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EFFECTS OF TIME OF SEASON AND COTTONSEED MEAL AND LASALOCID SUPPLEMENTATION ON STEERS GRAZING RYE PASTURES¹

Edisto Research and Education Center, Clemson University Blackville, SC 29817

	Abstract

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A study was conducted to estimate changes in rye (Secale cereale L.) forage quality during the grazing season and to determine the effects of supplemental cottonseed meal (CSM) with and without the ionophore lasalocid on the growth rate of steers grazing rye pastures. During 1986 to 1987, pasture samples were collected on December 19, January 20, February 19 and March 16 to estimate changes in fiber, N and in vitro OM disappearance (IVOMD) of rye forage. Significant seasonal differences in NDF, ADF, permanganate lignin and IVOMD were detected. Rye forage IVOMD ranged from 74.5 to 83.6% of DM, indicating that quality was high at all sampling times. Nitrogen content also was high and ranged from 2.1 to 4.6% of DM. Ruminal N degradation rate, percentage of N degraded and percentage of escape N indicated that the rye protein was rapidly degraded with less than 25% of the total plant N escaping ruminal breakdown. Ninety-six crossbred yearling steers were assigned to either control, .45 kg CSM/d or .45 kg CSM/d + 150 mg lasalocid/d treatments to determine the effects of supplementation on growth rate while steers grazed rye pastures. Protein supplementation (CSM) increased (P < .05) ADG early in the grazing season but not later. Addition of lasalocid to the protein supplement increased (P < .05) ADG during the spring grazing period and for the entire grazing season. Rye forage is a high-quality feedstuff containing rapidly degraded N throughout the grazing season. Supplementing steers with CSM containing lasalocid improved seasonal weight gains.

Key Words: Rye • Cottonseed Oilmeal • Ionophores • Steers • Protein

	Introduction

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Supplementing cattle grazing immature forages with escape protein sometimes has increased ADG. In a wheat pasture grazing study, Anderson et al. (1987) reported that steers receiving a feather meal plus meat and bone meal supplement gained 8.1% faster than steers receiving only a mineral supplement. Similar increases in ADG have been reported in steers grazing bromegrass pastures and offered increasing levels of escape protein (Anderson et al., 1988). With pastured animals, others have reported increases in growth rate either by feeding protected proteins (Stobbs et al., 1977) or by abomasal infusion of protein (Barry, 1980, 1981). These results suggest that, at the duodenum, essential amino acids may be insufficient for maximum gains.

With concentrate diets, addition of monensin decreases feed consumption with no change in ADG of cattle. But with high-roughage diets, monensin and lasalocid increased ADG (Potter et al., 1976; Faulkner et al., 1985; Anderson et al., 1987) even though lasalocid did not reduce feed consumption (Anderson and Horn, 1987).

This study was conducted to 1) estimate changes in forage quality of rye (Secale cereale L.) during the grazing season, 2) characterize rye protein fractions and 3) determine the effects of supplemental cottonseed meal (CSM) with and without the ionophore lasalocid on growth rate of steers grazing rye pastures.

	Experimental Procedure

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Pastures

Six 3.2-ha pastures were planted to rye (Secale cereale L., cv. Wrens Abruzzi) at the rate of 130 kg/ha on September 25, 1986. All pastures were fertilized uniformly with 330 kg of a granular 6–18–36 (N–P–K)/ha before planting and with 56 kg N/ha 2 wk postplanting. Additional N was applied at 56 kg/ha approximately midway through the grazing season (January 29, 1987).

Six .5-m² areas were hand-clipped to ground level in each of the six pastures on December 19, January 20, February 19 and March 16. This clipped forage was dried at 60°C in a forced-air oven for 48 h for subsequent laboratory analyses.

