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CROP QUALITY & UTILIZATION

Nitrogen Fertilization and Supplementation Effects on Performance of Beef Heifers Grazing Limpograss

^a EMPARN, C.P. 188, 59020-390, Natal, Brazil
^b Agronomy Dep., Univ. of Florida, Gainesville, FL 32611-0300 USA
^c Animal Science Dep., Univ. of Florida, Gainesville, FL 32611-0910 USA
^d USDA, ARS, 800 Buchanan St., Albany, CA 94710 USA

les{at}gnv.ifas.ufl.edu

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion Summary and conclusions REFERENCES

 
Seasonally low N concentrations in `Floralta' limpograss [Hemarthria altissima (Poir.) Stapf & C.E. Hubb.] limit intake and weight gain of growing animals. This research evaluated management alternatives for increasing summer weight gains of beef replacement heifers (Bos spp.) on limpograss pastures in Florida. Soils were sandy, siliceous, hyperthermic, aeric (Smyrna series) or ultic (Pomona series) Haplaquods. During 1992 and 1993, a factorial arrangement of two pasture N fertilization rates (50 and 150 kg ha^-1) and three diet supplements (NONE, corn [Zea mays L.] plus urea [CU], and CU plus rumen undegradable protein [CUUP]) were studied in two replications of a completely randomized design. Supplementation with CU increased average daily gain (ADG) from 0.06 (NONE) to 0.41 kg on pastures fertilized with 50 kg N ha^-1, but there was no ADG response to CU when N rate was 150 kg ha^-1. When no supplement was fed, increasing pasture N fertilization from 50 to 150 kg ha^-1 increased ADG from 0.06 to 0.36 kg. Grass crude protein (CP; 73 vs. 56 g kg^-1) and in vitro organic matter digestion (IVOMD; 542 vs. 509 g kg^-1) were greater at the higher N rate. Heifer plasma urea N (PUN) concentration was low (4.2 mg dL^-1) when no supplement was fed and pastures received 50 kg N ha^-1, suggesting that low CP was limiting ADG. These data indicate that N deficiencies of cattle grazing limpograss can be overcome by increasing rate of pasture N fertilization or by providing N supplements.

Abbreviations: ADG, average daily gain • CP, crude protein • CU, corn-urea supplement • CUUP, corn-urea-undegraded protein supplement • DM, dry matter • DOM/CP, in vitro digestible organic matter/crude protein ratio • IVOMD, in vitro organic matter digestion • NDF, neutral detergent fiber • NONE, no supplement • PUN, plasma urea N • TDN, total digestible nutrients

	INTRODUCTION

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WARM-SEASON PERENNIAL GRASSES

used as pasture or hay in Florida often do not meet the nutrient requirements of growing ruminants (Moore, 1992). Floralta limpograss is an important perennial forage in Florida because of its adaptation to poorly drained sites, relatively high digestibility, and better cool-season growth than other warm-season grasses. However, low CP concentration of Floralta limpograss limits weight gains of growing animals grazing pastures during the summer (Sollenberger et al., 1988, 1989; Holderbaum et al., 1991). High herbage digestible organic matter (DOM)/CP ratio and low PUN concentration of cattle grazing limpograss have been associated with lower than expected ADG (Holderbaum et al., 1991).

Several management alternatives have been tested to address the CP limitation in limpograss. Overseeding limpograss pastures with the legume aeschynomene (Aeschynomene americana L.) increased ADG from 0.39 (N-fertilized limpograss) to 0.70 kg (Rusland et al., 1988). Feeding a corn-urea supplement to livestock grazing limpograss improved ADG from 0.29 to 0.56 kg (Holderbaum et al., 1991).

