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Published in Crop Sci. 44:1348-1354 (2004).
© 2004 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

FORAGE & GRAZING LANDS

Nitrogen Fertilization and Stocking Rate Affect Stargrass Pasture and Cattle Performance

A. Hernández Garaya, L. E. Sollenbergerb,*, D. C. McDonaldc, G. J. Ruegseggerd, R. S. Kalmbachere and P. Mislevye

a Colegio de Postgraduados, Montecillo, Texcoco, Edo de México, Mexico CP56230
b P.O. Box 110300, Univ. of Florida, Gainesville, FL 32611-0300
c Bodles Res. Stn., Old Harbour P.O., Jamaica
d Dairy Nutrition Services, Chandler, AZ 85210
e Range Cattle Res. and Educ. Center, Ona, FL 33865

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


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Stargrass (Cynodon nlemfuensis Vanderyst) is an important tropical forage, but the interaction of stocking rate (SR) and N fertilizer rate on stargrass pastures is not well understood. The objective was to determine the effects of three SR (2.5, 5.0, and 7.5 bulls ha–1) and three N rates (112, 224, and 336 kg ha–1 yr–1) on stargrass pasture characteristics and performance of Jamaica Red Poll (Bos taurus x B. indicus) weanling bulls at St. Ann, Jamaica. Soil was a bauxitic clay loam, and pastures were rotationally stocked (7-d grazing and 21-d rest period). Pregraze herbage mass increased as SR decreased (2.0-4.8 Mg ha–1 in Year 1 and 3.3-8.3 Mg ha–1 in Year 2). Herbage crude protein (CP) and in vitro digestible organic matter (IVDOM) generally increased with increasing SR and N rate. Bull daily gain decreased curvilinearly from 0.70 to 0.26 kg in Year 1 and 0.65 to 0.35 kg in Year 2 as SR increased from 2.5 to 7.5 head ha–1. Daily gain increased linearly as N rate increased from 112 to 336 kg ha–1. The N fertilizer rate had little effect on gain per hectare at the lowest SR, but gain increased with fertilization up to 224 kg N ha–1 for a SR of 5 head ha–1 and up to 336 kg N ha–1 for a SR of 7.5. In conclusion, economic return from N fertilization of stargrass pastures is dependent upon SR, with greater N rates more likely to be profitable if SR is high.

Abbreviations: ADG, average daily gain • CP, crude protein • DM, dry matter • IVDOM, in vitro digestible organic matter • NDF, neutral detergent fiber • SR, stocking rate


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
STARGRASS is widely used throughout the Caribbean, Latin America, and southern Florida because of its productivity, ease of establishment, and persistence under grazing (Caro-Costas et al., 1976; Pitman et al., 1984; Mislevy et al., 1989). It is considered one of the most important forage grass species in Puerto Rico (Caro-Costas et al., 1976), Costa Rica (Rozas, 1975), Guatemala (Rodriguez et al., 1991), Jamaica (McDonald, 1993), and Mexico (Meléndez et al., 1980; Pérez-Pérez et al., 1997). Although forage quality of tropical grasses is generally inferior to that of temperate species, animal production per unit land area can be high on stargrass pastures because of their high dry matter (DM) production potential (Mislevy et al., 1989). To take advantage of this potential, pasture management is critical, and SR and N fertilization are important pasture management tools that affect animal performance.

Stocking rate is widely considered the most important grazing management factor affecting animal performance. Across the range of SR typically used in production systems, an increase in SR decreases daily animal performance (Mott, 1960; Bransby et al., 1988). Lower gain per animal at high SR is due in part to decreasing herbage allowance (Mott, 1960) and herbage mass (Hardy et al., 1997), resulting in less opportunity for selection, and in some cases inadequate quantity of forage. In contrast, animal production per hectare increases as SR increases, plateaus at moderate SR, and then decreases as SR is maximized (Mott, 1960). The optimum SR for a particular forage-livestock system is dependent upon the production goals of the system and must take into account the compromise between maximum individual animal production and animal production per hectare (Maraschin, 2001).