Cattle were placed on pasture on December 17, 1986, after the rye had reached a height of 30 cm and was well rooted. All pastures were grazed continuously throughout the season and treatment groups were rotated weekly through the six pastures to ensure that pasture differences did not influence animal gain. Grazing was terminated for a 3-wk period between January 28 and February 18 because of limited forage availability that resulted from the extreme cold (–4 to –7°C) weather in mid-January. Cattle were placed back onto pasture (February 18) after the rye forage had regrown sufficiently.

Laboratory Analyses

Six forage samples from within each pasture were ground in a Wiley mill (1-mm screen) and pooled randomly, which resulted in two composite samples (each composed of three forage samples) per pasture per clipping date. Because all six pastures were treated alike, there were 12 pasture replicates for each of the four clipping dates. Subsequent laboratory analyses were done in duplicate. Neutral detergent fiber and ADF were determined by the procedure of Goering and Van Soest (1970). Permanganate lignin (PL) was determined by the procedure of Van Soest and Wine (1968). In vitro OM disappearance (IVOMD) was determined by the procedure of Moore (1970). Ruminal fluid for determining IVOMD was obtained from a fistulated steer maintained on a bermudagrass hay (Cynodon dactylon [L.] Pers. cv. Coastal; 8.2% CP) diet supplemented with .45 kg of soybean meal (SBM, 48% CP) 1 h prior to collection. Crude protein (AOAC, 1975), acid detergent insoluble N (ADIN; Goering et al., 1970), N solubility in a bicarbonate plus phosphate buffer (Poos-Floyd et al., 1985) and nitrate (Baker and Smith, 1969) were determined to characterize the N fractions of the rye forage. Estimations of the protein degradation rate, percentage degraded at 12 h and percentage of the N escaping ruminal breakdown were determined by the in situ procedure of Anderson et al. (1988) modified by maintaining the fistulated steers on a bermudagrass hay instead of a corncob diet. Rates were determined using an exponential decay model (Y = k₀^–kdt with time (t) being 6, 12, 18 and 24 h of ruminal incubation and Y as the percentage of N remaining at time = t, corrected for microbial fiber attachment and ADIN, k₀ = computer generated intercept and k_d = rate of protein degradation. Nitrogen from microbial attachment was estimated as the percentage of N of rye ADF after it had been incubated in the rumen for time = t minus ADIN of the initial rye ADF. In situ estimations of protein degradation were determined on 10 replicates of composited monthly samples. Estimates of escape N were calculated using the following model proposed by Ellis (1978): Escape N = [k_p/(k_p+k_d)] x insoluble potentially degraded N, where k_p = 6%/h (from Pond et al., 1981; Ellis and DeLaney, 1982), k_d = rate of protein degradation and insoluble potentially degraded N = total N minus soluble N minus ADIN.

Animals

Ninety-six yearling crossbred (sire breeds: Brahman, Simbrah, Beefmaster, Santa Gertrudis and Senepol) steers out of Angus dams and averaging 250 kg in BW were assigned randomly within sire breed to one of three treatments: control (no supplement); .45 kg/d of mechanically extracted cottonseed meal (CSM); and .45 kg/d CSM with 150 mg/d lasalocid. Each treatment group (n = 32 hd) was stratified by weight and allotted to two of the six rye pastures, resulting in two field replications per treatment (n = 16 hd per pasture). All steers were shrunk (18 h without feed or water) and weighed prior to the initiation of grazing.

The trial was terminated on January 28, 1987 due to cold weather (–4 to –7°C) during the month of January that resulted in little growth of the forage. The cattle were shrunk and weighed; ADG for the 39-d fall grazing period was the difference in shrunk weights at the beginning and end of grazing divided by days. Steers were fed bermudagrass hay (Cynodon dactylon (L.) Pers. cv. Coastal) for a 3-wk period (January 28 to February 18), during which time all supplementation was discontinued. After forage had regrown sufficiently, steers were shrunk, weighed and placed back onto pastures and their respective supplement treatments for the duration of the grazing season (spring). Cattle were shrunk off-test on March 31, 1987, and ADG for the spring period (41 d) was calculated. Average daily gain for the entire grazing season was calculated by dividing the total weight gain during the two grazing periods by 80 d.

