completely randomized design compared the effects of monensin or lasalocid vs. no ionophore on wheat pasture bloat. Monensin decreased (P < 0.05) both the incidence and severity of bloat and was more efficacious than lasalocid for prevention of bloat. A second strategy was to feed two types of energy supplements (i.e., high-starch, corn-based supplement vs. high-fiber by-product feed-based supplement) at a level of 0.75% of BW. Over the 3-yr study, mean daily supplement consumption was 0.65% of BW. This energy supplementation program increased ADG by 0.15 kg and allowed stocking rate to be increased by one-third. Type of supplement did not influence ADG, supplement conversion, or the substitution ratio of supplement for forage. Supplement conversion was approximately 5 kg of as-fed supplement•kg of increased gain^–1•ha^–1 and was substantially less than conversions of 9 to 10 that have traditionally been used in evaluating the economics of energy supplementation programs for wheat pasture stocker cattle.

Key Words: Growing Cattle • Monensin • Supplementation • Wheat Pasture

	Introduction

Top Abstract Introduction Mineral Content of Wheat... Protein Supplementation Energy Supplementation Threshold Herbage Allowance for... Development of a Small-Package... Implications Literature Cited

 
Winter wheat pasture is a unique and economically important renewable resource in Oklahoma and the southern Great Plains. Income is derived from both grain and the increased value that is added, as weight gain, to growing cattle that are grazed on wheat pasture. The potential for profit from grazing stocker cattle on wheat pasture is exceptionally good because of the high quality of the forage and the very favorable seasonality of prices for stocker and feeder cattle that favor price appreciation of the cattle.

Supplementation of cattle grazing wheat pasture is of interest to 1) provide a more balanced nutrient supply and feed additives such as ionophores and bloat preventive compounds, 2) substitute supplement for forage where it is desirable to increase stocking rate in relation to grazing management and/or marketing decisions, and 3) substitute supplement for forage under conditions of low forage standing crops. Production risk with respect to rate of live weight gain of growing cattle in stocker or backgrounding programs decreases the value of cattle (i.e., purchase price at the beginning of the programs). Predicting performance of wheat pasture stocker cattle is particularly challenging because of the potentially large variation in weather and forage standing crop. If ADG by growing cattle cannot be predicted with some degree of accuracy, realistic breakevens, which are prerequisite to sound marketing decisions, cannot be calculated. Additionally, the ability to predict cattle performance will become more important as the feedlot and stocker segments of the industry compete for supplies of stocker/feeder cattle and as coordinated beef productions systems come to fruition. Results of several supplementation studies that have been conducted with growing cattle grazing wheat pasture are reported herein. The initial discussion of the mineral content of wheat forage is included to establish the rationale for including additional calcium in supplements that are provided to growing cattle on wheat pasture.

	Mineral Content of Wheat Forage

Top Abstract Introduction Mineral Content of Wheat... Protein Supplementation Energy Supplementation Threshold Herbage Allowance for... Development of a Small-Package... Implications Literature Cited

 
Wheat pasture poisoning is a noninfectious metabolic disorder of cows grazed on wheat pasture. It occurs most frequently in mature cows that are in the latter stages of pregnancy or are nursing calves and that have been grazing wheat pasture for 60 d or more. Cows with wheat pasture poisoning have low blood concentrations of both Ca and Mg. Although a similar, tetany-like condition may occur in stocker cattle, its incidence is extremely low. Considerable variation occurs in the mineral composition of wheat forage. Until more complete data are available, the data in Table 1 have been selected to indicate the Ca, P, Mg, and K concentrations in wheat forage in relation to the requirements for the same minerals of a 225-kg steer gaining 1.0 kg/d.

