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* Instituto Nacional de Investigaciones Forestales y Agropecuarias, Melchor Ocampo 234, Veracruz, VE 91700, México;
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Departments of Animal Science,
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Applied Economics and Management, and
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Crop and Soil Sciences, Cornell University, Ithaca, NY 14853; and
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# Centro de Pesquisa Agroflorestal EMBRAPA-Acre, Caixa Postal 321, Rio Branco, AC 69908-970, Brazil
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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Key Words: Amazonia • Brachiaria • Nellore • Simulation Models • Sustainability
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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). One-fourth of them reside in the Amazon region (INPE, 2000), where extensive systems prevail due to limitations of soil fertility, seasonal variation in rainfall and other climatic and economic factors (Faminow, 1998). Nearly all milk is from dual-purpose cows and most beef is from Nellore steers grazing without dietary supplementation.
Pasture grasses in the Amazon region typically are Brachiaria spp. selected for acid soils (Vera et al., 1992; Fisher and Kerridge, 1996). Although the effects of temperature are known (Nelson and Moser, 1994; Van Soest, 1994), there is little published information about the influence of monthly fluctuations in forage composition and digestion rates on animal performance (Tedeschi et al., 1999).
The evaluation of cattle systems is aided by modeling to estimate nutrient requirements of animals (Fox et al., 2003) and nutrients from feeds and diets (Sniffen et al., 1992) to predict probable productivity. For example, Juarez et al. (1999) found large variations in milk production potentials of 15 grasses during a 6-wk period in Gulf Coast Mexico using animal and forage inputs into the Cornell Net Carbohydrate and Protein System model (CNCPS; Fox et al., 2003). More information of this kind across seasons is required to better manage cattle systems in the tropics.
The purpose of this study was to evaluate management strategies for cattle systems in the western Brazilian Amazon based on data collected for this purpose and knowledge of performance under tropical conditions. Specific objectives were to evaluate seasonal effects on the nutritional values of B. decumbens and B. brizantha grasses and Pueraria phaseoloides legume to support milk production by dual-purpose (Bos taurus x Bos indicus) cows and growth of Nellore steers and to use these data to evaluate the profitability of options to improve cattle productivity and land use.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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Study sites were private farms located within 80 km of Rio Branco, Acre (Brazil). This moist humid tropical region is located at lat 07°07' S and long 66°30' W. Annual mean rainfall is approximately 1,800 mm, which is especially concentrated from November to March (e.g., 200 to 300 mm/mo). The annual means for temperature and relative humidity are about 25°C and 85%, respectively. The predominant soils are Ultisols and Oxisols with pH 5.5, low P content (<5 ppm), and low cation exchange capacity (Rueda, 2002).
Forages were sampled and animal performance was recorded monthly during a 9-mo period at three collaborating farms (a dual-purpose herd with 90 ha of land and two beef herds with 1,600 ha and 3,000 ha). The period of study was from the end of the 1999 dry season until the end of the subsequent rainy season in June 2000. Initial measurements (October) were taken prior to first precipitation in the 1999 to 2000 rainy season, which corresponded to the annual nadir in soil moisture content and total pasture biomass. Consequently, we were able to infer probable forage nutrient supply and animal performance, thus completing an annual cycle, without actually monitoring them during the rest of the dry season (i.e., July, August, and September).