Nitrogen is the most limiting nutrient in a majority of tropical and subtropical grassland ecosystems (Sollenberger et al., 2002). Stargrass is responsive to N fertilizer, with yields increasing from 12.2 to 21.7 Mg ha–1 yr–1 (30-d cutting interval) and from 20.2 to 30.0 Mg ha–1 yr–1 (45-d cutting interval) in Puerto Rico when N application was increased from 0 to 900 kg ha–1 yr–1 (Caro-Costas et al., 1972). Stargrass herbage accumulation in Puerto Rico was 25.1, 31.3, and 40.6 Mg ha–1 yr–1 for N rates of 225, 450, and 900 kg ha–1 yr–1, respectively (Velez-Santiago and Arroyo-Aguilu, 1983). In Tabasco, Mexico, stargrass yields increased from 17.2 to 24.8 Mg ha–1 as N rate increased from 0 to 400 kg ha–1 yr–1 (Meléndez et al., 1980). No further increments in herbage production were observed with higher N rates.

The primary effects of N fertilization of grasses are to increase herbage production and forage N concentration (Topall et al., 2001), and SR should be increased accordingly to convert the extra DM into animal product. Because of the impact of N rate on productivity of stargrass pastures, SR effects on animal performance are likely to be dependent on N rate. Data are needed in the Caribbean region to assess the effects of SR, pasture N rate, and their interaction on pasture and animal performance. The objectives of this study were to determine (i) the effect of three SR and three N fertilization rates of stargrass pastures on herbage accumulation, mass, allowance, and nutritive value, and on performance of weanling bulls, and (ii) the relationships between pasture characteristics and animal daily gain.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The experiment was performed in St. Ann Parish, Jamaica (18°15' N; 77°5' W) from October 1991 to July 1993. The trial area was a 14.4-ha stargrass (local ecotype) pasture that was established in 1987. The soil was a bauxitic clay loam with an average pH of 5.4 and Mehlich I extractable P and K of 3 and 85 mg kg–1. Rainfall during the experimental period (October–July) was 883 and 1500 mm, respectively, for Years 1 (1991–1992) and 2 (1992–1993), respectively (Table 1).


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  		Table 1. Monthly rainfall measured at the experimental site in Moneague, St. Ann, Jamaica.

 
Treatments were the factorial combinations of three fixed SR (2.5, 5.0, and 7.5 bulls ha–1; subsequently referred to as SR2.5, SR5.0, and SR7.5) and three N fertilizer rates (112, 224, 336 kg N ha–1 yr–1; subsequently referred to as N112, N224, and N336) and were allocated to two replicates of a completely randomized design. Pasture size was 0.8 ha and each of 18 pastures were subdivided into four, 0.2-ha paddocks for rotational stocking. The grazing cycle was 28 d with a 7-d grazing period and 21-d rest period for each paddock. In each paddock, animals had free access to water and a commercial mineral mix.

During June 1991, all pastures were staged by mowing to a stubble height of 20 cm, and on 1 July they were fertilized with 50 kg N ha–1 using ammonium sulfate. Uniform grazing occurred across all pastures until treatments were imposed in October. Nitrogen fertilizer was applied as urea in November, February, and June of each grazing year, with one-third of the treatment total applied at each date. Pastures received 30 kg ha–1 of P and 115 kg ha–1 of K annually in November.

Seventy-two Jamaica Red Poll male weanling bulls ({approx}8 mo old) were used in the experiment each year. The Jamaica Red Poll breed was developed from Red Poll (B. taurus) cattle with limited amounts of B. indicus breeding introduced to increase adaptation to warm climates. The bulls were assigned to pastures on 16 Oct. 1991 and grazed until 29 July 1992 (Year 1, 287 d). A new group was assigned on 21 Oct. 1992 and grazed to 28 July 1993 (Year 2, 280 d). Two, four, and six animals, weighing an average of 209 ± 12 kg (Year 1) and 200 ± 10 kg (Year 2), were assigned to the 2.5, 5.0, and 7.5 bulls ha–1 treatments, respectively. All animals remained on pasture during the entire experiment and each animal was considered a sampling unit for measurement of average daily gain (ADG). They were weighed at the beginning and end of the experiment and every 28 d during the experiment following a 16-h period without food and water. All animals were treated for control of internal and external parasites.