The lower values for P content of wheat forage (Table 1) are from Bushland, TX (Stewart et al., 1981). In this area, and perhaps the panhandle of Oklahoma and Southwestern Kansas, wheat pasture stocker cattle should also receive supplemental P depending on soil type and actual mineral analysis of wheat forage. More recently a case of P deficiency was encountered in a group of growing steers grazing wheat pasture near Loyal, OK (i.e., north-central Oklahoma). The farm had been in alfalfa for approximately 6 yr before putting it into wheat. The application of P fertilizer for the wheat crop was less than recommended from soil test results. Phosphorus, Ca, Mg, and K contents of wheat forage samples collected on January 14 were 0.16, 0.26, 0.16, and 1.72% of DM, respectively. The Angus steers appeared healthy and were in good body condition, but seemed to crave bones, which were present in a native grass area adjacent to the wheat pasture, from carcasses of cows that had died in previous years. Depraved appetite or pica is a classical sign of P deficiency in beef cattle. The mineral mixture that was being fed was changed from a low-P mineral (4.0%) to a mineral mixture that contained 12% Ca, 12% P, and 12% salt, which resolved the problems with bone chewing.

The question relative to the effect of feeding mineral mixtures (often high-Mg mineral mixtures) to wheat pasture stockers on the incidence of bloat is commonly raised. However, there is no evidence to support the suggestion that supplemental Mg will decrease the incidence and/or severity of bloat of stocker cattle on wheat pasture. There may be a relationship between ruminal motility (and the ability of stocker cattle to eructate ruminal fermentation gases) and the Ca status of the cattle. Ruminal and gut motility is greatly compromised by subclinical deficiencies of Ca (Huber et al., 1981). Therefore, the concern of providing additional calcium to growing cattle on wheat pasture is two-fold: 1) to meet requirements for growth and 2) to perhaps decrease the bloat problem by an effect on ruminal motility. An interesting question is whether the "dry bloat" problems that are sometimes observed in wheat pasture stocker cattle might be related to a subclinical deficiency of Ca.

	Protein Supplementation

Top Abstract Introduction Mineral Content of Wheat... Protein Supplementation Energy Supplementation Threshold Herbage Allowance for... Development of a Small-Package... Implications Literature Cited

 
This section is included to briefly summarize some of the rationale and results of research conducted at Oklahoma State University relative to the idea of providing undegradable intake protein to growing cattle on wheat pasture and not as a thorough review of this subject. Beever (1984) presented data at the National Wheat Pasture Symposium that was interpreted to suggest that performance of rapidly growing cattle on wheat pasture may be limited by flow of inadequate amounts of nonammonia nitrogen (NAN) to the small intestine. Johnson et al. (1974) reported CP values of wheat forage of 25 to 31% of DM during January to April, with 17 to 33% of the N being in the form of nonprotein nitrogen (NPN). Horn et al. (1977) observed total soluble N and soluble NPN concentrations of wheat forage of 45 to 62% and 25 to 37% of total N, respectively. Beever et al. (1976), in studying different conservation methods for perennial ryegrass, observed a negative relationship (r = –0.98, P < 0.001) between the amount of N flowing to the small intestine (g/100 g of N consumed) and solubility of forage N in 0.01% (wt/vol) pepsin in 0.1 N HCl. Egan (1974), Ulyatt and Egan (1979), and Egan and Ulyatt (1980) reported large losses (i.e., 40 to 45%) of ingested N from the rumen of sheep fed high-protein ryegrass. Studies by Vogel et al. (1989a) showed that N of immature and mature wheat forage exist kinetically as two distinct pools with different rates of in situ ruminal disappearance. Approximately 50 to 75% of total forage N disappeared from a very rapid disappearance pool at rates of 16 to 19%/h. Broderick (1984) also suggested the presence of two degradation fractions of N of alfalfa hay. MacRae and Ulyatt (1974) reported that 63% of the variation in live weight gain of sheep grazing ryegrass or white clover pasture was associated with the amount of NAN absorbed from the small intestine and that there was no relationship between live weight gain and energy absorbed as VFA. These data indicated that the traditional concept that performance of growing cattle on wheat pasture is not limited by protein status should be reevaluated.

Vogel et al. (1989b) and Smith et al. (1989) conducted studies over 4 yr to determine the effect of feeding additional escape protein on weight gains of stocker cattle grazing wheat pasture. Cattle received no supplement (other than free-choice access to a commercial mineral mixture) or were fed daily 0.91 kg (as fed) of a corn-based energy supplement or supplements that provided approximately 0.25 kg of protein from high-escape protein as cottonseed meal produced by mechanical extraction, meat meal, meat and bone meal, or corn gluten meal. The 0.25 kg of protein from high-escape protein is very similar to the levels used by Anderson et al. (1988), in which supplemental escape protein increased gains of steers grazing smooth brome pastures. The supplements were isocaloric and contained similar amounts of Ca, P, and Mg. Monensin was included in the supplements to supply 130 to 150 mg•steer^–1•d^–1.

