ELSEVIER Soil & Tillage Research 31 (1994) 11-22

Soil &

Ridge tillage for continuous grain sorghum production with limited irrigation

Paul W. Unger United States Department of Agriculture (USDA), Agricultural Research Service, Conservation and

Production Research Laboratory, P.O. Drawer 10, Bushland, TX 79012, USA

(Accepted 28 September 1993)

Abstract

Improved soil and water conservation are basic goals for crop production systems in the semi-arid southern Great Plains (SGP) of the USA and similar regions, and managing crop residues on the soil surface plays a key role in attaining these goals. Residue produc- tion by dryland grain sorghum (Sorghum bicolor (L.) Moench) is often too low in semi- arid regions to improve soil and water conservation, but much greater amounts are pro- duced by irrigated sorghum. However, irrigation water may be limited, as in the SGP, and a limited-irrigation approach is sometimes used for sorghum, which responds well to ti- mely irrigations. This study, conducted on Pullman clay loam (fine, mixed, thermic Tor- rertic Paleustoll) from 1986 to 1992, determined effects of residue management treat- ments involving ridge tillage on continuous production of two grain sorghum hybrids with limited irrigation. Treatments were conventional (ConvT) tillage, which involved stalk shredding, disking, and rebuilding of the ridges soon after sorghum harvest, and two ridge tillage methods (residues shredded early (at harvest) - - ConsTE; residues shredded late (at planting) - - ConsTL). Soil water storage between crops was greatest with the ConsTL treatment and least with the ConvT treatment. Soil water use by sorghum was greater (66 mm) with ConsTL than with ConvT (51 mm). Mean initial plant populations and pani- cles harvested were greater with ConvT than with other treatments, but grain yields were less with ConvT (4.21 Mg ha - l ) than with ConsTE (4.42 Mg ha - l ) and ConsTL (4.52 Mg ha-~ ). Mean stover yields and seed weight (mg per seed) were not affected by tillage treatments. Although differences were relatively small, mean grain and stover yields were greater for the 'Pioneer' cultivar (4.53 and 5.68 Mg ha - l ) than for the 'Funk' cultivar (4.24 and 5.25 Mg ha - l ) . Mean water use efficiency differences among treatments and between hybrids were small. Ridge tillage was better than conventional tillage for contin- uous grain sorghum production with limited irrigation in the semi-arid SGP. Ridge tillage that retained residues on the surface until planting increased soil water storage due to re- duced evaporation and increased snow trapping, which resulted in the greater yields than where residues were incorporated with soil soon after crop harvest.

SSDI0167-1987(93)00371-7

12 P. W. Unger / Soil & Tillage Research 31 (I 994) 11-22

Keywords: Irrigation; Ridge tillage; Water conservation; Residue management; Sorghum bicolor

1. Introduction

Improved soil erosion control and water conservation are basic goals for crop production systems in the semi-arid southern Great Plains (SGP) of the USA and other semi-arid regions. These goals can be achieved when crops produce adequate residues and the residues are retained on the soil surface through use of appropriate tillage practices. Whereas residue production by dryland grain sorghum is often low when grown continuously (annually) in semi-arid regions, production is much greater when it is irrigated. However, water for irrigation may be limited, as in the SGP. Hence, because grain sorghum responds well to timely irrigations (Musick and Dusek, 1969, 1971 ), limited irrigation is some- times practiced. Sorghum-fallow and wheat-sorghum-fallow rotation systems that increase water storage during fallow are common on dryland in the SGP, but use of fallow reduces cropping intensity and, therefore, may not be desirable. Be- cause many producers require production from their land each year, the limited- irrigation approach affords an opportunity for obtaining a crop each year without the use of full irrigation, and with minimal danger of crop failure.

A problem with continuous sorghum production by conservation tillage meth- ods, especially with no-tillage, is the control of volunteer sorghum. The problem arises when seed from the previous year's crop germinates at about the same time as the planted seed germinates. As a result, plant populations may be excessive (Allen et al., 1975). The volunteer sorghum problem can largely be avoided by using conventional intensive (clean) tillage, but such a system does not provide the soil and water conservation benefits associated with surface residues.