Pastures were sampled every 28 d to determine herbage mass and nutritive value. Pregraze and postgraze herbage mass were determined immediately before and after grazing by clipping herbage from four, 0.25-m2 quadrats per pasture to a 10-cm stubble height. These sites were selected to represent the average for the paddock. Herbage was dried at 60°C to constant weight. At each pregraze sampling date, 20 hand-plucked samples were taken. Herbage for pregraze hand-plucked samples was severed at the postgrazing stubble height of the preceding paddock in the rotation. This was done so that the hand-plucked herbage was representative of the forage consumed. Samples were dried to a constant weight at 60°C and ground to pass a 1-mm screen. This material was analyzed to determine CP by a macro-Kjeldahl technique, neutral detergent fiber (NDF) by the method of Golding et al. (1985), and in vitro digestible organic matter (IVDOM) by a modified two-stage procedure (Moore and Mott, 1974).

Response variables calculated from herbage mass measurements include average pregraze herbage mass, total herbage accumulation, and average herbage allowance (on a total pasture, not individual paddock, basis). These terms were defined by the Forage and Grazing Terminology Committee (1992). Average pregraze herbage mass for a given experimental unit was calculated as pregraze herbage mass summed across sampling dates within a grazing year (October–July) and divided by the number of sampling dates per year. Cycle 1 herbage accumulation was considered to be pregraze herbage mass of the first grazing cycle. In all subsequent cycles, accumulation was calculated as the difference between pregraze herbage mass of the current cycle and postgraze herbage mass of the previous cycle, plus herbage accumulation during the grazing period. To calculate herbage accumulation during a given grazing period, herbage accumulation rate (kg ha–1 d–1) during the preceding 21-d rest period was multiplied by number of days in the grazing period (7 d). This technique was an alternative to use of restriction cages because data from the cage technique are highly variable (Sollenberger and Cherney, 1995). Total herbage accumulation was the sum across cycles within a year. Herbage allowance was calculated from pregraze and postgraze herbage mass samplings for each grazing cycle. For any pregraze or postgraze sampling date, allowance was calculated as herbage mass (kg ha–1) divided by average animal liveweight (kg ha–1) during that 28-d grazing cycle. Average herbage allowance per grazing cycle was calculated by averaging pregraze and postgraze measures of allowance within a grazing cycle. For an entire grazing year, average herbage allowance was calculated as the sum across individual grazing cycles divided by the number of cycles.

Data were analyzed by the mixed procedure of SAS (SAS Institute, 1999). Year was considered a subplot and N rate x SR treatments as the main plot in a split-plot arrangement of a completely randomized design. Nitrogen and SR were fixed effects and year a random effect. Orthogonal polynomial contrasts were used to determine the nature of treatment responses to SR and N rate. The regression procedure of SAS was used to determine the relationship between ADG and herbage mass and allowance. The values for ADG, herbage mass, and allowance used in these regressions were experimental unit averages for the year, so regression relationships within a year included a total of 18 observations. Similarly, data for all experimental units were included in the regressions of gain per hectare on SR (done by N rate), and each curve within a year represents six observations. Unless otherwise stated in the text, all differences referred to are significant at P < 0.05.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Herbage Accumulation
There were N rate main effects and SR x year interaction effects on herbage accumulation. When analyzed by year, there were main effects of SR in both years. Herbage accumulation increased as the SR decreased from 7.5 to 2.5 bulls ha–1 in both years (Table 2). There were linear and quadratic effects in Years 1 and 2, with differences being least between SR5.0 and SR7.5 in Year 1 and between SR5.0 and SR2.5 in Year 2. Herbage accumulation was greater in Year 2 than Year 1 for all but the SR2.5 treatment. Across years, there were linear (P < 0.01) and quadratic (P < 0.05) effects of N rate on herbage accumulation. The quadratic effect occurred because herbage accumulation varied little for the N112 and N224 treatments (20.0 and 19.4 Mg ha–1, respectively). The linear effect was due to increased herbage accumulation (20.0–24.1 Mg ha–1) from the lowest to the highest N rate.


