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Published online 8 January 2009
Published in Agron J 101:150-158 (2009)
DOI: 10.2134/agronj2008.0098
© 2009 American Society of Agronomy
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Right arrow Grazing Management

Cattle Gain and Crop Yield for a Dryland Wheat-Sorghum-Fallow Rotation

R. L. Baumhardta,*, R. C. Schwartza, L. W. Greeneb and J. C. MacDonaldc

a USDA-Agricultural Research Service, Conservation and Production Research Laboratory, P.O. Drawer 10, Bushland, TX 79012-0010
b Dep. Animal Science, 209 Animal Sciences, Auburn Univ., Auburn, AL. 36849
c Texas AgriLife Research and Extension Center, 6500 Amarillo Blvd. W., Amarillo, TX, 79106

* Corresponding author (r.louis.baumhardt{at}ars.usda.gov).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
Increasing pumping costs and declining well capacities in the U.S. Southern High Plains have led to greater reliance on less productive and inherently riskier dryland cropping systems. Dryland wheat (Triticum aestivum L.) and grain sorghum [Sorghum bicolor (L.) Moench] are typically grown in a 3-yr wheat-sorghum-fallow (WSF) rotation that may be intensified by integrating cattle (Bos taurus) grazing. Suitability of grazing dryland crops in the WSF rotation has not been evaluated. Our objectives were to quantify (i) cattle gain during limited grazing of dryland wheat and sorghum stover, and (ii) grazing effects on the growth and yield of the grazed wheat and subsequent sorghum crop. We established, concurrently, all WSF rotation phases in duplicate ungrazed and grazed plots in three replicated paddocks on a gently sloping Pullman silty clay loam (fine, mixed, superactive, thermic Torrertic Paleustoll) at the USDA-ARS, Conservation and Production Research Laboratory, Bushland, TX (35°11' N, 102°5' W). Cattle gain, fallow soil water storage, and the growth and yield of wheat and subsequent grain sorghum were compared from 2000 to 2007 within a randomized complete block. Dryland wheat was grazed an average of 31 d during 7 of 8 test years by cattle stocked at 1.7 Mg ha–1 and produced a mean gain of 123 kg ha–1. Wheat grain yield averaged 1.72 Mg ha–1 without grazing and was not different from the 1.57 Mg ha–1 grain yield with grazing. Grazing decreased wheat straw yield, but subsequent soil water storage was unaffected. Sorghum grain yields of 2.26 Mg ha–1 in ungrazed plots were not different from grazed plots averaging 2.20 Mg ha–1. Overall productivity of the WSF cropping system was increased using limited grazing of dryland wheat forage and sorghum stover with no significant reduction in wheat or sorghum grain yields.

Abbreviations: G, grazed • LAI, leaf area index (m2 m–2) • UG, ungrazed • WSF, wheat-sorghum-fallow rotation

Received for publication March 31, 2008.
   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
INTENSIVE PRODUCTION of cultivated crops in the Texas High Plains depends on supplemental water from irrigation that is supplied from the declining Ogallala Aquifer (Musick et al., 1990). As a result, irrigated land area has decreased from a peak of 2.42 million ha in 1974 to 1.87 million ha in 2000 (Colaizzi et al., 2008). Increasing profitability of sustainable dryland cropping systems is critical for continued success of the southern Great Plains agriculture. For example, dryland wheat and grain sorghum are grown using the WSF crop rotation (Fig. 1 ). The WSF rotation begins during September of the first year with planting of winter wheat, which is harvested for grain 10 mo later in July. Grain sorghum is planted in June of the second year and harvested for grain in November. The land is, subsequently, fallowed through the next summer cropping season until wheat is planted at the end of the third year and the rotation cycle is repeated. This rotation uses the soil water stored during fallow plus seasonal precipitation to consistently produce two dryland crops in a 3-yr cycle with mean grain yields of 3.1 Mg ha–1 for sorghum and 1.2 Mg ha–1 for wheat (Jones and Popham, 1997). The dryland WSF cropping system can be intensified by integrating cattle grazing wheat forage and nearby sorghum stover during all or part of a period from November to March (Fig. 1). This adds cattle gain for greater productivity and profitability as a potential avenue to transition from irrigated crops.