Animal and Environmental Measurements
Lactating dual-purpose cows, growing steers (210 kg at 16 mo of age), and finishing steers (385 kg at 24 mo of age) were monitored monthly under field conditions. These measurements were used to establish baseline (typical observed) milk and growth responses to rotationally grazed forages from which multiple samples were also taken for chemical and kinetic analysis. Yield of saleable milk from 15 Bos taurus (mostly Holstein and Brown Swiss) x Bos indicus (mostly Nellore) crossbred cows was estimated from individual daily yields weighed at monthly intervals with calibrated scales. Multiparous lactating cows weighing about 400 kg (Table 1) and approximately 60 d in milk at the beginning of the study grazed a mixed stand of B. decumbens and P. phaseoloides (hereafter referred to as "kudzu"). Kudzu provided about 10% of available forage DMI. Individual body weights of 50 Nellore steers (10 growing steers grazing B. brizantha cv. Marandu and 40 finishing steers grazing B. brizantha or B. decumbens) were taken during morning routines using calibrated farmer-owned scales. Growing steers and dual-purpose cows received different mineral supplements (Ca, 13.5 and 25%; P, 0.8 and 13.5%; Mg, 0.71 and 1.1%; K, 0.19 and 0.65%; Na, 0.98 and 16.8%; Fe, 2,780 and 3,310 ppm; Zn, 1,120 and 1,290 ppm; Co, 201 and 350 ppm; and Mn, 305 and 500 ppm, respectively on a DM basis) and were visually scored for body condition on a nine-point scale (NRC, 2000). Temperature, relative humidity, rainfall, wind speed, and other environmental information were obtained throughout the period of study from collaborating farms and the EMBRAPA-Acre research station. Animal performance and environmental data were summarized for use as inputs for CNCPS prediction of animal requirements as shown in Table 1.
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Total accumulated plant biomass was estimated monthly for each of four paddocks by the cutting-quadrants method, with 20 microplot (0.25 m2) cuttings per paddock (Mannetje, 2000) and DM determination of a representative sample. Grasses were cut 20 cm above the soil surface from swards with a measured height of 80 cm. Paddocks were grazed with two animal units per hectare, which is the average stocking rate in this region [one animal unit = 450 kg of live weight] (Faminow, 1998). Two paddocks were sown with B. decumbens (14 and 20 yr after establishment) and two were sown with B. brizantha cv. Marandu (4 and 8 yr after establishment). Grasses were sampled biweekly, from which 36 grass samples (nine from each paddock) were selected for laboratory analysis. These samples were selected because they were collected closest to the monthly dates when milk production and BW were measured. In addition, one paddock containing kudzu and B. decumbens (6 yr after establishment) provided nine monthly samples of kudzu to assess the dietary contribution of this legume. Following recommendations by Van Soest (1994), grass samples were obtained by observing grazing behavior and simultaneously hand plucking plant parts comparable to the ones selected. The top 20 cm of plants in the sward, which was the portion grazed (Rueda, 2002), was also clipped, and green leaves were separated from stems for separate weighing.
Chemical Composition and Digestion Kinetics of Feeds
Immediately after collection, 500 g of each forage sample was dried at 55°C in a forced-air oven for 72 h and ground in a Wiley-type mill to pass through a 1-mm screen. Four samples of commercial protein/energy supplements also were obtained from Rio Branco vendors to assess their nutritional composition. Contents of CP, neutral detergent insoluble protein (NDIP), and acid detergent insoluble protein (ADIP) were determined by a modified Kjeldahl procedure using a boric acid solution (AOAC, 1990). Soluble CP was determined using the sodium borate–sodium phosphate method (Roe and Sniffen, 1990). Acid detergent fiber, acid detergent lignin, ash (AOAC, 1990), and NDF were also determined (Van Soest et al., 1991). The neutral detergent soluble fiber (ß-glucans and fructans) was estimated by the ethanol insoluble residue procedure (Hall et al., 1997). The amount of ether extract was determined and minerals were extracted with acid solution (1.5N HNO3 + 0.5N HCl) and determined by radial spectrometry (Sirois et al., 1994).
Carbohydrate Fractions and Digestion Rates of Feeds.
The CNCPS model estimates the ME content of feeds based on expected dietary intake, chemical composition, and digestion and passage rates of the carbohydrate fractions. The carbohydrate fractions, in order of decreasing rate of digestion, are: A fraction, which is fully water-soluble (e.g., simple sugars, organic acids, and fructans); B1 fraction, which is polymerized but partially water-soluble (e.g., pectin and starch); B2 fraction, which is insoluble fiber (e.g., cellulose and hemicellulose); and C fraction, which is insoluble and indigestible (e.g., lignin; Sniffen et al., 1992). Digestion kinetics of carbohydrate fractions in the grasses was estimated from water-soluble carbohydrates, in vitro digestion, and the rates of digestion. Digestion rates for kudzu were those reported by Tedeschi et al. (2002a), which are in the CNCPS feed library, because equipment was not available to lyophilize samples.
