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Published in Agron. J. 95:1157-1162 (2003).
© American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA

PRODUCTION PAPERS

Grazing Pressure on Beef and Grain Production of Dual-Purpose Wheat in Argentina

Martín J. Arzadun*,a, José I. Arroquyb, Hugo E. Labordec and Roberto E. Brevedanc

a Estación Experimental Coronel Suárez, MAA, CC 204, 7540 Cnel. Suárez (BA) Argentina
b Comisión de Investigaciones Científicas de la Provincia de Buenos Aires
c Departamento de Agronomía CERZOS, Universidad Nacional del Sur, 8000 Bahía Blanca (BA) Argentina

* Corresponding author (marzadun{at}faa.unicen.edu.ar).

Received for publication December 17, 2001.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The effects of three different grazing pressures on beef (Bos taurus) and wheat (Triticum aestivum L.) grain production were studied at Pasman (Argentina) during a 3-yr study. Thirty hectares of the variety ‘Pincen’ wheat were grazed with Angus heifers during 3 yr (1995–1997) until apex differentiation. The different grazing pressures were obtained through different stocking rates, adjusted every 21 d according to forage mass, predicted forage production rate, and an estimated forage allowance of 10, 15 and 20 kg dry matter (DM) heifer-1 d-1 for high, medium, and low pressures, respectively. The three treatments were randomly allocated within a complete block design with two replications. Every 21 d forage samples were clipped and forage mass and plant composition were recorded and in vitro dry matter digestibility (IVDMD), crude protein (CP), and neutral detergent fiber (NDF) were determined. Spike density and grain production were measured at harvest. Average daily gain (ADG) was obtained from 10 heifers per plot, present throughout the entire grazing period. Gain per hectare (GH) was obtained in each plot from the mean ADG and animal days per hectare computed during stocking rate adjustments. During grazing period the levels of CP and IVDMD decreased and NDF content increased, and these trends were more pronounced in 1996 and 1997 because of rust infection. Increased grazing pressure caused increased GH and reduced ADG, forage mass at the end of the grazing period, and grain production. Means of treatments over the years for GH were 283, 225, and 176 kg ha-1, and for grain production were 955, 1150, and 1351 kg ha-1 for high, medium, and low pressures, respectively. The increase in grazing pressure (from low to high treatment) produced an increase of 107 kg ha-1 in beef production and a 396 kg ha-1 reduction in grain production. Taking into account the traditional price of beef with respect to grain in Argentina, this beef gain over grain relation at increased grazing pressure is economically profitable for farmers.

Abbreviations: ADG, average daily gain • CP, crude protein • DM, dry matter • DP, dual purpose • GH, animal weight gain ha-1 • IVDMD, in vitro dry matter digestibility • NDF, neutral detergent fiber


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
BEEF PRODUCTION AND WINTER CROPS, predominantly wheat, are the mainstay of the agricultural economy of a wide region of the southern Argentine Pampas and the southern Great Plains of the USA. In this area of Argentina, wheat grain is the main source of producers' income (accounting for >50%) (Encuesta Nacional Agropecuaria, 1994). Forage availability of perennial pastures and native grazing lands for beef production is reduced during the winter period, so grazed cereals—mainly oat (Avena sativa L.), rye (Secale cereale L.), or barley (Hordeum vulgare L.)—are used to provide good quality forage during the 3 to 4 mo of reduced forage production. Over the last two decades an increase in the cropping area devoted to grain production has gradually replaced pasturelands. Currently, owing to low profits from grain crops, many farmers are attempting to rebuild their cattle stocks. Crops for grain and pastures compete for land use and make it more difficult to return to livestock production.

The use of wheat as a forage and grain dual-purpose (DP) crop is aimed at reducing competition between areas devoted to grain and forage crops. The income stability of this system should be higher because both beef and wheat commodities are available for market (Díaz et al., 1986; Redmon et al., 1995a).

In several countries the DP system has been used extensively (Díaz-Rosello et al., 1993). Farmers in the USA use DP winter wheat to increase income in relation to grain-only crop production (Horn et al., 1994). Redmon et al. (1995a) demonstrated the contribution of each product (beef and grain) to the stability of farm income. In Argentina, DP wheat was a common practice during the 1960s, reaching 28% of the total wheat crop (Coscia, 1967). More recent use of new varieties with potentially higher grain yields and no vernalization requirements combined with low beef prices caused a decrease in the amount of DP wheat sown and its replacement by grain-only crops in Argentina.