Figure 1
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  		Fig. 1. The wheat-sorghum-fallow (WSF) crop sequence diagrammed as a repeating 3-yr cycle beginning with planting wheat in September. Vegetative wheat growth provides forage during the first rotation phase in Year 1 and sorghum stover is supplied during the postgrain-harvest fallow after sorghum rotation phase in Year 3. Potential grazing of sorghum stover and vegetative wheat growth could be possible as early as December and continued through March.

 
Wheat pasture and adjoining fields of sorghum stover can supply high quality forage capable of meeting protein, energy, and fiber demand for grazing cattle (Shroyer et al., 1993). Grazing rapidly growing wheat often supplies sufficient protein to promote frothy bloat, which was detected during approximately 66% of the observation periods at various times of day, plant growth stages, and forage amounts (Min et al., 2005). These researchers concluded that condensed tannins had the potential to control bloat. The WSF rotation permits grazing nearby sorghum stover as a source of roughage, which can effectively control bloat (Shroyer et al., 1993) and supplement wheat forage for potentially greater stocking rates. Their suggested stocking rate of 0.25 to 0.50 Mg liveweight ha–1 achieved mean daily gains up to 0.75 kg with negligible effects on wheat grain yield.

In contrast, results from several studies evaluating wheat cultivars and stocking strategies at Bushland, TX, indicated an undesirable reduction in wheat grain yield, especially if grazing extended into the reproductive stage of growth (Winter and Thompson, 1987, 1990a, 1990b; Winter and Musick, 1991; Winter, 1994). Winter and Unger (2001) reported surface soil compaction and reduced irrigated wheat residue with cattle grazing, which decreased the subsequent grain yield of dryland sorghum grown in the WSF rotation. Soil density increased and more bare soil was exposed to raindrop impact where cattle consumed 25 to 30% of the sorghum residue and trampled the soil, which reduced infiltration (Bari et al., 1993). These factors govern the storage of precipitation as soil water and are critical for sustaining dryland crop yields.

In a review article, Redmon et al. (1995) illustrated the viability of grazing wheat established under rainfed conditions. Irrigation is used to establish and promote winter wheat growth for similar grazing conditions in semiarid regions and increase profitability and sustainability of integrated crop-livestock systems noted by Russelle et al. (2007) for rainfed conditions. Winter and Unger (2001) evaluated grain yields of dryland sorghum grown after grazed irrigated wheat; however, we found no similar study quantifying cattle grazing effects on the growth and yield of dryland wheat and grain sorghum grown under semiarid conditions. We hypothesized that integrated crop-livestock systems have the potential to increase production intensity and profitability of the WSF rotation used in semiarid dryland production. Our objectives were to quantify (i) cattle gain during limited grazing of dryland crops, and (ii) the effects of grazing on the growth and yield of the grazed and subsequent rotation crops.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
We quantified weight gain potential for cattle grazing dryland cropping systems and the effects of grazing on crop forage and grain yields at the USDA-ARS Conservation and Production Research Laboratory, Bushland, TX (35°10.25' N, 102°5' W). Bushland features a semiarid climate with 496 mm mean annual precipitation, an average 2100 mm annual pan evaporation, and approximately 180 d frost free growing season (Baumhardt and Salinas-Garcia, 2006). The experiment was conducted from 1999 to 2007 on a 330-m wide by 500-m long (16.5 ha) field of gently sloping (1.5%) Pullman clay loam described by Unger and Pringle (1981). To control variation due to slope, the field was divided into three blocks that averaged 166-m length and were drained by conveyance ditches along the contour. Each block contained randomly assigned grazed or ungrazed treatments in two 165-m wide paddocks. Paddocks were divided into three, 55-m wide plots that were randomly assigned one of three phases of the WSF rotation (Jones and Popham, 1997) so that all wheat, sorghum, or fallow rotation phases appeared every year. In this way, we maximized comparisons of treatment effects during a multi-year rotation, but repeated observations from unique experimental units only appear every 3 yr.