Average daily gains by the cattle were increased (P < 0.03) approximately 0.1 kg by the overall effect of supplementation. Provision of additional ruminal escape protein as cottonseed meal, meat meal, meat and bone meal, or corn gluten meal did not increase ADG (P = 0.30) compared with the corn-based energy supplement. Our conclusion was that although wheat forage contains large amounts of N that is rapidly degraded in the rumen, intakes of fermentable OM seem to provide energy for sufficient microbial protein synthesis in the rumen for growth of stocker cattle. In a later study reported by Phillips et al. (1995), N retention of lambs fed freshly harvested wheat forage in metabolism stalls was not improved by supplemental undegradable intake protein from cottonseed meal, feather meal plus corn gluten meal, or blood meal compared with a corn-based energy supplement. Interestingly, the Level 1 beef cattle model of the NRC (1996), with the default microbial efficiency of 13% and predicted DMI, indicated that ME intake was first-limiting with respect to ADG by stocker cattle grazing wheat (Reuter and Horn, 2000).

	Energy Supplementation

Top Abstract Introduction Mineral Content of Wheat... Protein Supplementation Energy Supplementation Threshold Herbage Allowance for... Development of a Small-Package... Implications Literature Cited

 
Feeding moderate amounts of an energy supplement to growing cattle on wheat pasture is one way of increasing the stability of the enterprise, improving the predictability of cattle performance (i.e., decreasing production risk), and increasing stocking rate and flexibility by having more cattle on hand for grazing during the graze-out period. Because of the seasonality of stocker/feeder cattle prices and the dynamics of breakeven selling prices in stocker cattle budgets, the latter of these can be particularly important to the economics of growing cattle on wheat pasture.

Silage
There are areas of the southern Great Plains where silage is used very successfully to substitute for available wheat forage and/or allow initial stocking densities on wheat pasture to be increased. In studies reported by Vogel et al. (1987, 1989c), use of supplemental corn silage or sorghum silage allowed initial stocking density on wheat pasture to be doubled without decreasing weight gains by stocker cattle. Supplemental silage decreased wheat forage intake linearly (P < 0.10). Each kilogram of added silage DM decreased DMI of wheat forage by 0.66 kg. Extent of ruminal digestion of DM and NDF of wheat forage was increased by feeding silage, indicating that silage had a positive associative effect on use of wheat forage (Vogel et al., 1989c).

High-Starch vs. High-Fiber By-product Feed-Based Supplements
The response of growing cattle on wheat and/or other small grain pastures to supplemental grain has been variable. In studies reported by Elder (1967), Lowrey et al. (1976a, b), Utley and McCormick (1975, 1976), and Gulbransen (1976), steer grazing days/ha or stocking densities were increased 1.25-to 2-fold, and ADG were increased by 0.05 to 0.30 kg by feeding grain at levels of 1 to 1.5% of BW. Supplement conversions (kg of supplement•kg of increased gain^–1•ha^–1) ranged from 6.7 to 10.3. To prevent adverse effects of starch on ruminal fiber digestion, high-fiber by-product feeds, such as wheat middlings, soybean hulls, and corn gluten feed, offer alternatives in formulating energy supplements with fairly high energy densities. The potential for using these by-product feeds in supplementing growing cattle on wheat pasture is particularly good because of the rapid rate of ruminal degradation of wheat forage (Zorrilla-Rios et al., 1985) and the relatively low ruminal pH (Andersen and Horn, 1987).

During the years of 1989/1990, 1990/1991, and 1991/1992, we conducted studies to evaluate type of energy supplement (i.e., a corn-based, high-starch vs. a high-fiber by-product feed-based energy supplement) for growing cattle on wheat pasture. The high-fiber energy supplement contained approximately 47% soybean hulls and 42% wheat middlings (as-fed basis). Supplements were hand-fed 6 d/wk at a level of approximately 0.75% of BW (i.e., 1.8 kg/d for a 242-kg steer), and stocking rate was increased 22 to 44%. Nonsupplemented control cattle had free-choice access to a high-Ca commercial mineral mixture throughout the study. The objective of this supplementation program with respect to increasing stocking density was much different from that of Grigsby et al. (1991), Rouquette et al. (1990), and Branine and Galyean (1990), who fed energy supplements at levels of 0.15 to 0.20% of BW to cattle grazing rye-ryegrass or wheat pastures without increasing stocking density. Conversions of a corn-based energy supplement of 1.3 to 3 kg•kg of increased gain^–1•animal^–1 were reported by Grigsby et al. (1991). Details of our studies have been reported by Horn et al. (1995). General results are described in the following sections.