Steers receiving the CSM supplements were fed once daily each morning by pasture group. All steers were implanted with zeranol (36 mg) at the initiation of the fall period (December 17, 1986) and again on March 6, 1987. In addition, all cattle had ad libitum access to a complete mineral supplement⁴ throughout the grazing season.

Statistical analyses

In vitro and in vivo data were analyzed using a completely randomized split plot in time design with effects for time of sampling (month), pasture, month x pasture, and forage sample within pasture as the sources of variation in the model. Month was tested by the month x pasture interaction term. Means among months were separated by least significant differences comparison protected by a significant F-test (SAS, 1987) with the month x pasture interaction used as the error term.

The animal response data for fall, spring and across season were analyzed as a completely randomized design with effects for treatment and pasture nested within treatment as the sources of variation in the model (SAS, 1987). The pasture group rather than animal was the experimental unit in the statistical analysis, although trough space (.73 m/hd) was considered more than adequate to ensure equal accessibility to the supplement. No time effect was included in the model because animal responses were averaged (ADG) across the time periods of interest. Treatment effects were tested by the pasture within treatment component of the model; treatment means were separated by a least significant difference comparison protected by a significant F-test.

	Results and Discussion

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Quality

Forage NDF and ADF were greatest (P < .05) during the months of December and January, least (P < .05) during February and intermediate (P < .05) during March (Table 1). Permanganate lignin was greatest (P < .05) during the month of March, least (P < .05) in February and intermediate (P < .05) during the months of December and January.

Nitrogen

Total N content decreased (P < .05) from December to January and again from February to March (Table 2). The decline in N for both of these time periods apparently was the result of increasing plant maturity and removal of leaves by grazing. February values were highest (P < .05), which can be attributed to the less mature forage and re-application of N fertilizer on January 29.

There were significant differences in bicarbonate-phosphate soluble N (Table 2) among the four clipping dates; soluble N ranged from 47 to 55% of total N. Somewhat less than expected (J. C. Burns, personal communication), these values may have been decreased by the high temperature (60°C) used in drying the clipped forage. Abdalla et al. (1988) showed that drying bromegrass (Bromus inermis Leyss.), birdsfoot trefoil (Lotus corniculatus L.) and alfalfa (Medicago saliva L.) at room temperature (20°C) or higher decreased protein solubility compared with freeze-drying. Therefore, values for protein solubility and degradation reported later probably were underestimated and escape N values were overestimated. Differences in soluble N among the four clipping dates in this study were associated inversely with plant growth rate. Friedrich and Huffaker (1980) reported that, 9 d after cell formation, soluble protein began to decrease; 85% of this decrease in soluble protein attributed to maturation and senescence of barley (Hordeum vulgare L.) was due to reduced ribulose biphosphate carboxylase. Therefore, the values for soluble N should be greatest during rapid growth and least when growth is the slowest. However, estimates of soluble N obtained from our study would not support that conclusion (Table 2). Acid detergent insoluble N (ADIN) values were low throughout the grazing season, ranging from .06 to .14% of DM. Forage ADIN was greater (P < .05) in the December and January samples than in the February and March samples, which may be related to the greater (P <.05) fiber content of the more mature, early-season samples.

Rates of protein degradation were quite rapid, ranging from 6.1 to 12.1%/h (Table 3). These fast rates of degradation suggest that only small amounts of dietary protein would escape ruminal breakdown. This model predicts that 84 to 88% of the total plant N will be degraded, or at least solubilized, within 12 h. Escape N estimates for the rye forage ranged from 16.7 to 25.6% of the total available N. Estimates for escape N of the CSM ranged from 42 to 48%. Escape N was greater (P <.05) for the CSM containing the lasalocid than for the CSM alone. The cause of this difference is not known because all the CSM came from a single batch of which half was mixed with lasalocid.