Agronomic
Winter wheat (cultivar TAM-110, Foundation Seed, College Station, TX)1 was sown at a 45 kg ha–1 rate to achieve 2.0 x 106 plants ha–1 using a high-clearance grain drill with hoe openers and press wheels at a 0.3-m row spacing during early September to late October when soil water was adequate for wheat establishment. With irrigation, wheat for grazing is typically planted during early September and ungrazed wheat is planted 1 mo later; however, because of limited September rains we used dual planting dates only in 2002 to minimize crop establishment risks. Seasonal weed control in wheat was primarily for flixweed [Descurainia sophia (L.) Webb ex Prantl] using 0.6 kg a.i. ha–1 2,4-D [(2,4-dichlorophenoxy) acetic acid] after grazing as recommended when using synthetic auxins on pastures (Hartzler and Owen, 2005). During the approximately 11-mo fallow after wheat harvest, weeds were controlled as needed (three to four tillage operations) using a 4.6-m-wide Richardson (Sunflower Manufacturing Co., Beloit, KS) sweep-plow. The sweep-plow has one 1.5- and two 1.8-m-wide overlapping V-shaped blades operated at a depth of 0.10 m and was fitted with a trailing mulch treader. Fallow after wheat ended in early to mid-June. Grain sorghum, Pioneer hybrid "8699" (Johnston, IA), was then seeded in 0.75 m rows at 80,000 seeds ha–1, using Max-Emerge (John Deere, East Moline, IL) unit planters. Sorghum seed was safened with fluxofenim [1-(4-chlorophenyl)-2,2,2-trifluoroethanone O-(1,3-dioxolan-2-ylmethyl)oxime] as required for weed control during the growing season using a commercially available mixture of atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine] and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide] applied postplant at rates of 1.3 kg a.i. ha–1 and 1.0 kg a.i. ha–1, respectively. No supplemental N fertilizer was applied to meet expected dryland yields, because the needed 50 kg ha–1 N is mineralized during the fallow period of this cropping system (Eck and Jones, 1992; Jones et al., 1997). No P or K fertilizers were applied because the Pullman clay mineralogy supplies sufficient K to meet crop demand (Johnson et al., 1983) and dryland crop response to broadcast applied P fertilizer has been limited (Eck, 1969, 1988).

Grazing
Dryland wheat growth was initially slow and delayed stocking cattle in electrically fenced paddocks until January or February. British by European-exotic crossbred steers that weighed approximately 0.25 Mg were stocked at a target rate of 1.8 Mg ha–1 (wheat area) depending on available wheat forage. Cattle were allowed to graze the entire paddock, including stover on the sorghum plots, for approximately 1 mo, when wheat forage was judged insufficient to maintain gain. We terminated grazing before hollow stem development in ungrazed wheat because hollow stem signals the beginning of seed head formation (Redmon et al., 1996). Grazing of dryland wheat in 2001 was deferred until April because delayed fall 2000 precipitation postponed wheat establishment. Wheat was grazed out (no grain yield) in 2001. Cattle gain was calculated as the difference between stocking and pull-off weights, determined after 1-d shrinkage, and related to the grazed wheat area.

Measurements
Wheat grain yields were taken from 0.76 m2 hand-harvested samples and corrected to standardized 120 g kg–1 water content. From these wheat samples, straw yield was determined and the number of seed heads counted. We hand harvested sorghum samples from a 1.54 m2 area during the boot growth stage to determine tiller number and the maximum leaf area index, LAI (m2 m–2), calculated from leaf area measured using a model 3100 leaf area meter (LI-COR, Lincoln, NE). Sorghum seed head number and grain yields, standardized to 130 g kg–1 water content, were estimated at the end of the growing season in November from 1.54-m2 hand-harvested samples. During 2002, variability in the sorghum stand required much larger combine samples for accurate yield estimation. Mean seed weight for both wheat and sorghum was determined from 200 seed. We measured wheat and sorghum forage amounts using triplicate oven dried 0.76-m2 hand samples harvested from each plot immediately after cattle stocking and pull-off for starting and ending forage conditions (respectively).

Precipitation was measured using the official location standard rain gauge. Soil water content was sampled gravimetrically at planting (after fallow) and at harvest using duplicate soil cores taken to a depth of 1.8 m in 0.3-m increments. Volumetric soil water, reported as plant-available soil water (water held between 0.03 and 1.5 MPa matric water potential), was calculated from these gravimetric samples and previously measured soil density as described by Jones et al. (1994). Precipitation storage during fallow was computed as the difference in initial soil water content at harvest and at the end of the fallow period.