Water-Soluble Carbohydrate Fractions (A+B1).
Approximately 0.75 g of ground forage was stirred for 15 h at 39°C with 75 mL water containing 10% (vol/vol) t-butanol. The solids were collected by filtration through a 47-µm nylon mesh screen, washed with cold water, briefly rinsed with 95% ethanol, and dried at 45°C in a vacuum oven. The DM and NDF compositions of this washed preparation were then determined (Pell and Schofield, 1993).
In Vitro Digestion.
Digestion was estimated using the method of Pell and Schofield (1993), which measures gas produced during fermentation as a pressure increase in a closed bottle (50 mL). Each digestion utilized 100 mg of ground forage plus 8 mL of buffer and 2 mL of fresh ruminal fluid. Ruminal fluid was obtained approximately 2 h after morning feeding from an 8-yr-old, nonlactating Holstein cow weighing 650 kg, which had been fed medium-quality grass hay twice daily, with continuous access to water. Six or seven duplicate samples, a blank, and an alfalfa standard were digested in each incubator. The samples were arranged in the incubators so that both the whole and the water-extracted samples were fermented using the same ruminal inoculum. Gas accumulation was recorded at 30-min intervals during a 48-h period. When digestion was completed, buffer pH was recorded and the medium was analyzed for VFA content. Fiber disappearance was measured using the micro-NDF method described by Pell and Schofield (1993).
Digestion Rates.
The gas data from 45 samples (36 grass and nine kudzu) were fitted to an exponential growth equation (Schofield and Pell, 1995) using the TableCurve software (SPSS Science, Chicago, IL). Digestion rates for the B3 protein fraction were from published values for B. decumbens and B. brizantha from Gulf Coast Mexico (Juarez et al., 1999), which are in the CNCPS tropical feeds library (Tedeschi et al., 2002a).
Statistical Analysis
The primary objective was to evaluate the effects of season (rainiest vs. less rainy months) and grass species on the quality of grazed forage because grass is the predominant feed. Preliminary analysis, which included measurements from the end of the 1999 dry season (October), indicated that these grass species were similar in chemical composition. Except for biomass availability (P < 0.06), the interaction of species with season was not significant (P > 0.50) and was pooled in the residual. Therefore, least squares methods using the GLM procedure of SAS (version 8.1, SAS Inst., Inc., Cary, NC) were used to evaluate these effects with the following fixed effects model:
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where Yijk = the observed total available biomass and chemical composition (DM, NDF, lignin, CP, soluble CP, NDIP, ADIP, fat, ash), the estimated carbohydrate digestion rates (A+B1 and B2 fractions), estimated ME content, and predicted ME and metabolizable protein (MP) available for milk production and steer growth of the jth grass collected in the ith season of year; µ = the overall mean; S = the fixed effect of the ith season (rainiest months of December, January, February, and March vs. less rainy months of April, May, June, October, and November surrounding the dry season); F = the fixed effect of the jth grass (B. brizantha vs. B. decumbens); and eijk = random residual.
Comparisons of average chemical composition between grasses and kudzu were approximated by t-test of the unadjusted means.
Baseline Prediction of Animal Requirements and Nutritive Values of Forages
The CNCPS model (version 4.0; Fox et al., 2003) was used to predict nutritive values of forages (ME content), available ME and MP, and animal requirements for baseline calibration. The MP required for maintenance was assumed to be equal to the beef cattle requirements of 3.8 g/kg of BW0.75 (NRC, 2000). Response variables for lactating cows grazing B. decumbens with 10% kudzu were used to predict milk production from dietary supplies of ME (i.e., ME available for milk) and MP (i.e., MP available for milk). Response variables for steers were the predicted gains in BW from ME and MP (i.e., ME and MP available for gain) of B. brizantha for growing steers and B. decumbens for finishing steers. The feed DMI required to achieve the observed, or baseline, animal performance was obtained by equating average daily forage intake to the amount matching the predicted ME requirement (Juarez et al., 1999) because actual DMI could not be measured.