Increasing pasture N fertilization rate may be an alternative to feeding N supplement. Additionally, supplementation efforts have focused on rumen degradable N sources. The potential benefit of including rumen undegradable protein is not known. Objectives of this study were (i) to determine the effect of increasing N fertilization rate of limpograss pastures on herbage CP concentration, IVOMD, and DOM/CP ratio; (ii) to quantify yearling heifer ADG, gain per hectare, and PUN concentration responses to pasture N fertilization and supplementation; and (iii) to quantify the feed and fertilizer cost per kg of gain above that achieved by heifers receiving no supplement and grazing pastures fertilized with 50 kg ha^-1 of N.

	Materials and methods

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Site Description
The experiment was carried out during July to October of 1992 and 1993 at the University of Florida Beef Research Unit located northeast of Gainesville, FL. The experimental area was 4.8 ha with 12 experimental units measuring 0.4 ha each. Soils at the research site were poorly drained, sandy Spodosols of low fertility, primarily belonging to the Pomona and Smyrna series. Soil pH ranged from 5.3 to 5.8 and Mehlich I extractable P and K each were approximately 20 mg kg^-1. Rainfall during June through October averages 750 mm in Gainesville. During these months in 1992 and 1993, rainfall was 860 and 517 mm, respectively.

Treatments and Experimental Design
Treatments compared were the factorial combinations of two rates of N fertilization (50 and 150 kg ha^-1) and three supplements (none, corn-urea, and corn-urea-undegradable protein) in two replications of a completely randomized design. Sources of undegradable protein were corn gluten meal and blood meal. Nitrogen fertilizer was applied as ammonium nitrate in three equal applications to achieve treatment totals. Applications were made weekly to the paddock (0.08 ha) on which grazing had ended that week. These applications occurred after the staging grazing, and after the first and second grazing cycles of the experiment.

Supplement Formulation
Ground corn (95 g CP kg^-1 and 870 g total digestible nutrients [TDN] kg^-1), corn gluten meal (700 g CP kg^-1 and 870 g TDN kg^-1), blood meal (850 g CP kg^-1 and 680 g TDN kg^-1), feed grade urea (2870 g CP kg^-1), S (from Dynamate, International Minerals Corp., Bannockburn, IL), Ca, and P were used in formulating the protein supplements. The supplements were formulated to be isocaloric and to provide approximately the same ruminally degradable protein (0.27 kg d^-1), and 0.05 (CU) and 0.14 kg d^-1 (CUUP) of ruminally undegradable protein (NRC, 1989). Supplements were fed at 0.7 kg dry matter (DM) per heifer d^-1 and had CP concentrations of 409 (CU) and 575 g kg^-1 (CUUP). Tables 1 and 2 describe the composition and ingredients for each supplement.

Animal and Pasture Measurements
Shrunk weights (16-h feed and water fast) of all animals were taken at the start of the trial and at 28-d intervals. Put and take animals were weighed when added to or removed from pasture. On weigh dates, blood samples were collected (10 mL) from tester animals. The plasma was separated by centrifugation, frozen, and stored at -20°C for PUN analysis. Samples were analyzed using an automated colorimetric procedure (Technicon Autoanalyzer II Industrial Method no. 339-01, Technicon Instruments Corp., Tarrytown, NY) based on the diacetyl monoxime method of Marsh et al. (1965). Weight gains of testers were used to calculate ADG. Animal grazing days per hectare was determined using both tester and put and take animals. Live weight gain per hectare was calculated by multiplying ADG of tester heifers by animal grazing days per hectare.

Pregraze and postgraze herbage mass were measured on the second and fifth paddocks during each grazing cycle. On each date, eight 0.25-m² quadrats were sampled per paddock. Sites were selected that, based on visual observation, represented average herbage mass of the paddock. Herbage was clipped to a 15-cm stubble height. Herbage allowance was calculated as average herbage mass (pregraze plus postgraze herbage mass divided by two) during a grazing period divided by average live weight of heifers on the pasture during that period. At the time of pregraze sampling, hand-plucked samples were taken at 20 sites per paddock. A second hand-plucked sample from Paddock 5 was taken in each grazing cycle and separated into limpograss leaf blade and limpograss stem plus sheath fractions. Crude protein, IVOMD, and neutral detergent fiber (NDF) were determined on the hand-plucked samples and for the plant-part fractions. In vitro OM digestion was determined by a modified two-stage digestion procedure (Moore and Mott, 1974). A modified aluminum block digestion procedure (Gallaher et al., 1975) and semi-automated colorimetry (Hambleton, 1977) were used for analysis of N concentration. Determination of NDF followed the procedure described by Golding et al. (1985).