Analyses
Crop growth and yield factors including maximum LAI, tiller and head number, mean seed weight, forage and stover yield, and grain and straw yield were analyzed according to a factorial arrangement of a randomized complete block design using SAS general linear models ANOVA procedures (SAS Institute, 1988). That preliminary analysis identified significant year effects and heteroscedastic experimental errors that varied, in some cases, by an order of magnitude possibly due to annual variation in precipitation. We, therefore, analyzed grazing effects on dependent parameters within years. In this way, we also compared grazing effects on soil water contents at planting and harvest and the water stored during fallow for both wheat and grain sorghum.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
SUMMARY AND CONCLUSIONS
 REFERENCES
 
Climatic Conditions
Our experiment was conducted during all or part of the years 1999 through 2007, which had a representative mean annual temperature of 13.6 ± 1.7°C compared with the long-term (1939–2007) mean of 13.4 ± 1.7°C. The first freeze date in fall averaged 22 October for both the long-term record and test period, and the latest spring freeze date of 14 April for the experimental period was 5 d earlier than the long-term record. Mean annual precipitation during our experiment was 439 mm and varied from 312 to 602 mm. This amount was equal to a range of approximately 63 to 121% of the long-term average reported by Baumhardt and Salinas-Garcia (2006). In contrast to irrigated conditions where planting date and wheat cultivar determined grain yield (Winter and Musick, 1993), it is the variability in precipitation amount and seasonal distribution that constitutes the greatest challenge for establishing dryland crops and successful integration of cattle grazing.

Irrigation is often used to ensure wheat establishment and sufficient vegetative growth for subsequent and timely cattle grazing under semiarid climatic conditions. We grew dryland wheat within the WSF cropping sequence using seasonal precipitation and soil water stored during the fallow period following sorghum. Cumulative precipitation during October through June (wheat growing season) exceeded the long-term (1938–2000) average of 300 mm in 4 of the 8 yr (Fig. 2 ), and varied from a low of 119 mm in 2006 to a high of 497 mm in 2005. Planting wheat in October depends on rain during September and October to replenish surface soil water for seed germination. Precipitation during this period, averaged 73 mm during the study or approximately 90% of the long-term average (data not shown). However, the difficult wheat growing seasons ending in 2002 and 2006 began with initial rainfalls of 22 and 26 mm, respectively. With mid-July grain harvest, the vegetative recovery after grazing and subsequent grain yield are governed by March to June precipitation, which averaged 185 mm for the study or approximately 88% of the 211 mm long-term average. Precipitation during the critical March to June period for 2001, 2004, 2005, and 2007 exceeded the long-term average by approximately 5 to 40 mm. Nevertheless, the long-term average fall and spring precipitation would have the potential for improved wheat establishment and increased spring vegetative recovery and subsequent grain yield, compared with our experimental period.


Figure 2
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  		Fig. 2. Cumulative monthly precipitation during the October through June growing season for wheat is plotted (dashed lines) by the grain harvest years 2000 through 2007. The long-term average cumulative precipitation is shown as a solid line.

 
Cumulative monthly precipitation during the June through November grain sorghum growing season for our 7-yr experiment appears with the long-term mean in Fig. 3 . During our study, growing season precipitation ranged from a low of 128 mm in 2001 to a high of 485 mm in 2004 with precipitation totals in 2002 and 2006 that approximated the 336 mm long-term mean. The long-term cumulative precipitation features an even distribution during June through August totaling 240 mm. This amount exceeds the 183 mm average during our study by approximately 60 mm. June rains that are critical for dryland sorghum establishment were less than half of the long-term average in 2001, 2002, and 2006. Subsequent precipitation during July and August averaged 50 and 80% (respectively) of the long-term average, and therefore, limited expected sorghum growth and grain yields.


Figure 3
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  		Fig. 3. Cumulative monthly precipitation during the June through November growing season for sorghum is plotted (dashed lines) for the years 2000 through 2007. The long-term average cumulative precipitation is shown as a solid line.