Potential for Increased Productivity
Potentials for overcoming dietary limitations of the baseline scenario were evaluated by simulation. Baseline and alternative scenarios to obtain daily milk production and BW 25 or 50% greater than currently observed means were examined in nine CNCPS simulations (three each for cows and growing and finishing steers) using monthly chemical composition and digestion kinetics of the grasses and animal descriptions in Table 1
. Nutritional requirements and needed supplementation were estimated monthly for each group of animals. Supplementation options included sorghum grain (85% TDN or 3 Mcal of ME/kg), which is readily available in Acre, and kudzu, a protein-rich (18% CP) forage already in use. Commercial concentrate was not considered because the energy content from our analysis of locally obtained samples (<67% TDN) was only modestly (<20%) better than the grazed portion of pasture grasses, similar to a finding in Venezuela (Townsend et al., 1990).
Net Economic Returns from Intensification Strategies
Net economic returns from alternative nutritional and pasture management were approximated by partial budget analysis of marginal costs and returns. Strategies were to increase individual animal production or land productivity by raising stocking density. Individual animal options were to increase milking or growth performance per animal by 25 or 50% by dietary supplementation. Land intensification options were from increasing stocking from 2 to 4 animal units/ha, which represents the range in observed stocking densities in the Amazon region. For the increased animal performance strategy, it was assumed that additional beef could be produced in the same paddocks with animals marketed 4 to 6 mo earlier than in the baseline case (additional beef sales were included in gross revenues; Table 2). Additional inputs were supplemental feed and labor to increase animal performance or additional fertilizer, electric fencing, and labor to increase stocking density (herd size). Our analysis did not account for requirements to achieve improved reproduction to enlarge the herd and its associated effects on forage availability and variable costs.
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). A grass-legume strategy combined current EMBRAPA-Acre recommendations with companion findings (Rueda, 2002), fertilizing with 14 or 18 kg/ha of superphosphate for 3 or 4 animal units/ha.
The data for partial budget analyses were quantities of required inputs, expected productivity responses, and prices of inputs and outputs. Technical coefficients for these assessments are summarized in Table 2. This information was obtained through direct interviews with farmers, local agrochemical store managers, milk collectors, intermediary buyers and sellers, and abattoir managers. Benefits and costs (undiscounted) were calculated using average farm-gate prices during the period of study in Rio Branco, Acre. Herbage allowances for alternative stocking densities were estimated assuming more forage of the same quality from more intensively grazed, younger plant regrowth under fertilization (Rueda, 2002).
Partial budgeting with sensitivity analysis helps identify situations where technology change has potential value (Boehlje and Eidman, 1984; Mutsaers et al., 1986). This method considers a restricted range of possibilities and ignores the costs required for transition from the current scenario to a more intensive alternative. Thus, our estimates are initial evaluations designed to identify options with economic potential and to eliminate those that are clearly unprofitable under current conditions. Farmers typically do not consider adoption unless the gain in net margin, where net margin is equal to total revenue minus total variable cost, exceeds some threshold in marginal variable cost (e.g., cash costs of feed, land, and labor) of the alternative technology. The ratio of additional net margin to additional cash costs (the marginal rate of return) often needs to be greater than 0.5 to interest farmers to adopt technologies with which they are familiar (CIMMYT, 1988); an increase equal to the amount of additional investment (marginal rate of return of 1.0) may be required to attract farmers to a new practice or technology. Average returns to labor (net margin plus hired labor costs/units of labor) were compared to the baseline scenario because technologies with higher returns would presumably be most desirable where labor is scarce. Marginal returns to hired labor ([change in net margin plus hired labor costs]/[change in required labor]), an additional indicator of desirability of adoption, was calculated for alternative management.
