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Agronomy Journal 94:1375-1380 (2002)
© 2002 American Society of Agronomy

PRODUCTION PAPER

Canopy Height and Nitrogen Supplementation Effects on Performance of Heifers Grazing Limpograss

Y. C. Newmana, L. E. Sollenberger*,a, W. E. Kunkleb and C. G. Chamblissa

a Agron. Dep., P.O. Box 110300, Univ. of Florida, Gainesville, FL 32611-0300
b Dep. of Anim. Sci., P.O. Box 110910, Univ. of Florida, Gainesville, FL 32611-0910

* Corresponding author (les{at}mail.ifas.ufl.edu)

Received for publication December 10, 2001.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
Leaf percentage and herbage crude protein (CP) are greater in the upper than lower half of limpograss [Hemarthria altissima (Poir.) Stapf & Hubb.] canopies; thus, canopy height may affect N status and cattle (Bos spp.) weight gain. Average daily gain (ADG) of beef heifers and sward characteristics were measured on continuously stocked limpograss pastures grazed to heights of 20, 40, and 60 cm, with cattle receiving 0 or 0.8 kg d-1 of a 44% CP corn (Zea mays L.)–urea mixture. Cattle grazing 20- and 60-cm pastures increased ADG by 200 g when supplemented (to {cong} 590 g); however, cattle grazing 40-cm pastures tended to have greater ADG when not supplemented (644 g) vs. when supplemented (536 g). Herbage mass and allowance increased with increasing canopy height, partially explaining greater ADG of unsupplemented cattle grazing 40- vs. 20-cm swards. The 40-cm canopies also had lower herbage bulk density, allowing greater opportunity for selection of leaf than in 20-cm canopies. Stocking rate increased linearly (5.9–8.5 head ha-1) as canopy height decreased from 60 to 20 cm, and gain per hectare increased linearly with decreasing canopy height for both unsupplemented (161–352 kg) and supplemented (273–378 kg) treatments. Data suggest stubble height is an important determinant of heifer performance. A 40-cm canopy height may be near optimum because of greater herbage allowance and opportunity to select leaf than in dense 20-cm canopies and less trampling and greater leaf proportion and accessibility than in 60-cm swards.

Abbreviations: ADG, average daily gain • BUN, blood urea nitrogen • CP, crude protein • DOM, digestible organic matter • DM, dry matter • HA, herbage allowance • HM, herbage mass • IVOMD, in vitro organic matter digestibility


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
DESPITE HIGH in vitro organic matter digestibility of forage, the performance of animals grazing limpograss pastures has been relatively low, attributable in some cases to N deficiency (Sollenberger et al., 1988, 1989). Average daily gains of animals grazing limpograss have ranged from 0.06 to 0.6 kg, with lowest gains associated with low herbage CP and cattle blood urea N concentrations (Rusland et al., 1988; Sollenberger et al., 1989; Holderbaum et al., 1991; Lima et al., 1999; Postiglioni et al., 2001). Different options have been used to overcome N deficiency, including association of legumes with limpograss (Rusland et al., 1988), the feeding of N supplements (Holderbaum et al., 1991; Lima et al., 1999), and increased N fertilization (Lima et al., 1999). These alternatives provide additional N to the animals and increase liveweight gains; however, less costly alternatives like grazing management remain to be tested. Grazing intensity has been recognized as the most important grazing management factor affecting the availability of pasture nutrients to grazing livestock (Chacon and Stobbs, 1976; McGillway et al., 1999). Likewise, it is the primary determinant of animal production, whether on a per animal or per unit land area basis (Jones and Jones, 1997). More intensively grazed tropical grass pastures have less biomass but greater herbage bulk density than taller pastures (Sollenberger and Burns, 2001), often resulting in less opportunity for livestock to prehend leaf and negatively affecting performance (Chacon and Stobbs, 1976).