 
Cattle Gain
We stocked cattle to graze wheat forage and sorghum stover during our 7-yr experiment in all years except 2006 when the October through February precipitation of 36 mm was insufficient for wheat establishment and growth on any plot. Slow wheat growth during the fall delayed rooting to adequately anchor the crop or produce sufficient forage for grazing so the target stocking date was shifted to February 1 ± 10 d. Delayed wheat planting in November 2000, shifted subsequent cattle stocking to 15 Apr. 2001 for wheat graze-out, otherwise our stocking dates ranged from 22 January to 22 February (Table 1 ). Stocking rates for wheat pasture averaged approximately 1.7 Mg ha–1 and ranged from a low of 0.91 Mg ha–1 in 2002 upward to 2.06 Mg ha–1. Eighteen animals were stocked in all years except 2002 when 11 cattle were grazed because dry growing conditions limited wheat forage. One animal fatality in 2004 reduced stocking rates to 17 head.


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  		Table 1. Yearly summary of stocking date, number of animals stocked, and grazing period with the mean ± standard error for the wheat stocking rate, total gain, daily gain, and grazing income based on $0.77 kg–1 gain.

 
Grazing was generally discontinued before hollow stem emergence because the available wheat forage was insufficient to maintain gain; however, the 27 Mar. 2007 termination date occurred at approximately hollow stem. Except in 2006 when wheat failed to grow in all plots, cattle were stocked for a period ranging from 27 to 39 d that averaged 31.4 d (Table 1). Daily gains averaged 3.8 kg ha–1 d–1 and ranged from a high of 6.5 kg ha–1 d–1 in 2000 to 2.3 kg ha–1 d–1 in 2005. Total cattle gain from grazing ranged from a minimum of 62 kg ha–1 to a maximum of 228 kg ha–1 and averaged 123 kg ha–1. Our results show that dryland sorghum and wheat grown using the WSF cropping sequence produced sufficient stover and forage to support cattle grazing for approximately 1 mo per year in 7 out of 8 yr.

Grazing income, calculated using the average 2000 to 2007 contract rate of $0.77 kg–1 gain typical for this region, varied in a manner similar to the overall gain. Grazing income ranged from lows near $50 ha–1 in 2002, 2005, and 2007 to a maximum of $175 ha–1 in 2000. The average income generated by grazing gain was $95 ha–1. In comparison with wheat marketed using the 2000 to 2005 average price of $112 Mg–1, this income equated to a grain yield of 0.85 Mg ha–1, or approximately half the average dryland wheat yield for the WSF crop rotation (Jones and Popham, 1997; Baumhardt and Jones, 2002). The volatile market for wheat grain during 2006 and 2007 achieved prices >$300 Mg–1, which decreased both grain yield required for an income equivalent to cattle gain and the tolerance for possible depression of grain yield due to grazing. Suitability of integrating livestock grazing into the dryland WSF rotation depends on stability of forage yield to support grazing and subsequent maintenance of grain yields.

Wheat and Sorghum Forage
Cattle gain depends on the availability of adequate forage that may be limited by seasonal precipitation, or precipitation stored as soil water during the intervening fallow periods of the WSF rotation. For example, these factors combined in 2006 to prevent dryland wheat establishment and production of forage. Available wheat forage at the start of grazing during 2000 through 2002 ranged from 0.32 to 0.39 Mg ha–1 and did not vary with grazing treatment (Table 2 ). None of these wheat plots followed previously grazed wheat before the first cycle of our 3-yr WSF rotation, although grazed sorghum plots from 2000 and 2001 were cropped to wheat in 2001 and 2002. In contrast, wheat plots entering the second rotation cycle from 2003 to 2005 and third cycle during 2006 to 2007 followed grazed wheat of the previous rotation cycles. The starting wheat forage yield in 2003 and 2004 was significantly (P ≤ 0.01) greater in grazed plots. We attributed these differences to an earlier planting date and more growth for grazed wheat in 2003 and wheat growth response to greater initial soil water in 2004 following the poor 2002 sorghum crop. The greater starting wheat forage for ungrazed plots in 2005 and 2007 followed better stand establishment and initial growth than in the grazed plots possibly because of better seed–soil contact in the ungrazed plots.