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  		Table 2. Total herbage accumulation of stargrass pastures as affected by stocking rate during 2 yr of grazing. Data are means across three N rates and two replicates (n = 6).

 
The marked decline in herbage accumulation with increasing SR, especially in Year 1, was likely because of the decreasing levels of residual DM (Hernández-Garay et al., 2000), residual leaf area (Chapman and Lemaire, 1993), and possibly carbohydrate reserves (Pitman, 1991). In a sward with good energy reserves and where substantial leaf mass remains after grazing (SR2.5), the rate of regrowth may be maintained at a high level. When grazing intensity is high (SR5.0 and SR7.5 in the dry Year 1 and SR7.5 in the wet Year 2), growth rate may be lower because of insufficient leaf area and carbohydrate reserves. The overall 37% greater herbage accumulation in Year 2 than Year 1 (Table 2) was likely because of the 70% greater rainfall in Year 2. Gilbert and Clarkson (1993) found that grass yield was curvilinearly related to summer rainfall and N fertilizer (R2 = 0.75) in the tropics and subtropics of northeastern Australia.

The primary effect of N fertilization of grasses is to increase herbage production (Topall et al., 2001), and SR should be increased accordingly to convert this extra DM into animal product. In this study, however, the effect of N rate on herbage accumulation was relatively small. Greatest accumulation was obtained with N336 supporting the results of Meléndez et al. (1980) who reported that across the range of 0 to 400 kg of N ha–1 yr–1, greatest stargrass yield occurred in the range from 300 to 400 kg of N ha–1 yr–1.

Herbage Mass and Allowance
Pregraze herbage mass was affected by the SR main effect and N x year interaction. Herbage mass decreased linearly with increasing SR in both years (Table 3), although the magnitude of the response was greater in wetter Year 2. In both years, pregraze herbage mass was more than twice as great for SR2.5 than SR7.5. For all SR, Year 2 pregraze herbage mass was greater than in Year 1. In the drier first year, pregraze herbage mass varied <0.4 Mg ha–1 among N-rate treatments (average of 3.3 Mg ha–1), while in the second year it increased linearly from 5.1 (N112) to 6.8 Mg ha–1 (N336). The effects of year and N rate on pregraze herbage mass appeared to be primarily a function of greater rainfall in Year 2, which resulted in greater overall herbage accumulation that year and allowed greater expression of the effect of increasing N rate on stargrass growth.


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  		Table 3. Average pregraze herbage mass and average herbage allowance as affected by stocking rate of stargrass pastures during 2 yr of grazing. Data are means across three N rates and two replicates (n = 6).

 
Average herbage allowance was affected by the SR x year interaction. Allowance increased as the SR decreased from SR7.5 to SR2.5 (P < 0.01; Table 3), and there were both linear and quadratic effects (P < 0.01) of SR on allowance in both years. Quadratic effects occurred because differences between SR7.5 and SR5.0 were much less than those between SR5.0 and SR2.5 (Table 3). Average herbage allowance was five and seven times greater for SR2.5 than SR7.5 in Years 1 and 2, respectively. Year x SR interaction occurred because year effects on herbage allowance were significant for SR2.5 and SR5.0 but not for SR7.5. There was no effect of N rate on herbage allowance, as the range in response averaged across years was only 3.64 (N112) to 4.12 (N336).

Nutritive Value
There were year main effects and SR x N rate interaction effects on hand-plucked herbage CP. Herbage CP was greater in Year 2 than in Year 1 (145 vs. 138 g kg–1). Averaged across years, CP increased linearly with increasing SR for all N rates, but the magnitude of the increase was less for N336 than the other N rates (Table 4). The response to N rate also depended upon SR, with CP increasing with increasing N rate for SR2.5 and SR5.0 but not for SR7.5. Increasing herbage CP with increasing N rate is well documented in the literature (Caro-Costas et al., 1976), while greater CP for higher SR treatments is likely a function of utilization of the pastures and its effect on average maturity of herbage mass (Newman et al., 2002). The greater the SR, the greater the degree of pasture utilization, thus a greater proportion of herbage mass, especially that in the upper canopy, is new growth produced since the most recent defoliation event.