Characterization of the nutritive value of limpograss canopies (Holderbaum et al., 1992) showed that leaf percentage, herbage CP concentration, and in vitro organic matter digestibility (IVOMD) are variable throughout the canopy profile. Leaf percentage was three times greater in the upper than lower half of the canopy, CP twice as great, and IVOMD approximately 10% greater. These data suggest that decreasing grazing intensity, such that grazing occurs higher in the canopy, may allow for a diet higher in leaf and CP and avoid CP deficiency of grazing livestock. Thus, the objectives of this study were to evaluate the effects of different grazing heights of limpograss pastures on canopy characteristics and liveweight gain responses of yearling beef heifers.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
Site Characterization and Experimental Design
The study was conducted from 15 July to 7 Oct. 1998 and 24 June to 16 Sept. 1999 on established stands of ‘Floralta’ limpograss located at the Forage Evaluation Field Laboratory–Beef Research Unit, University of Florida, Gainesville (29°38' N, 82°22' W). Soils were of the Smyrna (sandy, siliceous, hyperthermic Aeric Alaquod) and the Pomona and Wauchula series (both sandy, siliceous Ultic Alaquods). These are low-organic-matter, poorly drained soils. The pH at the site averaged 5.8. Average nutrient concentrations were Mehlich-I extractable P, 9 mg kg-1; K, 18; Ca, 716; and Mg, 81. Based on soil analysis and the intended use of the land, all pastures were fertilized annually with 160, 17, and 66 kg ha-1 N, P, and K, respectively. All P and K and 40 kg N ha-1 were applied on 15 Apr. 1998 and 4 May 1999 to promote grass growth. The remaining N (120 kg ha-1) was split in three equal applications made on 26 June, 29 July, and 27 Aug. 1998 and on 10 June, 8 July, and 12 Aug. 1999.

Six treatments were replicated twice in a completely randomized design. Treatments were all combinations of three canopy heights (20, 40, and 60 cm) of the continuously stocked swards and two N supplement levels (with and without). The supplement was a corn–urea mixture, with 44% CP (40% digestible intake protein), and was fed daily in a ration of 640 g dry matter (DM) per animal (Table 1). The experimental units in the study were 0.5-ha pastures. Sampling units for blood urea N (BUN) and gain were the individual tester animals; for plant-related measurements, sampling units were 0.5-m2 quadrats.


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  		Table 1. Composition [dry matter (DM) basis] and amount fed of the corn–urea supplement.

 
Grazing Management
Two crossbred (three-fourths Angus, one-fourth Brahman) yearling heifers of similar initial weight (average of 355 and 335 kg for 1998 and 1999, respectively) and medium frame were assigned to each pasture as testers; they remained on that pasture during the entire season. A variable stocking rate was used, and additional animals of the same breed and similar weight as the testers were added or removed as needed to achieve the target treatment canopy height for each pasture. Tester animals were assigned to pastures 1 wk before the beginning of the experimental period each year. During this time, they were fed gradually increasing amounts of supplement until the full daily ration was being consumed. Pastures were supplied with a portable shade, tubs for minerals and supplement, and fresh water. A mineral mix was provided ad libitum, which contained Ca, 140 to 160 g kg-1; P, >60; K, >10; Na, 220 to 230; Mg, >6.5; Fe, >9; Zn, >1.9; Cu, >0.8; Mn, >2.5; Co, >0.3; and Se, <0.03.

Plant Measurements
Canopy height was monitored weekly at 50 random sites per pasture. Based on the average height, stocking rate was adjusted as needed; however, most adjustments were made in conjunction with weigh days (every 28 d). Herbage mass (HM) was measured biweekly at five representative 0.5-m2 sites per pasture, and samples were cut to a 7-cm stubble height and dried at 60°C. Herbage allowance (HA) was calculated as HM divided by the total kilograms per hectare of animal liveweight. Concurrently, a hand-plucked sample (approximately top 5 cm of the canopy) was taken from 20 random sites in each pasture to represent the nutritive value of the diet selected by the animal. Additionally, the canopy was characterized for herbage density and leaf percentage twice in 1998 (13 August and 15 September) and three times in 1999 (14 July, 9 August, and 7 September). For this characterization, the upper 25% layer by height was sampled, and percentage leaf and total and leaf herbage bulk density (kg DM ha-1 cm-1) were calculated. The upper layer was the top 5 cm in the 20-cm pastures, top 10 cm in the 40-cm pastures, and top 15 cm in the 60-cm pastures.

Micro-Kjeldahl CP and IVOMD (Moore and Mott, 1974) analyses were conducted on hand-plucked samples. Ratios of digestible organic matter (DOM) to CP were calculated. Herbage mass and HA are the means across sampling dates within a year. Percentage leaf, herbage CP, IVOMD, and bulk density are the weighted means across sampling dates within a year.