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  		Table 2. Summary of grazed (G) and ungrazed (UG) treatment effects on the starting and ending wheat forage and sorghum stover biomass, and wheat tiller number. Significance levels of treatment effects are designated for means calculated within years and for the overall study mean.

 
The wheat forage remaining at termination of grazing averaged 0.49 Mg ha–1 and was significantly (P ≤ 0.01) less than the 2.23 Mg ha–1 for ungrazed wheat (Table 2). Removal of wheat forage by grazing, calculated as the difference in forage at termination plus the initial forage difference, averaged approximately 1.9 Mg ha–1. The difference in ending forage mass between grazed (1.21 Mg ha–1) and ungrazed (1.61 Mg ha–1) plots was not as great in 2007 as in other years. This may have been due to rapid vegetative wheat growth in response to favorable spring precipitation and the need to terminate grazing when reproductive growth began. Wheat tiller number can indicate grazing impact on production potential for seed heads and, consequently, grain yield. The graze-out condition in 2001 and crop failure in 2006 precluded comparison of tiller numbers between treatments. Tiller numbers for grazed and ungrazed wheat were not significantly different for the remaining years except 2007 when grazing reduced tiller number.

In our grazing system, cattle supplemented growing wheat forage with standing stover from the previous sorghum harvest. Grazing effects on sorghum stover in 2002 were not reported because of sample bias in sparse stands, or in 2006 because dry growing conditions leading to wheat failure prevented grazing treatments. The mean yield of sorghum stover at the start of the grazing period during the remaining 6 yr, averaged 3.40 Mg ha–1 for the grazed plots and 3.24 Mg ha–1 in the ungrazed plots (Table 2) and did not vary significantly by year except in 2007. In contrast, sorghum stover remaining after the grazing period was significantly greater in ungrazed plots during all years except 2007. The remaining sorghum stover after grazing averaged 2.26 Mg ha–1 in grazed plots compared with the significantly (P ≤ 0.01) higher 3.09 Mg ha–1 stover in the ungrazed plots. Using the difference between the starting and ending mean stover mass for ungrazed plots to estimate any loss due to weathering, our data show that grazing cattle consumed an average of 0.99 Mg ha–1 of the sorghum stover to supplement wheat forage.

Wheat Growth and Yield
Grazing wheat forage may shift water use to support vegetative growth at the expense of reproductive growth and grain yield. Grazing treatment effects on seed head number, mean seed weight, and the yields of grain and straw (Table 3 ) were compared for all years when grain was harvested, thus excluding 2001 (graze-out) and 2006 (crop failure). Graze-out is an infrequently used practice that eliminates both grain for harvest and straw for residue management; nevertheless, we implemented it during our study to maintain the grazing treatment. During the remaining 6 yr, the number of seed heads averaged 491 heads m–2 in grazed plots compared with 523 heads m–2 in ungrazed plots and were significantly lower with grazing in 2005. This indicates that grazing did not depress the potential for grain production except in 2005, which had generally favorable precipitation while cattle grazed and trampled the wheat. In contrast, grazing significantly (P ≤ 0.01) decreased overall mean seed weight during the study from 27.4 mg kernel–1 for ungrazed wheat to 26.0 mg kernel–1 for grazed wheat, and specifically in 2000, 2004, and 2007 during years with favorable precipitation during grain fill.


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  		Table 3. Summary of grazed (G) and ungrazed (UG) treatment effects on wheat variables, including heads m–2, mean seed weight, grain yield, and straw yield. The overall mean and treatment effects do not include 2001 (graze-out year) or 2006 (wheat failure). Significance levels of treatment effects are designated for means calculated within years and for the overall study mean.