Information from grazing experiments shows that grain production decreases when animal weight gain per hectare increases, by effect of increases of stocking rate (Horn et al., 1994; Redmon et al., 1995a) or by increases in grazing duration (Hernández, 1969). In a 5-yr study, Hernández (1969) reported that grazing management producing 88 kg ha-1 of beef caused a 211 kg ha-1 reduction in grain production. In the same trial, intensified grazing produced 136 kg ha-1 of beef, but grain production decreased by 590 kg ha-1. Redmon et al. (1996) reported that grain yield decreased 83 kg ha-1 d-1 as cattle grazed past the first hollow stem stage of maturity and considered this stage the critical time for grazing termination. This morphological stage occurs prior to the growing point (head) reaching the soil surface and these authors recommended the first hollow stem detection in an ungrazed area of the crop as a key indicator of when to terminate grazing.

While apex removal due to an extended grazing period can substantially reduce grain yield, significant reductions in response to defoliation can occur without apex removal (Winter and Musick, 1991). Grain yields of winter wheat are affected by the extent of defoliation created by different clipping heights (Dunphy et al., 1982) or by different stocking rates (Horn et al., 1994). In this work, decreasing grain production was likely a consequence of decreasing forage mass at the end of grazing period, although this parameter was not measured.

Winter and Musick (1991) found a positive correlation between leaf area index at anthesis and grain yield of winter wheat that had been grazed. Development of the wheat crop between the end of grazing and the onset of flowering appears to be crucial for grain production. Forage mass at the end of grazing determines the leaf area of the crop at the beginning of the recovery period, whereas water and N availability in the soil and genotype determine plant recovery until flowering (Redmon et al., 1995a) and, ultimately, grain yield.

Although experiments with different levels of stocking rate can represent management practices of grazing and provide information relevant for economic analysis, the effects of the treatments on the animal and on the pasture can be different in each year and site. In relation to the variations in forage production and consequently in forage mass, the same stocking rate applied through different years could result in different animal performance, forage mass at the end of grazing, and grain production. In contrast, grazing pressure (relation between animal units and forage mass) (ASA, 1992) as a treatment could give more useful information to understand animal–plant relationships and their effects on animal and crop production because of a more stable reference unit.

Considering the lack of information about the use of modern adapted varieties of winter wheat as a dual purpose crop in environmental and management conditions of the subhumid plains of Argentina, the objective of our research was to evaluate beef and grain production under different grazing pressures.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The research was conducted at the Experimental Station of the Agriculture Ministry of Buenos Aires Province in Pasman, Argentina, located 37°11'S, 62°08'W, and 180 m above sea level. Analyses of forage composition were made by the Animal Nutrition Laboratory of the Universidad Nacional del Sur, Bahia Blanca, Argentina.

In 3 consecutive years, 1995, 1996 and 1997, adjacent areas were sown during the first week of March with ‘Pincen’ winter wheat at 90 kg ha-1 of live seed. The soil in the experimental area was a Typic Hapludoll with moderate fertility and represented the soils in a large area of the southwest subhumid region of the Pampa. Nitrogen (46 kg ha-1) in the form of urea was applied before mid-March each year. In May 1996 and 1997, propiconazole (1[[2-(2,4-dichlorophenyl)-4-propyl-1,3-dioxolan-2-yl]methyl]-1H-1,2,4-triazole) fungicide was applied to prevent rust (Puccinia recondita) attack.

In a randomized complete block design with two blocks, treatments were three different grazing pressures—high, medium and low—obtained through stocking adjustments (put-and-take method) on paddocks of different area. These areas were inversely related to the level of grazing pressure—3.33 ha for high, 4.99 ha for medium, and 6.66 ha for low. Different paddock sizes supported an equal but as high as possible number of tester animals per paddock among the treatments and optimized use of the experimental area. A continuous grazing method was used, with stocking rate adjustments every 21 d during the grazing period. Forage mass and expected production for the remaining vegetative period were taken into consideration in calculating the stocking rate. Information from a forage evaluation experiment with different clipping heights, and including Pincen among others varieties (Arroquy, 1999), was used to estimate an expected production rate of 12 kg DM ha-1 d-1 and 1 September as the end date for the vegetative stage. Taking this available plus expected forage supply into account, animals were assigned to each paddock according to a forage allowance of 10, 15, and 20 kg DM animal-1 d-1 for high, medium, and low treatments, respectively. Forage mass was estimated from clipping eight 0.25-m2 quadrats to ground level per paddock at 21-d intervals, before each stocking adjustment and at termination of grazing. A similar sampling method was applied at anthesis in 1996 and 1997 to estimate dry matter recovery postgrazing.