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  		Table 4. Stocking rate x N fertilizer rate interaction means for crude protein concentration of hand-plucked stargrass herbage. Data are means across 2 yr and two replicates (n = 4).

 
In vitro digestible organic matter concentration was affected by the SR x N rate interaction (Table 5). Interaction occurred because IVDOM increased as the SR increased for N112 and N224 but not for N336 (Table 5). Similar results were observed by Adjei et al. (1980) in Florida, who reported that IVDOM of stargrass increased as SR increased. Herbage IVDOM also increased with increasing N rate for SR2.5 and SR5.0, but not for SR7.5. Johnson et al. (2001) reported a linear increase in stargrass IVDOM with increasing N fertilization rate; however, Caro-Costas et al. (1976) found that neither calculated apparent digestibility nor in vitro digestibility of stargrass was affected by changes in fertilization rate.


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  		Table 5. Stocking rate x N fertilizer rate interaction means for in vitro digestible organic matter concentration of hand-plucked stargrass herbage. Data are means across 2 yr and two replicates (n = 4).

 
Stargrass NDF was affected by the SR x year interaction. Stargrass NDF was high for all treatments, ranging from 740 to 775 g kg–1, and it decreased linearly in both years with increasing SR (Table 6). Interaction occurred because the rate of decline was greater in Year 2 than in Year 1. No N rate effects were observed in either year.


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  		Table 6. Stocking rate effects on neutral detergent fiber concentration of hand-plucked stargrass herbage during 2 yr of grazing. Data are means across three N rates and two replicates (n = 6).

 
Hand-plucked herbage nutritive value was generally greater for high than low SR treatments, but this did not translate into greater gains for high SR. Forage mass on high SR pastures was likely not sufficient for ad libitum intake of forage. Relationships between measures of pasture quantity and ADG will be explored in more detail later.

Average Daily Gain
There were N rate main effects and SR x year interaction effects on ADG. There was a linear increase in ADG with increasing N fertilizer rate both within and across years (P < 0.05; Table 7). The range in response to N rates was only 0.07 kg d–1 in both years, indicating that SR had much greater impact on animal performance than did N rate.


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  		Table 7. Stocking rate and N rate effects on average daily gain of Jamaica Red Poll weanling bulls grazing stargrass pastures during 2 yr of grazing. Stocking rate data are means across three N rates and two replicates (n = 6) and N rate data are means across three stocking rates and two replicates (n = 6).

 
The ADG response to SR had linear and quadratic terms in both years, with ADG decreasing at a faster rate between SR5.0 and SR7.5 than between SR2.5 and SR5.0 (Table 7). Interaction occurred because the magnitude of the difference in ADG between SR2.5 and SR7.5 was greater in Year 1 (0.44 kg d–1) than in Year 2 (0.30 kg d–1). In Year 1, ADG was 2.7 times greater for SR2.5 than for SR7.5 (0.70 vs. 0.26 kg d–1), while in Year 2 the proportion was 1.9 (0.65 vs. 0.35 kg d–1). The larger response in Year 1 could be attributed in part to lower rainfall, which resulted in average pregraze herbage mass and average herbage allowance on the SR7.5 treatment being only 60% as great as in Year 2.

Increased SR increases consumption of herbage mass per unit land area, but there is a shift in use of consumed energy from maximum daily animal growth at low SR, toward maintenance of the animals at moderate to high SR. The consequence is generally reduced production per animal with increasing SR, as was observed in this study. Animal daily gain in the current study was similar to that reported for young Holstein heifers (Bos taurus) on rotationally stocked stargrass pastures in Puerto Rico (Caro-Costas et al., 1976). During 168-d seasons in Florida, ADG of yearling beef steers grazing stargrass (average of three cultivars and 2 yr) was 0.47, 0.38, and 0.21 kg d–1 for SR treatments of 7.5, 10, and 15 cattle ha–1 (initial weight of 230–250 kg) (Adjei et al., 1980).

Relationships between Pasture Characteristics and Average Daily Gain
Relationships between herbage nutritive value and ADG were not significant in either year, but measures of pasture quantity had strong relationships with ADG. Pregraze herbage mass explained 88 and 75% of the variation in ADG in Years 1 and 2 (Fig. 1) , respectively. In Year 1, the equation included a quadratic term, but in Year 2 the response was linear. When data for SR2.5 were removed from the regression equation, the response to herbage mass was linear in both Year 1 (R2 = 0.75) and Year 2 (R2 = 0.70).