Animal Measurements
At the end of the preliminary period and every 28 d during the experimental period, shrunk weights were obtained and blood samples collected from all animals by tail venipuncture. Blood samples were centrifuged for 20 min at 3000 rpm using a Sorvall RC 3B centrifuge (Dupont Co., Wilmington, DE), and plasma was stored at -20°C for analysis of BUN by automated colorimetric determination (Marsh et al., 1965; Hammond, 1983). Weight gains of tester animals were used to calculate ADG. Total heifer grazing days per hectare for the entire experimental period were calculated using both testers and put-and-take animals and were expressed on the basis of a 350-kg animal. Average stocking rate was determined by dividing total heifer days per hectare by 84 (days in experimental period). Gain per hectare was determined by multiplying ADG of the testers on a given pasture by the number of heifer grazing days per hectare.

Statistical Analyses
Data were analyzed using mixed-model methodology through PROC MIXED (SAS Inst., 1996). In all models, effects of canopy height, supplementation, and their interaction were considered fixed effects. The nature of the canopy height effect was assessed using orthogonal polynomial contrasts. All means reported in the text are least square means. Average daily gain was regressed on HM and HA, and the least-squares regression parameters were calculated using PROC REG (SAS Inst., 1996).


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
SUMMARY AND CONCLUSIONS
 REFERENCES
 
Average maximum temperatures for the experimental period were 32 and 34°C in 1998 and 1999, respectively, and the average minimum was 22°C for both years. In 1999, the highest temperature was 39°C, and there were 14 d with temperatures >=35°C. In 1998, the highest temperature was 36°C, and there were only 4 d with temperatures >=35°C. Total annual rainfall was 1300 and 962 mm, respectively, for 1998 and 1999 (30-yr average of 1342 mm); rainfall during the experimental period (84 d) was 550 and 373 mm for 1998 and 1999, respectively (30-yr average of 525 mm).

Toward the end of the grazing period in 1998 (20 September), damage to limpograss from two-lined spittle bug (Prosapia bicincta Fay) and southern chinchbug (Blissus insularis Barber) was observed. Also present was a blight, diagnosed as Gaeumannomyces graminis Sacc. var. graminis, that causes a root rot for which there is no control in pastures (Speakman et al., 1978). These pests caused browning of herbage, but no control efforts were attempted because they are often ineffective after symptoms appear.

Average Daily Gain
There was no year effect (P = 0.25) on heifer ADG nor was there a year x canopy height or year x supplement treatment interaction (P = 0.61 and P = 0.88, respectively). There was an interaction (P <= 0.05) between canopy height and supplementation treatments (Table 2). Interaction occurred because there was a positive response to supplement at the 20- (40% increase) and 60-cm (70% increase) grazing heights but not at 40 cm. Heifers grazing either 20- or 60-cm pastures and receiving supplement gained nearly 200 g d-1 more than those receiving no supplement. For the 40-cm height, supplementation tended to decrease ADG (P = 0.10), but the reason for this response is not clear. Gains of supplemented heifers were not affected by canopy height. Unsupplemented heifers had increased ADG as canopy height increased from 20 to 40 cm, but ADG decreased from 40 (644 g) to 60 cm (327 g) (Table 2). Heifer ADG on the unsupplemented 40-cm treatment was similar to that reported for cattle on continuously stocked limpograss pastures in Brazil (Postiglioni et al., 2001) and slightly higher than the 551 g reported in Colombia (Tergas et al., 1982).


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  		Table 2. Average daily gains (ADG) and blood urea N concentrations (BUN) of cattle grazing limpograss pastures in response to canopy height and supplementation (suppl). Data are means across 2 yr and two replicates (n = 4).

 
Heifer Blood Urea Nitrogen
There was no canopy height x supplement treatment interaction (P = 0.25) for BUN. There were, however, supplement effects (P <= 0.05) with greater BUN for supplemented vs. unsupplemented (18 and 15 mg 100 mL-1, respectively) heifers. There were also canopy height effects (P <= 0.05), and BUN decreased linearly as height increased (Table 2). Cattle BUN concentrations of 9 to 12 mg 100 mL-1 are in a transition range below which daily gain response to N supplementation has been greater and above which the daily gain response has been less (Hammond et al., 1993). In this study, BUN levels were all above 12 mg 100 mL-1, and the relationship between heifer BUN and ADG was not consistent across treatments. Lowest BUN levels did occur for heifers with lowest ADG (unsupplemented 20- and 60-cm treatments), and when heifers were supplemented, both BUN and ADG increased. The 40-cm treatment deviated from this pattern as greater BUN of supplemented animals was associated with lower ADG.