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Results and Discussion |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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Mean milk production from B. decumbens and kudzu, with 2.5 kg/d assumed for calf consumption (Nicholson et al., 1994), ranged from 5.4 ± 0.26 kg/d (May) to 8.1 ± 0.26 kg/d (December) during the period of this study. Lowest yields were at the end of lactation in May and June, which was similar to other reports for Brazil based on Brizantha spp. without supplementation (Abdalla et al., 1999; Euclides et al., 1993; 2000). Mean milk yield in October was 7.0 ± 0.26 kg/d, indicating similar forage and body tissue support of milk production at the end of the dry season compared to months with rainfall. Mean weight gains were 0.5 ± 0.02 kg/d throughout the study period for growing and finishing steers grazing B. brizantha, including October. Finishing steers grazing B. decumbens gained an average of 0.4 ± 0.03 kg/d. The BCS of animals were essentially invariant throughout the study period, averaging 4.6 for cows, 5 for growing steers, and 6 for finishing steers. This is further evidence that forage supply during the dry season was sufficient for stocking with 2 animal units/ha.
Pasture Biomass Accumulation and Grazing Behavior
Total plant biomass accumulation during the period of study averaged 2,550 kg of DM•ha-1•mo-1 (SD =133) for B. decumbens and 3,700 kg of DM•ha-1•mo-1 (SD = 215) for B. brizantha. These DM accumulations were within the observed range for Brachiaria spp. (2,500 to 4,400 kg of DM/ha) under continuous grazing and similar low stocking in eastern and southern Brazil (Debeux, 1995; Jannoti, 1997; Euclides, 2000). An interaction of grass species x season (P < 0.06) indicated unequal biomass response throughout the year and probable unequal amounts of under-grazing.
More DM was accumulated from B. decumbens in the rainiest season (especially December to February), whereas the drier months (especially June and October) favored pasture DM from B. brizantha (P < 0.02). The observed biomass production of B. decumbens on a Colombian Ultisol was greatest in the rainy season, regardless of stocking density (Velasquez and Cuesta, 1990). Less accumulated DM from B. brizantha in the rainiest season may be due to its intolerance of excess water and poorly drained soils (Dias-Filho and Reis de Carvalho, 2000).
Although total herbage allowances ranged from 7 to 12 kg of DM per 100 kg of BW for B. decumbens and 11 to 15 kg of DM per 100 kg of BW for B. brizantha, the forage was actually consumed was from the top 20 cm of plants. This portion of the sward, comprising 85% leaves and 15% stems, provided only about 3 kg of DM per 100 kg of BW. Correspondingly, the animals in this study selected these more digestible plant parts from the available pool of forage DM, which was approximately equal to the maximal daily intake that would be expected from unrestricted grazing. This indicates that current stocking in this region with 2 animal units/ha is essentially at the lower limit for pasture management because excessive accumulation of mature plant biomass is permitted.
Grazing management to ensure that the available amount of (more nutritive) herbage more closely matches the animal’s daily DM requirement would be expected to increase forage intake by reducing the time devoted to selective feeding (Stockdale and Trigg, 1985). Greatest weight gains from more time spent grazing have been found with 6 to 9 kg of DM per 100 kg of BW/d in (sub)tropical locations with Pennisetum spp. and Cynodon spp. (Adjei et al., 1980; Stockdale and King 1983; da Silva, 1994). In other studies, about 90% of the DM consumed by animals was leaves (Otoya, 1986; Euclides et al., 1992). Regardless of species in the forage portfolio, management is needed to maintain swards with high leaf (low stem) and low fiber content (Van Soest, 1994). Higher stocking in such a grazing system could help improve feeding conditions, especially with paddock rotation, which assures larger amounts of younger forage with high feeding value. Rotational grazing would require more paddocks and fencing, where the lowest installation cost is from electric fencing (Carvalho and Pinho, 2000).
Chemical Composition, Carbohydrate Digestion Rates, and Nutritional Values of Forages
These Brachiaria grasses did not differ (P > 0.05) in chemical composition (Table 3), indicating similar nutritive values, which agreed with the results of Juarez et al. (1999) for milk production in Gulf Coast Mexico. The contents of CP and NDF were similar to those reported from other parts of tropical America (Euclides et al., 1999; Juarez et al., 1999; Tedeschi et al., 2002a).