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  		Fig. 1. Weanling bull average daily gain (ADG) response to average pregraze herbage mass on stargrass pastures.

 
Herbage allowance incorporates both pasture and animal aspects in its calculation. For grazing trials in which treatments result in a relatively wide range of herbage mass or herbage mass is low, herbage allowance often can be useful in explaining animal performance responses. For this study, when ADG was plotted against average herbage allowance, the response was curvilinear in both years and allowance explained 92 and 85% of the variation in Years 1 and 2, respectively (Fig. 2) . When data for SR2.5 were removed from the regression equation, the response to average herbage allowance was linear in Year 1 (R2 = 0.86), but in Year 2 the model continued to include the quadratic term (R2 = 0.82).



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  		Fig. 2. Weanling bull average daily gain (ADG) response to average herbage allowance on stargrass pastures.

 
The close relationships of average herbage allowance and pregraze herbage mass with ADG support the conclusion that the major factor affecting gains, at least at SR7.5 and SR5.0, was herbage quantity. Parkin and Boultwood (1981) also observed that herbage allowance was the main factor determining animal production on stargrass pastures. In the current study, increasing average herbage allowance up to {approx}4 kg DM kg–1 of animal liveweight resulted in a linear increase in ADG, but as average herbage allowance increased above 4, the rate of increase in ADG slowed. For continuously stocked pearl millet [Pennisetum glaucum (L.) R. Br.], there was little further increase in ADG above an allowance of 3.3 (McCartor and Rouquette, 1977).

Gain per Hectare
Liveweight gain per hectare was affected by the three-way interaction of SR x N rate x year. When analyzed by year, there was a SR x N rate interaction only in Year 2, but for consistency of data presentation, the responses to SR will be plotted for each N rate in each year. The quadratic effect of SR on weight gain per hectare was significant for all N rates in both years (Fig. 3) , with weight gain per hectare increasing as SR increased from 2.5 to 5.0 and then decreasing, with the exception of N336 in Year 2. The rate of decline in weight gain per hectare between SR5.0 and SR7.5 was greater in the drier Year 1, especially for N336 (Fig. 3), and this can be attributed to lower herbage accumulation for SR5.0 and SR7.5 in Year 1 (Table 2). In Year 2, treatment N336 appeared to only approach its maximum at SR7.5. Although increasing SR from 2.5 to 5.0 caused a decline in ADG, it resulted in a large increase in animal production ha–1. This increase does not continue indefinitely, however, because at high compared with low SR a greater proportion of total energy consumed is being used to meet the maintenance requirement of the animals (Burns and Sollenberger, 2002). Other studies with stargrass have shown a similar response, as liveweight gain per hectare averaged across 2 yr in Florida was 470, 617, and 576 kg for SR of 7.5, 10, and 15 yearling beef cattle ha–1 (Adjei et al., 1980).



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  		Fig. 3. Weanling bull liveweight gain per hectare response to stocking rate (SR) on stargrass pastures fertilized at three rates of N. For N rates of 112, 224, and 336 kg N ha–1, respectively, fitted curves for Year 1 are Y = –78 + 305 SR – 30.8 SR2; Y = –462 + 499 SR – 49 SR2; and Y = –160 + 359 SR – 33.5 SR2. For N rates of 112, 224, and 336 kg N ha–1, respectively, fitted curves for Year 2 are Y = –26 + 242 SR – 20.8 SR2; Y = –488 + 501 SR – 46.7 SR2; and Y = –158 + 283 SR – 19 SR2.