Pasture Responses
Herbage Mass and Allowance
Herbage mass and HA increased linearly (P <= 0.01) with increasing canopy height (Table 3), but they were not affected by supplement treatment (P > 0.36). Herbage mass averaged >=3000 kg DM ha-1 for all canopy heights, suggesting that HM alone was not limiting animal performance. When ADG of unsupplemented cattle was plotted against HM, there was no linear relationship. The quadratic term was significant, however, because of lower ADG for low-HM 20-cm pastures and high-HM 60-cm pastures (Fig. 1A) . Removing the 60-cm data from the regression showed that ADG increased linearly with increasing HM for the 20- and 40-cm unsupplemented treatments, but HM explained only 33% of the response (Fig. 1B). Herbage mass on the 60-cm pastures was approximately 6000 kg DM ha-1, and Sollenberger et al. (1988) suggested that lenient grazing of limpograss results in a buildup of stem that may reduce DM intake and thereby limit gain. In addition, trampling of accumulated material occurred on 60-cm pastures as the season progressed, resulting in significant lodging. This observed trampling may result in limited access to leaf biomass and may be responsible, in part, for the lower gains observed for this treatment.


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  		Table 3. Herbage mass (HM) and herbage allowance (HA) in response to canopy height. Data are means across two supplement levels, 2 yr, and two replicates (n = 8).

 


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  		Fig. 1. Effect of herbage mass (HM) and herbage allowance (HA) on average daily gain (ADG) of heifers grazing unsupplemented limpograss pastures. (A) and (C) include data from all unsupplemented treatment–replicate combinations in both 1998 and 1999; (B) and (D) include data from unsupplemented 20- and 40-cm treatment–replicate combinations in both 1998 and 1999.

 
Similar to the ADG response to HM, there was a quadratic response of ADG to HA when all data for unsupplemented treatments were included in the model (Fig. 1C), but the response was linear if data from the 60-cm treatment were omitted (Fig. 1D). Herbage allowance (HM per kilogram of animal liveweight) means were >=1.27. Herbage allowance of >=1.0 was associated with highest gains of yearling beef heifers on continuously stocked bermudagrass [Cynodon dactylon (L.) Pers.] (Pedreira, 1995) and rotationally stocked stargrass pastures (C. nlemfuensis Vanderyst) (Sollenberger et al., 1997). Thus, only for the 20-cm treatment were HAs close to levels thought to limit ADG of cattle grazing other C4 grasses.

Upper-Layer Herbage Bulk Density and Leaf Percentage
There was a year x canopy height interaction (P < 0.01; Table 4) for limpograss herbage bulk density and leaf percentage in the upper layer. Total bulk density declined with increasing canopy height in both years (Table 4), but the magnitude of the decline was greater in 1998. Total herbage bulk density of 20-cm canopies was greater in 1998 than in 1999 (122 vs. 95 kg ha-1 cm-1, respectively) while for taller canopies, there was little effect of year (Table 4). Leaf bulk density also declined with increasing canopy height in both years, but the magnitude of the decline was greater in 1999 (Table 4). Leaf percentage tended to decline as canopy height increased from 40 to 60 cm in both years, but in 1998, leaf percentage was least at 20 cm, and in 1999, it was greatest (Table 4). The absence of a relationship between leaf bulk density or leaf percentage and ADG suggests that ease of leaf prehension may have been an important determinant of animal performance. Specifically, it seems likely that the greater total herbage bulk density for 20-cm canopies limited opportunity for leaf selection. Although leaf was present in relatively large amounts in 20-cm pastures, its close association with stem made it difficult for cattle to select the leaf without consuming the stem. Similar observations have been reported for other C4 grasses by Burns et al. (1991). Lower herbage bulk density for 40-cm canopies may have provided greater opportunity for selection of leaf in the pasture and likely contributed to the higher ADG on unsupplemented 40- vs. 20-cm pastures. Although bulk density was low for 60-cm canopies, lodging and trampling likely limited access to leaf and were responsible, in part, for the low gains observed at this canopy height.


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  		Table 4. Limpograss total herbage and leaf bulk density (BD) and leaf percentage in the upper layer (top 25% by height) in response to canopy height during 1998 and 1999. Data are means across two supplement levels and two replicates (n = 4).