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) with more lignin, CP, NDIP, and ADIP and less DM, NDF, and soluble CP. More lignin (as a percentage of NDF) suggests lower digestibility and faster fermentation of the unlignified fraction (Van Soest, 1994). The solubility of CP was less for kudzu than for the grasses (P < 0.05), which suggests greater availability of protein for microbial growth and, consequently, better degradation of fiber. Therefore, kudzu potentially complements Brachiaria spp. providing more dietary CP that is less soluble. The nutritional value of kudzu was similar to that of other tropical legumes (e.g., Leucaena leucocephala, Gliricidia sepium, Neonotonia wightii, and Macroptilium atropupureum) that typically range from 12 to 24% CP, 47 to 60% NDF, and 20 to 24% lignin (as a percentage of NDF; Tedeschi et al., 2002a).
Dietary intake and composition affect the ruminal microbial population that degrades the feeds consumed and thus, the rates of digestion and passage (Van Soest, 1994). The rates of digestion for carbohydrate fractions are in Table 3. Rates for the nonstructural carbohydrates (A+B1 fraction) of grasses and kudzu were similar (P > 0.05), ranging from 21 to 30%/h. However, the insoluble fiber (B2) fraction in grasses was digested more rapidly (P < 0.05) than in kudzu (6.5 vs. 4.5%/h). The more rapid B2 digestion of grasses would potentially yield more MP and AA for milk synthesis from a larger pool of ruminally available fiber (Cerosaletti, 1998; Juarez et al., 1999). Therefore, excessive substitution of kudzu for grasses would be expected to decrease the ME available for milk and growth. Digestion rates for the selected portions of grasses in this study were similar to those in young regrowth of B. brizantha in the Gulf Coast of Mexico (Juarez et al., 1999). Hence, the digestion rates and chemical composition of the forage selected by animals from relatively mature Acre pastures were similar in quality to intensively grazed, vegetatively younger plant material.
Digestible NDF (B2 fraction as a percentage of NDF) ranged from 53 to 63% in B. brizantha and from 0 to 22% in kudzu. Kudzu had three times more indigestible carbohydrate (C fraction) than grasses (33 vs. 10%). The CP content (DM basis) averaged 8% for the grasses and 17.5% for kudzu (Rueda, 2002).
Seasonal Differences
Season affected chemical composition of grasses (Table 4) with more NDF and NDIP (slowly degraded protein) and less energy content in the rainiest months (P < 0.01). Higher NDF content in grasses decreases the energy available for milk or gain. Hence, animal performance would be expected to diminish in the rainiest season because, even if intake were not depressed, diets with more structural carbohydrates would support less ruminal microbial growth. Nicholson et al. (1994) found that more energy and protein supplementation was required in the rainy months in dual-purpose systems in Venezuela. Figure 1 summarizes monthly variations in CNCPS prediction of forage digestibility (TDN) and dietary intake protein (IP) for finishing steers and lactating cows consuming these Brachiaria grasses and kudzu. Grasses averaged 55% TDN and 72% IP, which exceeded the biological values for kudzu (46% TDN and 50% IP). Digestibility (TDN) predicted from digestible DM (Van Soest, 1994) averaged 50% for Brachiaria spp. across tropical America (Lascano and Euclides, 1996), where most tropical grasses contained 48 to 55% TDN (Tedeschi et al., 2002a). Although grasses and kudzu differed in most elements, there was no significant (P > 0.90) seasonal effect on their mineral composition, except for Cu (P < 0.001). Predictions indicated that dietary supplies of Ca, P, Na, S, and Cu were substantially below the recommended daily allowances for all animal groups consuming only forage (NRC, 2000).
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Animal and Pasture Land Productivity (Baseline Scenario)
Table 5 shows that predictions of average expected voluntary DMI by the CNCPS model were only 4% less than the amounts that were predicted to be required to match the observed milk production. These results indicate good agreement between DMI based on the intake equation of Traxler (1997) and the amount expected for the observed animal production and forage composition. The required daily DMI was equal to approximately 2.2% of cow BW and contained approximately 2.0 Mcal of ME/kg of DM. For finishing steers, DMI predicted with an empirical equation was 10% less than the CNCPS-predicted DM requirement (2.5% of BW), but these predictions were similar (2.1% of BW) for growing steers (Rueda, 2002).