 
Differences among N rates in weight gain per hectare were most pronounced at SR5.0 and SR7.5. At SR2.5, there was no effect of N rate on weight gain per hectare, despite increasing herbage nutritive value with increasing N rate (Tables 4 and 5). With high herbage mass at SR2.5, opportunity for diet selection may have overridden the relatively small differences among N rates in herbage nutritive value and kept ADG similar across N rates. At SR5.0, weight gain per hectare was greater for N224 and N336 than for N112, mostly because of increasing herbage accumulation with increasing N rate. This conclusion is based on the fact that ADG increased relatively little with increasing N rate (Table 7) despite herbage nutritive value being modestly greater for the higher N rates (Tables 4 and 5). At SR7.5, weight gain per hectare was greatest for N336 in both years, again most likely because of greater herbage accumulation. A linear increase was reported in weight gain per hectare with increasing N fertilization of stargrass pastures in Puerto Rico (Caro-Costas et al., 1976).

To assess the return in liveweight gain due to N fertilization, additional gain per kg of N applied (above the 112 kg ha–1 rate) was calculated by level of SR. For SR2.5, increasing N rate from 112 to 224 kg ha–1 resulted in a liveweight change of –0.12 (Year 1) and 0.21 kg of liveweight gain per kg of N applied (Year 2), while increasing N rate from 224 to 336 kg ha–1 changed liveweight gain by 0.44 (drier Year 1) and –0.38 kg per kg of additional N (wetter Year 2). Thus, if stargrass pastures in this environment were stocked at 2.5 weanling bulls per hectare, there was little apparent benefit to increasing N rate above 112 kg ha–1, with a possible exception being in a dry year. For SR5.0, each kg of additional N between 112 and 224 kg ha–1 resulted in 1.15 (Year 1) and 1.68 kg of additional liveweight gain. For SR5.0, there were no further increases in gain between N224 and N336 (–0.09 and –0.60 kg liveweight change per kg of additional N). Thus, the response to increasing N fertilization from 112 to 224 kg ha–1 was strongly positive at SR5.0, but there appeared to be no advantage to increasing N rate above 224 kg ha–1. For SR7.5, the changes between N112 and N224 were 0.39 and 0.25 in Years 1 and 2, while between N224 and N336 the changes were 1.12 (drier year) and 2.27 (wetter year). Thus, the greatest response to the increase from N224 and N336 was achieved with the SR7.5 treatment. Salazar-Diaz (1977) found a response of about 1.05, 1.02, and 1.36 kg of liveweight gain per kg N applied, with low, medium, and high SR, respectively, of digitgrass (Digitaria eriantha Steud.) pastures. Nuthall and Whiteman (1972) reported a linear response of liveweight gain to N fertilizer for tropical grasses; the slope of the line was 1.88 up to 900 kg of N ha–1.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Stocking rate of stargrass pastures is a key determinant of pasture and animal production, and for most responses has a much greater impact than N fertilizer rate. In each of 2 yr, animal daily gains decreased curvilinearly with increasing SR from 2.5 to 7.5 weanling bulls per hectare. Greater decreases in daily gain were observed between SR5.0 and SR7.5 than between SR2.5 and SR5.0. Because the range in SR was large, bull daily gains were closely related to herbage mass and herbage allowance in both years, but forage nutritive value was not significantly related to gain responses in either year. There was a SR x N rate interaction for liveweight gain per hectare in 1 of 2 yr. In both years there was relatively little response of gain per hectare to N rate at SR2.5; however, at SR5.0, gain increased up to 224 kg N ha–1 and at SR7.5 gain increased up to 336 kg ha–1. In this environment and with this schedule of N applications, it is likely that N fertilization in excess of 112 kg N ha–1 will not be economical at low SR because herbage mass and allowance are not limiting, allowing the animal opportunity to select a high quality diet. As SR increases above 2.5, then greater N rates are necessary to increase herbage mass and allowance and consequently animal daily gains. In conclusion, across a wide range of SR of stargrass pastures, herbage mass and allowance were the major factors affecting animal performance. The recommended pasture N rate is dependent upon SR, with greater N rates more likely to be profitable if SR is high.


   ACKNOWLEDGMENTS
 
The financial support of Alcan Corporation, the Jamaican Agricultural Research Programme, and USAID was critical to the successful conduct of this research. Dr. Lyndon McLaren of the Jamaican Agricultural Development Foundation played a key role in establishing contacts with Alcan and facilitating the research effort.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Florida Agric. Exp. Stn. Journal Series No. R-09744.

Received for publication September 4, 2003.


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


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