Statistical Analyses
Data were analyzed by ANOVA in PROC GLM of the Statistical Analysis System (SAS, 1989). Year was included in the model as a subplot treatment in a split-plot arrangement of the completely randomized design. Nitrogen by supplement treatment combination was the main plot. To test specific hypotheses inherent to the research, treatment comparisons were made by preplanned single degree of freedom contrasts. Contrasts were (i) N rate 50 vs. 150 kg ha^-1, (ii) N rate 50 vs. N rate 150 kg ha^-1 when no supplement was fed, (iii) CU vs. CUUP, (iv) CU plus CUUP vs. NONE when N rate was 50 kg ha^-1, and (v) CU plus CUUP vs. NONE when N rate was 150 kg ha^-1. Treatments were considered different for animal responses when P value of the contrast was 0.10 and for plant responses when P value of the contrast was 0.05. When planned comparisons of CU vs. CUUP were not valid due to N rate by supplement interactions, additional single degree of freedom contrasts were run to test supplement effects within level of N rate.

	Results

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Pasture Responses
There were no treatment x year interactions (P > 0.203) for pasture responses. Data presented are means across the 2 yr.

Herbage allowance (kg herbage DM per kg heifer live weight) was not affected by treatments or their interaction in either year and averaged 0.94 kg DM per kg live weight across periods and years. Sollenberger et al. (1996) reported that ADG of growing cattle grazing stargrass (Cynodon nlemfuensis Vanderyst) increased linearly with increasing herbage allowance up to 0.9 kg of DM per kg of live weight, after which there was no further increase in ADG with increasing allowance. No comparable data are available for limpograss, but if the ADG vs. herbage allowance relationship is similar to that of stargrass, then quantity of forage likely was not limiting gain in the current experiment.

Nitrogen fertilizer increased pasture carrying capacity , but there was no effect of supplementation nor was there an N rate by supplement interaction. Averaged across years, the higher N rate had 667 heifer days per hectare per year compared to 599 for the low N rate.

Herbage nutritive value and leaf blade percentage were affected primarily by N fertilization. In several instances there were interactions of N rate and supplementation, but in all cases the interaction was due to a change in magnitude not in direction of the response. To simplify presentation of the data and to emphasize those differences that are thought to be of biological importance, this section will focus on the N fertilization effects.

Herbage CP concentration increased 17 g kg^-1 from the low to high N fertilizer rates (Table 3) . There was a trend toward greater CP in herbage from pastures on which heifers were supplemented (66 g kg^-1) than on those where no supplement was fed (62 g kg^-1). In vitro OM digestion was greater on pastures receiving 150 kg ha^-1 of N than on those receiving 50 kg ha^-1, while NDF was lower on pastures receiving the high N rate (Table 3). A proportionally greater response to N fertilizer was observed for CP than for IVOMD and this difference in response resulted in lower herbage DOM/CP ratio for pastures fertilized at the higher N rate (Table 3). More N fertilizer also increased leaf percentage of the herbage (Table 3).

There was an N rate by supplement interaction for live weight gain per hectare . Interaction occurred because there was a much greater increase in gain per hectare because of supplementation when pastures received 50 kg ha^-1 of N fertilizer than when they received 150 kg ha^-1 (Table 5). Heifers receiving no supplement gained nearly 200 kg ha^-1 more liveweight when the higher N fertilization rate was used. Heifers receiving the CUUP supplement averaged 90 kg ha^-1 greater live weight gain than those receiving CU (Table 5).