Supplementation Response.
Over the 3-yr period, ADG during the fall/winter and early spring grazing period (i.e., up to the time of jointing of wheat) was increased by energy supplementation (regardless of type of energy supplement) by an average of 0.15 kg/d, with values of 0.92, 1.05, and 1.08 kg/d for the control, high-starch, and high-fiber supplemented steers, respectively. The ADG response was similar at all stocking densities, which increases the scope of application of the results. Mean consumption of the supplements was 0.65% of BW, which was a little less than the target of 0.75.

Type of Energy Supplement.
Type of energy supplement (i.e., high-starch vs. high-fiber) did not affect ADG by the cattle. In general, one would expect the difference in response by cattle to high-fiber vs. high-starch energy supplements to decrease as the amount of supplement fed decreases and as crude protein content of the forage increases. The level of supplement fed in these studies was relatively small, and wheat forage contains excess CP. Substitution of the supplements (i.e., units change in forage OM intake per unit increase in supplement OM intake) was calculated by regression of forage intake on amounts of supplement consumed. Substitution did not differ (P = 0.60) between the two types of supplements and was –0.91 (Cravey, 1993). The mechanism for substitution of the supplements for forage has not been identified, but it would not be expected to be the result of a ruminal N deficiency as has often been the case in grazing studies, as discussed by Horn and McCollum (1987).

Supplement Conversion.
Mean conversion of the supplements (expressed as kg of as-fed supplement•kg of increased gain^–1•ha^–1) was approximately 5.0 for both types of supplement and did not differ (P = 0.95). This is substantially less than conversions of 9 to 10 that have traditionally been used in evaluating the economics of energy supplementation programs for wheat pasture stocker cattle.

Cattle Preference for Supplements.
Cattle seemed to prefer the high-fiber supplement and consume it much more readily than the corn-based high-starch supplement. Generally, the cattle consumed the high-fiber supplement in a matter of 10 to 30 min in the morning, whereas the corn-based supplement was eaten over at least two feeding periods during the day (morning and midafternoon). From a feed and bunk management standpoint, this difference in the supplements is extremely important on days of inclement weather (i.e., rain, snow, etc.) and in situations of bird predation, where contamination of feed bunks by bird excreta was substantial for the corn-based supplement. In addition, the potential for acidosis is much less for the high-fiber supplement provided that the wheat middlings used in the high-fiber supplement do not contain much fine particle starch.

Risk Aversion.
We addressed the issue of risk aversion and input decisions relative to energy supplementation of stocker cattle under various cattle and supplement price scenarios (Coulibaly et al., 1996) and concluded that, in general, supplementation decreases production risk.

Feedlot Performance.
Because wheat pasture cattle are typically of higher body condition than other cattle at the start of the feedlot finishing phase of production, we were interested in the potential effect of energy supplementation on subsequent feedlot performance, and we followed the cattle through the feedlot in two of the three years. Supplementation did not affect feed intake or G:F (P = 0.30) in one year; however, ADG was decreased by approximately 0.09 kg (P < 0.05). In the other year, supplementation did not (P = 0.80) affect feedlot ADG.