 
Herbage Nutritive Value
For herbage CP, IVOMD, and DOM/CP ratio, there was no effect of supplementation or year x supplementation treatment interaction (P > 0.49), but there was a year x canopy height interaction (P < 0.01; Table 5). Crude protein and IVOMD of hand-plucked herbage were higher and DOM/CP ratio lower in 1999 compared with 1998 (Table 5). As described earlier, during 1999, there was less rainfall on an annual basis and 173 mm less rainfall during the experimental period than in 1998. Mild drought, such as that in 1999, has been associated with increased herbage nutritive value (Wilson, 1983). Moreover, an accumulation of free amino acids from protein hydrolysis in the plant tissues seems to occur with water-stressed plants (Jones et al., 1980).


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  		Table 5. Limpograss hand-plucked herbage crude protein (CP), in vitro organic matter digestibility (IVOMD), and digestible organic matter (DOM)/CP ratio in response to canopy height during 1998 and 1999. Data are means across two supplement levels and two replicates (n = 4).

 
Herbage CP decreased with increasing canopy height in both years while IVOMD decreased only in 1999 (Table 5). In addition, there was much greater incidence of chinchbug damage in 1998 than in 1999. Associated browning of herbage likely was responsible in part for lower nutritive value in 1998. Although the differences are small, the lower CP at 60 cm was associated with lower BUN and may have been associated with the lower gains on that treatment. As noted earlier, however, BUN levels of cattle on all treatments were above those where response to supplement is expected. The herbage DOM/CP ratio increased with increasing canopy height in both years (Table 5) but was well below levels of 8.7 reported for rotationally stocked limpograss by Holderbaum et al. (1991) and 9.1 by Lima et al. (1999) when cattle grazing these forages responded to similar corn–urea supplements. A herbage DOM/CP ratio of 7 to 9 corresponds to a range above which daily gain response to N supplementation is expected and below which the daily gain response is not expected (Moore et al., 1999). In this study, DOM/CP ratios were below 7.0, indicating a favorable balance of protein and energy, due largely to herbage CP concentration that was somewhat greater than expected.

Average Stocking Rate and Gain per Hectare
For stocking rate, there was no effect of year (P = 0.14) or supplement (P = 0.45), but there was a canopy height effect (P <= 0.05). Greater stocking rate was achieved when swards were grazed to 20 compared with 40 or 60 cm, and the response was linear (Table 6).


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  		Table 6. Canopy height effects on average stocking rate and gain per hectare of yearling heifers grazing limpograss pastures. Data are means across 2 yr and two replicates (n = 4).

 
Seasonal gain per hectare of heifers was not different (P = 0.14) between the 2 yr. There was no year x canopy height or year x supplement treatment interaction (P = 0.97 and P = 0.72) for gain per hectare, but there was a trend toward interaction (P = 0.12) between canopy height and supplement treatments (Table 6). This trend was caused by the large response to supplement for the 60-cm treatment but the lack of response at 20 or 40 cm. In both years, there was a linear decline in gain per hectare as canopy height increased. This occurred due to greater stocking rates on shorter pastures. As an indication that none of the pastures were extremely overgrazed, gain per hectare was still increasing with the high stocking rate imposed to achieve a 20-cm canopy although less so for unsupplemented than supplemented animals (Table 6).


   SUMMARY AND CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
REFERENCES
 
Highest daily gains of unsupplemented heifers occurred with a canopy height of approximately 40 cm. Based on forage DOM/CP ratio and heifer BUN, there were no obvious instances of protein deficiency in this experiment. Greater HM and HA explained a portion of the increase in ADG on unsupplemented 40- vs. 20-cm treatments, but further increases in both HM and HA were associated with lower ADG for 60-cm pastures. High herbage bulk density in the 20-cm canopies likely limited opportunity for selection of leaf while lower bulk density and greater HM in the 40-cm canopies made selection of leaf possible, explaining in part the greater ADG on those pastures.

Greater average stocking rate and gain per hectare were obtained in heavily grazed pastures (20 cm) compared with those grazed at 40- or 60-cm heights, but grazing closely has associated risks, such as reduced limpograss persistence and encroachment by other grasses like common bermudagrass (Newman, 2001). Based on these data, grazing of limpograss to a 40-cm height is an acceptable compromise between production per animal and per hectare and increases the likelihood of sustaining the pasture resource.


   ACKNOWLEDGMENTS
 
Dr. Newman, Sollenberger, and Chambliss wish to honor the life and career of our friend and colleague, Dr. William E. Kunkle, who passed away on 21 Feb. 2002. Dr. Kunkle was an integral part of the forage–livestock research and extension programs of the University of Florida for many years, and he will be greatly missed.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
Florida Agric. Exp. Stn. Journal Ser. no. R-08547.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 




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