 
Wheat grain yields averaged over the 8-yr study were not significantly different for the grazing treatments, ranging from 1.57 Mg ha–1 for the grazed plots to 1.72 Mg ha–1 without grazing (Table 3). These averages did not include the 2001 graze-out year, with grain yields of 3.05 Mg ha–1 in ungrazed plots and no grain yield in grazed-out plots or the 2006 crop failure when drought prevented establishment in both grazing treatments. Inclusion of the 2006 nontreatment related zero yields would uniformly reduce study means 15% to 1.34 and 1.47 Mg ha–1 for the grazed and ungrazed plots (respectively). Wheat grain yields for the first four reported years were not significantly different. The possible effects of soil compaction due to trampling may have depressed the yields significantly in 2005 and to a lesser degree in 2007; however, the overall mean grain yields of wheat were unaffected by grazing. Our results are consistent with experiments from the northern Great Plains and Southern Piedmont that showed no significant reduction in grain yields of winter cereals due to grazing (Tanaka et al., 2005; Franzluebbers and Stuedemann, 2007). Straw yields, shown in Table 3, generally decreased with grazing and differed significantly (P < 0.01) in 2000, 2004, and 2005. The overall straw yield decreased from 5.85 Mg ha–1 for the ungrazed plots to 4.77 Mg ha–1 for grazed plots. Not shown was the 5.90 Mg ha–1 reduction in straw yield from grazed plots during 2001. This reduction in straw yield due to graze-out could reduce capture of precipitation during fallow for use by the subsequent sorghum crop.

Wheat recovery after grazing depends on rapid spring growth promoted by favorable soil water. The 8-yr study mean soil water content at wheat planting averaged similar 156 and 159 mm for the grazed and ungrazed plots, but were significantly different in 2007 (Table 4 ) when the grazed plots averaged 116 mm compared with 144 mm for ungrazed plots. That is, during most years, precipitation stored as soil water during the preceding fallow after grain sorghum was unaffected by grazing, in part, because sorghum stalks provide less soil cover to reduce evaporation compared with wheat straw. Except after the 2001 graze-out year, no significant difference in soil water at harvest was observed due to grazing. The increased water use by ungrazed wheat to produce grain in 2001 reduced soil water at harvest approximately 73 mm and decreased soil water when planting the subsequent 2002 sorghum crop.


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  		Table 4. Annual summary of grazed (G) and ungrazed (UG) treatment effects on plant available soil profile water content and precipitation storage to 1.8 m during fallow after wheat and sorghum. Significance levels of treatment effects are designated for means calculated within years and for the overall study mean.

 
Sorghum Growth and Yield
We compared grazing effects on grain sorghum growth indicated by the maximum LAI, tiller and head number m–2, mean seed weight, and grain yield for the years 2000 to 2006 (Table 5 ). Sorghum grown in 2000 followed wheat that had no previous grazing treatments; therefore, no wheat grazing effects were expected or observed. The first instance of residual effects of grazing wheat on sorghum growth appeared during the 2001 growing season, which also featured the lowest measured rain (128 mm) during the summer growing season. Rapid sorghum growth early in 2001 produced more tillers per meter and achieved a higher LAI in grazed plots probably because of favorable soil water at planting (Table 4). The greater storage of soil water during fallow after wheat grazed in 2000 may have reflected greater infiltration of spring rain by soils where compacted surface aggregates resisted consolidation and crust formation. Storage of soil water during fallow after wheat harvested in 2001 was <34 mm for ungrazed plots and resulted in soil water at sorghum planting (2002) of 128 and 82 mm for grazed and ungrazed plots, respectively. The limited available soil water and rain during June 2002 led to a sparse sorghum stand, which biased hand samples and rendered them unsuitable for LAI, mean seed weight, and tiller and head number m–2 (data not shown). Beginning with 2003, sorghum LAI was typically greater in previously ungrazed wheat plots (Table 5). Likewise, tiller number m–2 was generally greater in ungrazed wheat plots beginning in 2003, although not significant except in 2005. We attributed the greater LAI and, generally, greater tillering in ungrazed plots to improved seedling establishment and possibly greater initial rooting through untrampled soil. Sorghum seedling establishment and early growth was independent of the soil water at planting, which did not vary significantly with grazing treatment (Table 4).


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  		Table 5. Summary of grazed (G) and ungrazed (UG) treatment effects on sorghum growth and yield factors including: maximum leaf area index (LAI), tiller number, heads m–2, mean seed weight, and grain and stover yields. Significance levels of treatment effects are designated for means calculated within years and for the overall study mean.