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). Larger sacrifices would be expected if cows consumed more legume (from swards containing more of it). About 30% legume is currently recommended to reduce the N-fertilization cost of pastures in Acre (Wadt, 2002). Therefore, kudzu and other suitable legumes have a multipurpose role involving tradeoffs in making pasture-based systems more productive. Steers consuming B. brizantha were predicted to have sufficient ME to gain 0.50 kg/d. Finishing steers consuming B. decumbens had enough ME to gain 0.40 kg/d. Energy concentration in these grasses averaged 2.0 Mcal of ME/kg of DM. Surplus protein consumption was predicted in every month of the study period, probably because the supply of energy was not enough to support the amount of gain from the protein that was available.
Intake of energy was the most important dietary limitation for the animal groups monitored. This finding agrees with other studies in the American tropics that found energy more limiting than protein (Townsend et al., 1990; Nicholson et al., 1994), but differs from findings by Juarez et al. (1999), where MP was first limiting. In our case, the DMI predictions for dual-purpose cows were in good agreement with those measured in other tropical studies (Traxler, 1997; Euclides et al., 2000; Molina, 2002). In addition to evaluation of alternative feeding strategies (forages and supplements), calibrated nutritional models support farmer decision-making about land use and management of cattle systems in the western Amazon (e.g., supplementation to optimize ruminal fermentative digestion; van Houtert and Sykes, 1999).
At current stocking densities in Acre, excessive accumulation of mature plant biomass of low digestibility is a key constraint to animal and land productivity. This pasture management strategy probably minimizes labor and management inputs to maintain current herd productivity. However, it results in stable but low outputs of milk and beef because it sequesters most nutrients produced by these grasses in indigestible DM. This nutrient stock influences system productivity by slowly releasing nutrients from leaf litter and through periodic burns to maintain grass cover.
Net Economic Returns from Intensification Options
Biological potential exists to increase productivity of Acre cattle systems. Alternative supplies of energy and a higher grazing efficiency would be expected to increase animal and land productivity. However, distinct economic opportunities (a growing market for animal products) and constraints (limited labor and alternative feed source supplies) of Acre cattle systems need to be considered in interpreting these potentials.
Potential to Increase Animal Performance.
The dietary requirements for increasing milk production by 25 and 50% with a more nutrient-rich diet are summarized in Table 5. Milk production goals were predicted to be achievable with 1 to 2 kg/d of sorghum grain and 20% of total daily forage as kudzu (about 2 kg of DM/d) when grazing B. decumbens. Brachiaria brizantha required less kudzu (10% of total forage) than B. decumbens because of its greater CP content with lower solubility, which resulted in more MP available for production. Less sorghum, 0.4 to 1.8 kg/d, was required to similarly increase growth of steers also needing 1.3 kg of DM/d of kudzu for sufficient MP to achieve 50% greater weight gains.
Partial budgeting analysis indicated this approach would be less profitable than current practices to produce milk and beef, a clear disincentive for farmers to alter productivity of animals, land, and labor in this manner. Table 6 illustrates this result for the case of milk production, where the increase in feed costs alone nearly equals the increase in revenues. Sensitivity analysis indicated that an unrealistically higher milk price ($0.16/kg vs. $0.08/kg) or 60% lower grain cost for 50% more milk would be required to achieve a marginal rate of return of 1.0. Therefore, supplementation to increase individual cow productivity is financially unattractive with a low (unitary) milk:sorghum price ratio (Table 6). Net margins were also lower at current prices of beef and grain for more rapid growth of steers with grain supplementation (Table 7). The increase in feed cost for beef production was more than twice the assumed increase in revenues. Only the unrealistic combination of about 100% higher beef price ($0.76/kg vs. $0.38/kg) and 75% lower grain cost would provide adequate incentive (marginal rate of return of 0.5) for farmer investment in more rapid growth. These evaluations differ from those in Venezuela, where low-cost feeding strategies (supplementation with cassava, urea and molasses) for dual-purpose cows proved more profitable than the baseline forage-concentrate diet (Townsend et al., 1990; Nicholson et al., 1994). A key difference is the lower milk:supplement price ratio, which markedly limits the profitability of supplementation for more milk.