Nine-month-old Angus heifers were used, with mean ± SE initial weight of 142 ± 17, 164 ± 16, and 160 ± 18 kg, in 1995, 1996, and 1997, respectively. Before starting the experiment, animals grazed a similar wheat pasture for 10 d. Each paddock held a set of 10 animals (testers) for the duration of grazing and a variable number of similar animals (graziers) was used according to the stocking rate adjustments needed to attain forage allowance targets. Every 21 d throughout the grazing period the weights of the tester heifers were recorded early in the morning with no shrinkage. Weights from the first and the last date for the 10 tester animals per paddock were used to calculate average daily gain (ADG), in kg animal-1 d-1. The same number of testers results in a more homogeneous variance among animals within pastures (Bransby, 1989), and a high number of animals reduces the contribution of the differences between animals to experimental error (Petersen and Lucas, 1960). Animal weight gain ha-1 (GH) was calculated from the total number of heifers (testers and graziers) and the number of days that each heifer grazed, multiplied by the mean ADG of testers.

In each paddock, to avoid areas of unusually frequent animal traffic, an area of 50 by 50 m near the water supply and 20 m away from the fences was avoided in the collection of forage samples. Fresh forage from a pool per paddock was divided into two subsamples; one of them was hand separated into dead and green fractions and dried in a force air oven, at 60°C, until constant weight. The other was also dried and reserved for quality analysis.

Green leaf content of the forage was considered as a quality factor. Dried samples were ground in a mill to pass through a 1-mm screen, and in vitro dry matter digestibility (IVDMD) (Tilley and Terry, 1963), crude protein (CP) (AOAC, 1990), and neutral detergent fiber (NDF) (Goering and Van Soest, 1970) were determined. Two 1.5-yr old fistulated steers fed good quality alfalfa (Medicago sativa L.) hay were used as the source of rumen liquid used for IVDMD analysis.

In 1995 and 1996, grazing began when forage mass reached approximately 2.5 Mg ha-1 of DM. This level of forage mass was reached by 13 June 1995, and by 7 May 1996. In 1997, grazing began on 1 July, even though there was only approximately 1.0 Mg ha-1. The lower level of forage accumulation during the latter year was due to lower rainfall during the initial development of the crop in April and May (Table 1). In all 3 yr, grazing ended in September when reproductive apices began to emerge. The appearance of the first hollow stem, observed in an adjacent nongrazed area, was used to determine when grazing should cease (Redmon et al., 1996). In 1996 and 1997, at anthesis, eight additional samples per paddock were taken, 1.5 m apart of the site of a previous sample taken at grazing termination. The difference in forage mass between both samples, divided by the number of days between both sampling dates gave the DM accumulation rate considered a measure of crop recovery postgrazing pressure treatment.


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  		Table 1. Rainfall, and number of frosts per month during the experiment in comparison with the average of the period 1971–1990.

 
Grain yield was evaluated by harvesting three strips (each 4.8 m wide and 150 m long) per paddock using a conventional combine–harvester. Before harvest, spike density in eight areas of 1 m2 per strip was counted.

Analyses of variance were performed according to a randomized complete block design, combined over years, with MSTATC (Freed et al., 1991), and the grazing pressure within each year was analyzed as a single-factor. Year was considered a fixed effect to analyze the results of three different years on production, and the homogeneity of variances among years was verified using the Bartlett's test (Steel and Torrie, 1980). For forage quality parameters and green leaf content, which had heterogeneous variances among years and sampling dates, only the effect of treatment within each date was analyzed. When the F value was significant (P < 0.05), polynomial orthogonal contrasts were used to evaluate linear and quadratic effects of treatments, and the LSD test was used to compare means from different years.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Weather conditions during the 3 yr of the study differed considerably (Table 1). There were 15 more frosts (temperature below 0°C) during 1995 than the average of the last two decades, whereas there were 38 fewer frosts in 1997 than during the first year. Climatic conditions during April and May strongly affect forage production of annual winter crops throughout much of the Pampas plains. In 1997, rainfall during this period was <50% of that occurring the first 2 yr and the 20-yr average. The lower forage accumulation caused by this water deficit delayed the start of grazing until 1 July, when the forage mass was still only 1 Mg ha-1.