 
Grain yield and yield components reflect growing season conditions. For example, seed head numbers and, consequently, number of seed can indicate plant stress during the early growing season while seed weight will vary with late season stress (Krieg and Lascano, 1990). During our 8-yr experiment, the number of sorghum heads per m2 did not differ with grazing of the previous wheat crop (Table 5) that had no affect on early sorghum growth. No seed heads were exerted in 2003 for either grazing treatment, though we observed >19 tillers m–2 for both treatments. We suggest that favorable June rain (116 mm) supplemented by stored soil water encouraged tillering and, consequently, increased the LAI significantly in the ungrazed plots. The subsequent limited rains during July, August, and September of 7, 41, and 28 mm (respectively) were insufficient to meet the crop requirements for producing grain in 2003. Bandaru et al. (2006) also observed that favorable conditions early in the growing season promoted tillering and increased LAI often at the expense of grain yields. Mean seed weight did not vary significantly with grazing treatment during any year except for 2006, the last sorghum year of this study, and averaged 15.8 mg. No significant differences in seed weight of sorghum indicated that grazing wheat had no residual effect on sorghum grown before 2006.

Grain yields for both grazed and ungrazed treatments (Table 5.) ranged from no measurable yield in 2003 to approximately 4.2 Mg ha–1 in 2005 and reflect the combined effects of soil water at planting and precipitation during the growing season. Grain yield did not vary significantly with grazing treatment in 7 of 8 yr, averaging similar 2.20 Mg ha–1 with grazing and 2.26 Mg ha–1 in ungrazed plots. Similarly, Franzluebbers and Stuedemann (2007) reported no yield reduction from several years of conventionally tilled grain sorghum grown after grazed winter cover crops in Georgia. Grazing depressed the grain yield of sorghum in 2002 following graze-out wheat in 2001. This exception was because residue that increases soil water storage during fallow was removed and resulted in 7.1 and 33.4 mm of precipitation storage as soil water (Table 4) for grazed and ungrazed plots (respectively). This combined with 26 mm June rain was insufficient for crop establishment and resulted in a sparse stand and significantly lower yields. Nevertheless, the overall mean storage of precipitation as soil water during fallow after wheat or sorghum for the study did not vary with grazing treatment (Table 4). Also, we observed no cumulative grazing effect to decrease potential soil water storage by removing as forage the residue needed for increased rain infiltration and decreased evaporation.


   SUMMARY AND CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
REFERENCES
 
We evaluated the potential gain of grazing cattle on dryland wheat forage and sorghum stover produced in a WSF rotation. Additionally, grazing effects on soil water storage during fallow, and the growth and yield of grazed wheat and subsequent grain sorghum were quantified. Under the conditions of our dryland WSF rotation, sufficient wheat forage was produced in 7 of 8 yr to permit grazing an average of 31 d with a mean total gain of 123 kg ha–1, which had an estimated gross value of $95 ha–1. Excluding the atypical 2001 graze-out of wheat, our grazing treatments did not significantly decrease mean yield of wheat grain that averaged 1.72 Mg ha–1 for ungrazed plots compared with 1.57 Mg ha–1 for grazed treatments. Similarly, grazing the wheat forage did not significantly decrease grain yield of the subsequent sorghum crop, which averaged 2.26 Mg ha–1 for ungrazed plots compared with 2.20 Mg ha–1 when grazed. Compared with grazed plots, the greater tiller number and higher LAI for sorghum in ungrazed plots may have resulted in earlier soil water depletion before seedset, thus decreasing yield to levels consistent with those generated by grazed plots. Generally, yields of wheat straw decreased with grazing, but subsequent water storage during summer fallow was unaffected except for 2001 (wheat graze-out). The limited storage of soil water during 2001 resulted in poor stand establishment and near failure of the subsequent sorghum crop.

We conclude that limited grazing of dryland wheat successfully increased overall productivity of the WSF cropping system. Wheat and sorghum grain yields were maintained while adding the value of cattle gain generated by grazing. Success of this system depends on timely termination of grazing to prevent reduced grain yield by wheat and to retain adequate residues to promote storage of precipitation as soil water during fallow of the WSF rotation.


   ACKNOWLEDGMENTS
 
The technical assistance of Mr. Grant Johnson for plot management and Mr. Rex VanMeter and Mr. Gary Graham for animal management during all phases of this experiment are gratefully acknowledged.


   NOTES
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 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
1 The mention of trade names of commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. Back

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





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