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Potential to Increase the Productivity of Land.
Variable costs for fertilization strategies with alternative stocking densities exceeded those for the baseline scenario for both cattle systems due to greater requirements for labor, fencing, and especially, fertilizer (Table 8). Net margins at higher stocking densities in the dual-purpose system were always lower than for baseline management. None of these options are likely to be adopted for milk production because the increased labor and fertilizer costs exceeded the increase in revenues, all strategies having negative returns.
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In both production systems, the EMBRAPA-recommended maintenance fertilization strategy for the western Amazon had the lowest marginal rate of return, and also may be 20 to 60% deficient in P. According to our estimations (Rueda, 2002), the annual nutrient exports in animal products were 6 and 3 kg/ha of N and P, respectively, at baseline stocking with 2 animal units/ha. As stocking density increases, more nutrients would be exported in greater sales of milk and beef.
Although there is biological potential to increase the productivity of individual animals and land, only the more intensified use of land for beef production seemed profitable. This management strategy has the potential to benefit owners of both dual-purpose and beef herds. However, the transition process and associated capital cost to achieve larger herds and greater land productivity were not accounted, which may imply lower returns (higher costs) than indicated here. More farm-level research is needed to more fully evaluate the transition dynamics of these low equilibrium systems to more intensive ones.
The technological options evaluated are labor-intensive (or potentially land-saving), since under our assumptions, they require more than four times the yearly amount of labor used in baseline management. Given that labor is scarce in this region and farmers, especially those with limited resources, have restricted access to capital, it is unclear whether these more intensified activities would be adopted. However, the cost of forest clearing and seeding with grass is about $100/ha (Banco da Amazônia, 2000), which exceeds the approximate $80 cost of more intensive pasture use for beef production. Thus, more intensive use of land may discourage land clearing to maintain the same level of production. Technological change increases profits and the incentive to deforest (Carpentier et al., 2000). Labor intensity is an important factor affecting these outcomes. Profitable, labor-intensive agricultural technologies tend to reduce forest clearing because they exhaust available labor that otherwise could be used to deforest (Angelsen and Kaimowitz, 2001). Although this relationship is not the focus of this research, our analysis points out that some intensification strategies (supplementation, more milk per land area) are unlikely to affect forest clearing because their use is economically unattractive.
In conclusion, chemical composition of grasses differed between rainiest and less rainy seasons. Grasses were of poorer quality (more NDF and NDIP and less ME) during the rainiest months. Simulations indicated that ME is first limiting in the baseline scenario and MP is limiting at higher milk production and weight gain. Brachiaria brizantha provided slightly more MP than B. decumbens to support lactation. Seasonal variability in forage quality suggests that diet supplementation is needed during the months with highest precipitation, in contrast to the common recommendation to supplement only in the dry season.
Voluntary DMI predicted by empirical equations and the DMI required for the observed animal performance predicted with the CNCPS model were in close agreement. Animals grazed selectively because accumulated biomass exceeded the recommended herbage per animal, even in the months with lowest precipitation. Thus, there is opportunity for greater productivity in this region from managed grazing of less mature forage.
Partial budgeting analyses indicated that increases in individual animal performance by dietary supplementation are less profitable than baseline management. More intensive beef production by judicious fertilization of grass–legume pastures and greater stocking density is the preferable strategy for owners of both cattle systems to improve net economic returns under current conditions. It may also help reduce incentives for additional forest clearing. Further analysis is required to evaluate alternative practices to increase productivity in these cattle systems, including soil-plant-animal nutrient dynamics and nutritional support of herd reproduction and its effect on profit (Tedeschi et al., 2002b).
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2 Correspondence—phone: 607-255-2858; E-mail: rwb5{at}cornell.edu.
Received for publication February 19, 2003. Accepted for publication July 21, 2003.
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