	Discussion

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Nitrogen Fertilizer Effects
Pasture Responses
Applying 150 compared to 50 kg N ha^-1 resulted in an increase in carrying capacity of approximately 100 heifer days ha^-1 in 1992 when rainfall was near normal. In the drier than normal year of 1993 (58% of 70-yr average rainfall during July through September), the higher N rate promoted an increase in carrying capacity of only 37 heifer days ha^-1.

Increasing N fertilizer rate from 50 to 150 kg ha^-1 increased leaf percentage and total herbage and plant-part IVOMD and CP. Similar increases in CP have been reported for tropical grasses (Humphreys, 1987; Hodgson, 1990), but many experiments report little effect of N rate on IVOMD (Wilson, 1982; Messman et al., 1991). Crude protein of leaf blades was more than twice that of stem plus sheath, but IVOMD of the two fractions varied by only 6 to 12 g kg^-1. Thus, the increase in leaf proportion with higher N rate had a much greater impact on total herbage CP than IVOMD. Holderbaum et al. (1991) reported that limpograss stem plus sheath fractions had similar or greater IVOMD than leaf blade from the same stratum of the canopy.

The relationship between herbage IVOMD and CP concentrations expressed as DOM/CP ratio is very important in determining an animal's N status (Hogan and Weston, 1981; Moore, 1992). Nitrogen fertilization at the higher rate increased herbage CP by an average of 30% (17 g kg^-1); in contrast, herbage IVOMD increased only 6.5% (33 g kg^-1) causing herbage DOM/CP to decrease.

Animal Responses
Increasing N fertilizer rate from 50 to 150 kg ha^-1 increased heifer PUN from 4.2 to 9.2 mg dL^-1 and ADG from 0.06 to 0.36 kg. These animal responses were associated with changes in herbage composition caused by fertilization. One effect of higher N rate was to increase herbage leaf percentage. This may have contributed in part to greater ADG of unsupplemented heifers grazing the higher N rate pastures. Intake of leaf has been reported to be 42% greater than stem of the same digestibility in 26 comparisons using tropical grasses (Laredo and Minson, 1975). Our data suggest, however, that the most important factors driving the ADG and PUN responses were herbage CP concentration and DOM/CP ratio. Heifers grazing the low N fertilizer treatment consumed herbage with an average of 56 g CP kg^-1 and a DOM/CP of 9.1. Increasing N rate to 150 kg N ha^-1 increased CP to 73 g kg^-1 and decreased DOM/CP ratio to 7.4. When CP concentration of forages is less than 70 g kg^-1, there may be inadequate protein to supply the needs of rumen bacteria, and forage intake and animal performance may be decreased (Moore et al., 1991). Moore and Kunkle (1995) have suggested that cattle grazing forages with DOM/CP ratios greater than 7 to 8 should respond to N supplementation by increasing forage intake and performance. Cattle PUN concentrations of 9 to 12 mg dL^-1 are in a transition range below which ADG response to protein supplementation has been greater and above which the ADG response has been less (Hammond et al., 1993). Concentrations of 4.2 mg dL^-1 in cattle grazing the 50 kg N ha^-1 treatment are clearly in the range of PUN where response to additional protein is likely. In this case, additional protein was provided by increasing the N fertilization rate of the pasture and a resultant increase in leaf percentage and total herbage CP concentration.

For no supplement treatments, gain per hectare increased six fold as N fertilizer rate increased from 50 to 150 kg ha^-1. The greatest proportion of this increase was due to greater ADG for the higher N rate treatment.

Supplement and N Rate by Supplement Interaction Effects
The N fertilizer by supplement rate interaction for ADG occurred because the increase in weight gain due to supplementation (either CU or CUUP) was very large at the low N fertilizer rate and much smaller (CUUP vs. NONE) or did not occur (CU vs. NONE) at the high rate. These data suggest that increased N fertilization of limpograss is as effective or at least nearly as effective as CU supplementation in overcoming N deficiency.