Forage Mass
The grazing period was 85 d in 1995 (13 June–6 September), 136 d in 1996 (7 May–20 September), and 84 d in 1997 (1 July–23 September). Differences in the length of the grazing period among years were related to different levels of forage accumulation during the fall, which affected the starting date of grazing. The ending date of grazing depended on the appearance of the first hollow stem. The three levels of grazing pressure resulted in a different amount of forage mass at the end of grazing with a linear increase (P < 0.01) as grazing pressure decreased (Table 2 and 3). The productivity of forage during the grazing period was different each year (Fig. 1) , in relation to different weather conditions and the severity of rust. In the winter of 1997, improving climatic conditions during the grazing period caused the amount of available forage to increase beyond the initial levels of available forage in the medium and low grazing pressure treatments. Abundant forage availability in the fall of 1996 allowed an earlier start to grazing (7 May), but an intense rust attack during the winter caused considerable dead matter accumulation (650 g kg-1 of forage) by July.


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  		Table 2. Probability of >F in the analysis of variance of forage mass at the end of grazing, animal weight gain ha-1 (GH), average daily gain (ADG), spike density, and grain yield.

 

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  		Table 3. Total forage dry matter at the end of grazing, animal weight gain per hectare (GH), spike density, and grain yield of winter wheat grazed at three levels of pressure.

 


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  		Fig. 1. Forage mass in each treatment during 3 yr. Vertical bars indicate SE of means, average of two paddocks, of treatments on each sampling date.

 
Forage Quality
Forage quality traits of IVDMD, CP, NDF, and green leaf content are presented in Fig. 2 . According to the requirements of growing heifers such as used in this experiment (NRC, 1984), the CP content of the forage would not limit live weight gain. In contrast, IVDMD were sometimes <600 g kg-1 in 1996 and often about 700 g kg-1 in 1997, while NDF levels for both of these years equaled or exceeded about 600 g kg-1. This NDF concentration in the diet would limit DM intake because of rumen fill (Mertens, 1994). With continuous grazing of wheat pasture and other cool season grasses, morphological and quality changes in relation to age have been observed. During periods of continuous grazing, dead vegetative matter increased, the levels of CP and IVDMD decreased, and NDF content increased (Nelson and Moser, 1994). In our study, these trends in forage quality were observed in 1995 (Fig. 2), and were more pronounced in 1996 and 1997 because of rust infection. In May and June of these years CP, IVDMD, and green leaf content decreased markedly, especially in 1996 when the severity of rust infection was more intense. In both years, when rust infection went away IVDMD and green leaf levels raised again (Fig. 2). For the quality traits measured, significant (P < 0.05) differences occurring between treatments were observed usually only toward the end of grazing, except for IVDMD in July of 1996 (Fig. 2b). The decreased IVDMD observed in the rust affected forage during the 1996 period were consistent with lower in vitro digestion of rust infected tissues observed by Edwards et al. (1981) and may be the consequence of decreased soluble carbohydrate level in leaves infected with rust (Daly et al., 1962).



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  		Fig. 2. Three years of measurements for (a) forage crude protein (CP), (b) in vitro dry matter digestibility (IVDMD), (c) neutral detergent fiber (NDF), and (d) green leaf content of samples collected from pastures managed at three grazing pressures. Vertical bars are SE of means (n = 2), arrows indicate sampling dates with significant effect of treatment (L, linear contrast; Q, quadratic contrast; * 0.05 probability level, ** 0.01 probability level).