	Threshold Herbage Allowance for Initiation of Energy Supplementation Programs

Top Abstract Introduction Mineral Content of Wheat... Protein Supplementation Energy Supplementation Threshold Herbage Allowance for... Development of a Small-Package... Implications Literature Cited

 
Two studies were conducted (Redmon et al., 1995) to determine the relationship between in vitro OM disappearance in diets of cattle grazing wheat pasture and herbage allowance, the relationship between wheat forage intake (kg OM/100 kg BW) and herbage allowance, and the relationship between estimated ADG of growing beef cattle grazing wheat pasture and herbage allowance. Paddocks were differentially grazed with growing beef cattle to produce an array of different herbage masses (kg DM/ha). Each experimental paddock was then continuously stocked with three steers during each 7-d forage intake trial. Estimated ADG was calculated from forage intake and net energy values calculated from diet organic matter disappearance data. Forage intake, OM disappearance, and estimated ADG were related to daily herbage allowance (kg DM•100 kg BW^–1•d^–1) and herbage mass (kg/ha), using nonlinear regression procedures to fit quadratic models with a plateau function (SAS Inst., Inc., Cary, NC). Plateaus for diet OM disappearance, forage intake, and ADG were achieved at herbage allowances between 20 and 24 kg DM•100 kg BW^–1•d^–1, and decreased markedly at herbage allowances below this range. These data were interpreted to suggest that a herbage allowance of 20 to 24 kg DM•100 kg BW^–1•d^–1 may provide a threshold allowance for initiation of energy supplementation programs for growing cattle on wheat pasture. Similarly, Ellis et al. (1984) reported that DM digestibility by steers grazing annual ryegrass was progressively decreased (P < 0.01), as daily herbage allowance was decreased to less than 30 kg/100 kg BW.

	Development of a Small-Package Monensin-Containing Energy Supplement

Top Abstract Introduction Mineral Content of Wheat... Protein Supplementation Energy Supplementation Threshold Herbage Allowance for... Development of a Small-Package... Implications Literature Cited

 
Because of the large amounts of rumen degradable N in wheat forage, the underlying rationale for this supplementation program was to provide: 1) additional degradable OM relative to degradable N and increase nonammonia N supply per unit of ME, 2) monensin to improve the economics of the supplementation program and to decrease bloat (Branine and Galyean, 1990), and 3) additional Ca and a means from a management standpoint of getting other feed additives into cattle when needed. The target level of consumption of this supplement was 0.91 to 1.36 kg (as fed) •animal^–1•d^–1, and the supplement was initially designed to be self-limited.

Self-Limited Monensin-Containing Energy Supplement
Details of the individual year studies have been reported by Horn et al. (1990, 1992) and Beck et al. (1993). Because of the low targeted level of intake of the supplement, stocking densities were not changed when this supplement was fed. Composition of the supplement is shown in Table 2, and the mean (±SD) of supplement and monensin intakes are shown in Table 3 as reported by Beck (1993). The supplement was fed as a 0.5-cm pellet during the first two years and in meal form during the third and fourth years. Although we experienced some over-consumption (i.e., mean overall daily consumption of monensin greater than 200 mg) of the supplement by one group of cattle during each of the first two years, mean intakes were close to the target. In general, the target supplement intake of 0.91 to 1.36 kg (as fed) •animal^–1•d^–1 was more closely achieved by feeding the supplement in meal form. Feeding the supplement in meal form probably slowed rate of consumption of the supplement and may have increased the taste of salt.

Modifications of the Formula.
We had an additional opportunity to study this supplementation program during the 1995/1996 wheat pasture year. The "original" formula for this supplement contained approximately 63% ground sorghum grain and 21% wheat middlings. The wheat middlings were included primarily to improve pellet quality during the first two years of the study. The objective of the 1995/1996 study was to determine whether substitution of equal proportions of wheat middlings and soybean hulls (midds/hulls) for the ground sorghum grain and wheat middlings of the original formula affected intake of the self-limited supplement and cattle growth performance. The monensin concentration of the supplement was also decreased from 165 mg/kg ("original" formula) to 132 mg/kg to provide a greater margin in relation to the FDA approved level of monensin intake.

Mean intake of the sorghum grain-based ("original" formula) and the midds/hulls-based supplements from December 7, 1995 through March 13, 1996 (98 d) was 0.94 and 1.06 kg•steer^–1•d^–1, respectively. There was no difference between intake of the two supplements. This would allow for greater flexibility in formulating this supplement depending on the availability and cost of energy feedstuffs. Monensin consumption averaged 124 and 140 mg•steer^–1•d^–1 for the sorghum grain- and midds/hulls-based supplements, and was lower than the desired level of 180 to 200 mg.