Plasma urea N concentration was higher for heifers grazing pastures that received the higher N fertilizer rate and for supplemented heifers. The PUN concentration of 9.2 mg dL^-1 for the unsupplemented, 150 kg ha^-1 fertilization rate treatment falls in the transition zone of 9 to 12 mg dL^-1, in which response to N supplement is less predictable. In this case there was no measurable response to CU when PUN exceeded 9 mg dL^-1.

Ellis (1990) reported that inadequate daily intake of undegradable intake protein limited performance of growing heifers grazing warm-season pastures. In the current study, addition of undegradable intake protein (CUUP) increased ADG 37% over that observed for heifers receiving CU on pastures fertilized at the low N rate. This suggests that microbial protein synthesis plus any undegradable intake protein provided by the CU supplement and limpograss herbage may not have satisfied the metabolic protein requirement of the heifers. Owens et al. (1991) indicated that supplemental ruminally undegraded protein, by evading rumen fermentation, can directly increase the intestinal supply of amino acids and glucogenic compounds. At the high N rate in the current study, there was a slight trend favoring the CUUP treatment vs. . It remains unclear why ADG for the CUUP treatment was not greater than for CU at the high N rate. One possible explanation is linked with the higher herbage CP concentration on pastures receiving the high N fertilizer rate. Protein of warm-season grasses is considered more slowly degraded than that of cool-season grasses (Akin, 1989), a characteristic that can provide a greater proportion of protein for utilization in the small intestine (Karges et al., 1992). The 30% increase in herbage CP concentration provided by the higher N fertilizer rate may have increased passage of undegradable intake protein to the intestines and limited response to the CUUP treatment.

Gain per hectare responses closely paralleled those of ADG. One exception was that gain per hectare of heifers supplemented with CUUP was superior to CU at low and high N fertilizer rates. Kunkle (1993) evaluated several strategies for cost effective supplementation of beef cattle in Florida. He indicated that protein supplementation was the second priority after mineral supplementation and that protein supplementation generally was more cost effective than energy supplementation. In the present experiment, cost per kilogram of gain (N fertilizer and supplement materials only) above that observed on the 50 kg N ha^-1-no supplement treatment was calculated. Cost was least for the 150 kg N ha^-1-no supplement ($0.39/kg) and 50 kg N ha^-1-CUUP supplement treatments ($0.41). These results were similar to those reported by Holderbaum et al. (1991) when feeding a corn-urea supplement to yearling cattle grazing limpograss ($0.44/kg).

	Summary and conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion Summary and conclusions REFERENCES

 
Increasing N fertilization of limpograss pastures from 50 to 150 kg ha^-1 increased herbage IVOMD and CP, decreased DOM/CP ratio, increased heifer PUN concentration, and increased heifer ADG and gain per hectare. On pastures fertilized with 50 kg N ha^-1, heifer gain increased by 0.35 to 0.50 kg d^-1 when N supplements were fed, but when pastures received 150 kg N ha^-1, there was no increase in ADG for CU (compared to NONE) and the response to CUUP (relative to NONE) was 0.11 kg d^-1. Heifers receiving CUUP supplement had greater ADG than those receiving CU when fertilization rate was 50 kg N ha^-1 and tended to have greater ADG when fertilization rate was 150 kg N ha^-1. This response to undegradable intake protein suggests that neither protein synthesized in the rumen nor that from CU supplement or limpograss herbage were enough to meet the heifers' requirement. Of the management alternatives evaluated, cost per kilogram of additional gain (above that for 50 kg N ha^-1-NONE) was least for 150 kg N ha^-1-no supplement ($0.39/kg) and 50 kg N ha^-1-CUUP supplement treatments ($0.41). We conclude that both N fertilization and supplementation can be used to overcome N deficiencies of heifers grazing limpograss, and that at least in some cases heifers grazing limpograss will respond to addition of undegraded intake protein to a corn-urea supplement.SAS Institute 1989

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion Summary and conclusions REFERENCES

 
Florida Agric. Exp. Stn. Journal Series no. R-06861.

Received for publication February 1, 1999.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion Summary and conclusions REFERENCES

 

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