 
Animal Production
The effects of treatment and year were significant (P < 0.01) for ADG and GH, and the treatment x year interaction effect was significant (P < 0.05) only for ADG (Table 2). Except for 1995, a linear increase (P < 0.01) in ADG from high to low grazing pressure was observed (Table 4). In 1997, GH was lower than in the other years. As an average of the 3 yr, GH declined quadratically (P < 0.05) from high to low pressure, inverse to forage mass at the end of grazing period (Table 3). The lack of effect of grazing pressure on ADG in 1995 could be caused by a forage mass higher than 1 Mg DM ha-1 in high pressure. This level would not restrict forage intake (Chifflet de Verde et al., 1974). Although the stocking rate adjustments were made each year based on the total forage mass, in 1996 and 1997 it included dead material with lower nutritional value that was probably rejected by the animals. The level of forage mass at low grazing pressure in 1995 was greater than that of the same treatment in 1997 (Fig. 1) and the green leaf content in 1995 were greater than that of 1996 (Fig. 2d). Using data from all 3 yr, the correlation coefficient between ADG and green forage mass was greater (r = 0.76, P < 0.01) than that between ADG and whole forage mass (r = 0.63, P < 0.05). Consequently, calculation of stocking rate would likely be improved when only green forage mass is considered rather than total forage mass.


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  		Table 4. Average daily gain of heifers on winter wheat pasture at three levels of grazing pressure.

 
Postgrazing Dry Matter Accumulation and Grain Yield
The dry matter accumulation rate from end of grazing to anthesis, as estimated in 1996 and 1997, was not different between years (P > 0.05), but differences among grazing pressure treatments were significant (P < 0.05). Treatment means (±SE) (kg ha-1 d-1) were: 49.7 (±1.2) for high, 58.9 (±4.0) for medium, and 66.9 (±7.5) for low, with significant linear and quadratic trends (P < 0.05). The decreasing growth rate as grazing pressure increases could be due to decreasing residual leaf area, as observed with clipping treatments (Hubbard and Harper, 1949).

Spike density decreased linearly (P < 0.01) with increasing grazing pressure and was lower (P < 0.01) in 1997 than in the other years (Table 3). Low rainfall in 1997 during the period between the end of grazing and anthesis (Table 1) may have contributed to this difference. Grain yield was affected quadratically (P < 0.01) by grazing pressure, but differences between years were not significant. Grain yield increased nearly 400 kg ha-1 when the grazing pressure was reduced from a high to a low level and appears to be the consequence of differences in dry matter accumulation by anthesis. This result is consistent with clipping-induced decreases in preanthesis dry weight and carbohydrate accumulation of triticale (X Triticosecale Wittmack) that adversely affected grain yield (Royo et al. 1999).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The presence of rust disease in wheat pasture decreased beef production because the nutritional value of the forage was decreased. In 1996 and 1997, rust infection occurred despite the application of fungicide. Development of rust-resistant varieties should be one goal of a genetic improvement program for DP wheat grown in the Pampa region of Argentina.

For conditions encountered in this study, increasing the grazing pressure led to lower forage mass, which may have reduced animal voluntary intake. Intake of similar annual cool-season forages is often limited when forage availability falls <1000 kg DM ha-1 (Chifflet de Verde et al., 1974), or when the daily dry matter allowance falls <21 to 24 kg 100 kg -1 of body weight (Redmon et al., 1995b). In addition, nutrient intake is likely less with the intermixing of accumulated dead vegetative matter with green forage.

Adverse changes in wheat forage quality during the grazing period would be expected to amplify the negative effect of reduced forage intake when forage mass is low. Reduced digestibility of forage sampled from annual ryegrass (Lolium multiflorum Lam.) pastures with low forage availability was considered a main factor in reducing intake (Jamieson and Hodgson, 1979).

Our results demonstrate that increasing grazing pressure from low to high causes an increase of 107 kg ha-1 in GH and a decrease of 396 kg ha-1 in grain production. These values imply a grain per beef replacement rate of 3.7:1. With net incomes (U.S. dollars) of $0.09 and 0.78 kg-1 from selling grain and beef, respectively, on the Argentine domestic market, total income per hectare would potentially increase by $48 if grazing pressure were increased from the low to the high level of intensity. The decrease in ADG at the high grazing pressure, however, must be considered in relation to its delay in preparing the animals for marketing. In beef production systems based on grazing, this lower ADG would have a larger economic importance than the simultaneous drop in grain production.


   ACKNOWLEDGMENTS
 
Contributions to this work by statisticians Ricardo Camina and Nélida Winzer are sincerely appreciated and we thank Silvia Canelo and Rosana Palomo (UNS–Laboratorio de Nutrición Animal) for forage quality analysis. Support for this research came in part from the National Research Council of Argentina (Project no. 08-04516).


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




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