Daily weight gain by steers during the 98-d study averaged 0.96 (nonsupplemented, control), 1.10 (sorghum grain-based supplement), and 1.07 kg (midds/hulls-based supplement), and was increased by supplementation but not by type of supplement (i.e., sorghum grain- vs. midds/hulls-based supplement). The gain response to supplementation was substantially less than that of our previous studies, which likely reflected a lower mean intake of supplement and monensin, particularly during the early part of the study.

What Are the Limiters of Intake?
Salt.
During the first year of these studies, the level of salt in the supplement was sometimes increased from 4 to 6% when consumption was greater than desired and/or plain block salt was provided for the cattle. The salt levels of 4 to 6% were based on initial conversations with other researchers. In subsequent years, we used the 4% level of salt and have not conducted studies to evaluate the sensitivity of supplement consumption to salt level.

Monensin.
Paisley and Horn (1996a,b) used four wheat pastures equipped with Pinpointer (Pinpointer 5000; PLM Corp., Cookville, TN) feeders to examine the effect of monensin on voluntary consumption of the self-limited supplement. Each 9-ha pasture was grazed by 11 fall-weaned steer calves from a single beef cow herd. The pastures were approximately 681 x 125 m and had automatic waterers at the south end of each pasture. The Pinpointer feeders were located within 17 m of the waterers in each pasture. The steers had free-choice access to the sorghum grain-based supplement with 4% salt and either no monensin or 165 mg of monensin/kg as-fed. Supplements were fed in meal form and were sampled each time feed was added to the hopper bin to verify monensin concentrations. Supplement intakes of each group of steers from January 17 to April 12 (84 d) were calculated from feed and weekly weigh-backs because of large discrepancies between the calculated data and Pinpointer data. Thus, no data were obtained relative to frequency of supplement intake and meal size by the individual steers.

Supplement intakes were analyzed as a repeated measures design with week and treatment in the model. Because there were no week x treatment interactions (P = 0.16), the data were pooled across weeks. Mean (±SD) intakes of the supplements by steers of each pasture were: 2.15 ± 0.69 and 2.43 ± 0.42 kg DM (0 mg of monensin), and 0.61 ± 0.16 and 0.70 ± 0.24 kg DM (165 mg/kg of monensin), and were decreased by monensin (P < 0.001). Overall ADG by steers in the two treatments were not different (P = 0.24), even though steers consumed much lower amounts of the monensin-containing supplement.

Magnesium Oxide.
Similar studies as those described above for monensin were conducted by Paisley et al. (1997) and Paisley (1998) with the sorghum grain-based supplement containing 4% salt, 165 mg/kg of monensin, and four levels of magnesium oxide (BayMag; BayMag Inc., Calgary, Alberta, Canada) using the Pinpointer feeders. Levels of magnesium oxide were 0.25, 0.75., 1.25, and 1.75% of the as-fed supplement. Actual magnesium concentrations were 0.49, 0.85, 1.17, and 1.56% as-fed, and the supplements analyzed 176 to 187 mg/kg of monensin as-fed. Forty-eight fall-weaned steer calves from a single beef cow herd were initially weighed November 8 and assigned to one of four pastures. Cattle were fitted with Pinpointer collars on November 13 and allowed to adapt to feeders for 8 d before the intake measurements. Final weights were taken December 20, with both initial and final BW recorded after a 14-h shrink.

Because supplement intakes of individual animals were measured, steer weights and ADG were analyzed as a completely randomized design, with animal as the experimental unit. Supplement intakes were analyzed with treatment, steer within treatment, day, and treatment x day as sources of variation and was used to analyze all 1,344 intake observations (28 d x 48 steers). Preplanned linear, quadratic, and cubic orthogonal contrasts were used to interpret the effect of increasing levels of magnesium oxide on supplement intake and animal performance.

Final weights and ADG of steers were not affected (P = 0.11; Table 5) by increasing levels of MgO. Results indicated that supplement intake increased linearly (P < 0.01) with increasing levels of MgO (Table 5). Differences in intake among the supplements were small, and mean consumption of all supplements was less than the target of 0.91 to 1.36 kg (as-fed) •steer^–1•d^–1. Time (min/d) that the steers spent in the feeders was influenced in a quadratic manner (P < 0.01) by level of MgO. Although not significant (P = 0.09), visits per day seemed to follow the same quadratic trend as eating time. These results indicate that inclusion of MgO at levels up to 1.75% of supplement (as-fed basis) did not limit intake. Similar results reported by Coffey and Brazle (1994) showed that magnesium-mica at levels up to 50% did not limit intake of a ground sorghum grain supplement by steers grazing smooth bromegrass.