Both supplemental CSM treatments increased (P <.05) ADG of steers by 21% during the fall grazing period (Table 4). There are several possible explanations for the observed increase in ADG. First, the additional energy supplied by the CSM (approximately .62 Meal NE_g/d; NRC, 1984) would be sufficient to induce this .3 kg/d increase in ADG. Although no supplemental energy control treatment was included in this study, it is unlikely that the response reported herein from the CSM can be attributed to increases in energy intake because estimates of digestibility of the rye forage (IVOMD) were quite high (74.5 to 83.6%) throughout the grazing season. Also, forage availability (forage height between 13 and 18 cm) should not have limited intake. The high digestibility values for the rye forage in conjunction with sufficient forage availability suggest that energy intake by the steers should not limit growth rate (Blaxter et al., 1961; Conrad et al., 1964; Montgomery and Baumgardt, 1965). Therefore even though intakes were not measured, any additional energy derived from the CSM should have limited effect on total energy intake. Second, supplemental mechanically extracted CSM could elicit improvements in microbial fermentation function by increasing postruminal protein supply or increasing energy efficiency (Galbraith and Miller, 1973).

During the spring period, when the steers were heavier and the growth rate was reduced, the estimated MP requirements were 527 g, 537 g and 617 g for the control, CSM and CSM with lasalocid treatments, respectively. The MP supplied from the rye forage were in excess of the animal requirements which suggests that supplemental MP would not stimulate animal gain. No response in ADG by steers was noted with added CSM alone during the spring period.

Lasalocid did not increase ADG further during the fall period. However, during the spring grazing period, CSM with lasalocid increased (P <.05) gain by 22% above CSM alone and 25% above control. Whereas higher gains for the lasalocid group could be the result of improvements in N metabolism, this improvement also could have been the result of an increased feed efficiency from increases in ruminal propionate concentrations, as seen in a study by Davenport et al. (1988) with monensin. Other possible mechanisms could be 1) a reduction in sub-acute acidosis (Bergen and Bates, 1984) with a concomitant change in blood acid-base balance (Miller et al., 1988) or 2) an increase in forage DM intake. No evidence to date, however, supports the concept that ionophores increase forage intake (Anderson and Horn, 1987; Jacques et al., 1987). Further research on the mechanism of ionophore action on growth rate of cattle grazing pastures is needed.

Faster gains for the lasalocid group during the spring grazing period resulted in numerically (P >.05) greater gains for the entire grazing season compared with the group receiving only the CSM and greater (P <.05) gains than the control group. Average daily gain by steers receiving the CSM without lasalocid were intermediate to the control and lasalocid groups. Yet, differences in ADG among the three treatment groups were large and if repeatable would be important economically.

	Implications

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Rye forage is a high-quality feedstuff throughout most of the grazing season. Estimates of the N characteristics indicate that the majority of rye protein is degraded in the rumen with less than 25% escaping breakdown, especially at an early stage of maturity. This suggests that postruminal, nonammonia N (essential amino acids) could limit animal gain. Animal response data suggest that protein supply limited gain only early in the grazing season when animal protein requirements were highest. Feeding lasalocid had no effect on rate of animal gain early in the grazing season, but it stimulated rate of animal gain later in the season.

	Footnotes

 
¹ Journal paper No. 2889 of the South Carolina Agric. Exp. Sta., Clemson Univ., Clemson.

² Dept. of Anim. Sci.

³ Dept. of Exp. Stat.

⁴ Mineral composition (% of DM): Ca, 12 to 14; P, 7; NaCl, 17 to 19; K, 1; Mg, 2.5; S, 1.2; Fe, 1; Mn, .03; I, .01; Co, .01; Cu, .02; Zn, .02; Se, .001.

Received for publication April 7, 1989. Accepted for publication August 21, 1989.

	Literature Cited

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