	Implications

Top Abstract Introduction Mineral Content of Wheat... Protein Supplementation Energy Supplementation Threshold Herbage Allowance for... Development of a Small-Package... Implications Literature Cited

	Footnotes

 
¹ Part of this material is based upon work supported by the Cooperative State Research, Education, and Extension Service, USDA, under Agreement Nos. 89-34198-4288, 93-34198-8410, and 97-4198-3970.

² Presented at the ASAS Southern Sectional Meeting, 2004.

⁴ Present address: Univ. of Arkansas, Southwest Res. and Ext. Center, 362 Hwy 174 N, Hope 71801.

⁵ Present address: Crop and Soil Sci., 3111 Miller Plant Sciences, Univ. of Georgia, Athens 30602.

⁶ Present address: Dept. of Anim. Sci., Box 3684, Univ. of Wyoming, Laramie 82071.

³ Correspondence: 208 Animal Science (phone: 405-744-6621; fax: 405-744-7390; e-mail: horngw{at}okstate.edu).

Received for publication March 29, 2004. Accepted for publication July 1, 2004.

	Literature Cited

Top Abstract Introduction Mineral Content of Wheat... Protein Supplementation Energy Supplementation Threshold Herbage Allowance for... Development of a Small-Package... Implications Literature Cited

 


Andersen, M. A., and G. W. Horn. 1987. Effect of lasalocid on weight gains, ruminal fermentation and forage intake of stocker cattle grazing winter wheat pasture. J. Anim. Sci. 65:865–871.

Anderson, S. J., T. J. Klopfenstein, and V. A. Wilkerson. 1988. Escape protein supplementation of yearling steers grazing smooth brome pastures. J. Anim. Sci. 66:237–242.

Andrae, J. G., G. W. Horn, and G. Lowrey. 1994. Effect of alternate-day feeding of a monensin-containing energy supplement on weight gains and variation in supplement intake by wheat pasture stocker cattle. Anim. Sci. Res. Report No. P-939:158–161. Oklahoma Agric. Exp. Stn., Stillwater.

Beck, P. A. 1993. Development of a self-limited monensin-containing energy supplement for growing cattle on wheat pasture. M.S. Thesis, Oklahoma State Univ., Stillwater.

Beck, P. A., G. W. Horn, M. D. Cravey, and K. B. Poling. 1993. Effect of self-limited monensin-containing energy supplement and selenium bolus on performance of growing cattle grazing wheat pasture. Anim. Sci. Res. Report No. P-933:256–261. Oklahoma Agric. Exp. Stn., Stillwater.

Beever, D. E. 1984. Utilization of the energy and protein components of forages by ruminants—a United Kingdom perspective. Pages 65–97 in Proc. of the National Wheat Pasture Symposium. G.W. Horn, ed. Oklahoma Agric. Exp. Stn. MP-115, Stillwater.

Beever, D. E., D. J. Thomson, and S. B. Cammell. 1976. The digestion of frozen and dried grass by sheep. J. Agric. Sci. (Camb.) 86:443–452.

Branine, M. E., and M. L. Galyean. 1990. Influence of grain and monensin supplementation on ruminal fermentation, intake, digesta kinetics and incidence and severity of frothy bloat in steers grazing winter wheat pasture. J. Anim. Sci. 68:1139–1150.[Abstract/Free Full Text]

Broderick, G. A. 1984. In vitro determination of rates of ruminal protein degradation. Can. J. Anim. Sci. 64(Suppl.):31–32.

Coffey, K. P., and F. K. Brazle. 1994. Consumption of free-choice grain supplements containing salt or magnesium-mica. Page 10 in Rep. of the South East Kansas Exp. Stn., Parsons.

Coulibaly, N., D. J. Bernardo, and G. W. Horn. 1996. Energy supplementation strategies for wheat pasture stocker cattle under uncertain forage availability. J. Agric. Appl. Econ. 28:172–179.

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