The volunteer sorghum problem is reduced when an appropriate herbicide is applied for weed and volunteer sorghum control, and 'safened' sorghum seed is planted (Jones et al., 1988 ). 'Safened' seed is seed that is treated with a chemical so that the seedlings are not killed by herbicides that have been applied for con- trolling weeds or volunteer sorghum. If no-tillage is used, most residues are re- tained on the surface to enhance soil and water conservation. Residues can also be retained on the surface for most of the crop production cycle by using a ridge tillage system. Ridge tillage is a crop production system intermediate in intensity between no-tillage and conventional tillage (Regehr, 1985 ). With ridge tillage, residues and seed from the previous crop are moved from the seed zone at the time of planting, thus reducing the potential for excessive plant populations due to volunteer sorghum. One or two tillage operations are used to rebuild the ridges and control volunteer sorghum in the established crop. An additional benefit of using ridge tillage is the reduced herbicide requirement because only a band ap- plication may be needed to control weeds on the planted row with interrow weeds being controlled by cultivation.

The objective of this study was to determine the effects of different residue

P. W. Unger /Soil & Tillage Research 31 (I 994) 11-22 13

management practices on continuous grain sorghum production using ridge til- lage under limited-irrigation conditions in the southern Great Plains. Factors studied were soil water storage and use, plant population and panicle numbers, seed weight, and grain and stover yields.

2. Materials and methods

The study was conducted from 1986 to 1992 at the USDA-ARS Conservation and Production Research Laboratory, Bushland, Texas. The soil of the study area was Pullman clay loam, and plots were leveled and individually surrounded by berms to prevent water runon and runoff. All plots were uniformly managed dur- ing the interval from leveling to sorghum planting. Leveling Pullman clay loam, which has 0.3% slope, has little effect on surface soil conditions (Unger et al., 1990) when small plots, as used for this study, are involved. Plots were 8 m wide and 39 m long.

A conventional and two conservation tillage treatments were used to manage the sorghum residues in this study. Conventional tillage (ConvT) involved shredding sorghum stalks, disking, and rebuilding the ridges in the fall with a disk bedder soon after sorghum harvest, and planting in the spring. Conservation til- lage involved early stalk shredding (ConsTE) (soon after harvest) in the fall or late stalk shredding (ConsTL) (shortly before planting) in the spring, planting, and rebuilding ridges after the crop was sufficiently tall in early summer. Unit planters equipped with double disk openers, preceded by barring-off disks, were used for planting on the ConsTE and ConsTL treatment plots. The barring-off disks moved stalks and root crowns from the previous crop and dry surface soil from the seeding zone, thus permitting trouble-free planting in moist, residue- free soil. Barring off-disks were not used on the ConvT treatment plots. All plant- ing was on ridge tops. A rolling cultivator was used for rebuilding the ridges.

The treatments were replicated three times in the study that had a randomized block, split plot design. Residue management treatments were assigned to the main plots and the plots were split lengthwise for two sorghum hybrid subplot treatments. The sorghums (cultivars 'Funk 1498' and 'Pioneer 8493') were me- dium-maturity hybrids. Each was planted on one half (four rows) of the eight- row-wide ( 1 m row spacing) plots.

Propazine (6-chloro-N,N'-bis(1-methylethyl)-l,3,5-triazine-2,4-diamine) at 336 mg m - 2 active ingredient (a.i.) and terbutryn (N- ( 1,1-dimethylethyl )-N'- ethyl-6-(methylthio )-1,3,5-triazine-2,4-diamine ] at 112 mg m-2 a.i. were broad- cast applied for growing season weed control. Anhydrous ammonia was chiseled into the furrows at a rate of 150 kg N ha-~ before sorghum planting. Planting dates ranged from 25 May to 12 June. Planting rates were set to obtain a popula- tion of about 96 000 plants ha-1. Depth bands on planters were set for a 38 mm seeding depth. Sorghum was harvested between 15 October and 15 November.

Precipitation was measured with a standard (200 mm diameter) gauge situ- ated adjacent to the plot area. Irrigation water, which was applied through gated

14 P. W. Unger / Soil & Tillage Research 31 (1994.) 11-22

pipes at one end of the plots, flooded the entire plots. The amount applied was determined with in-line flow meters. Irrigations were made, if needed, for sorghum establishment and at the late-booting and grain-filling stages if no major rains occurred immediately before or at those stages.

Soil water contents were determined gravimetrically from samples obtained at two sites per subplot by 0.30-m increments to the 1.8 m depth at planting and harvest. Total water use was based on growing season precipitation, irrigation, and soil water use (net planting time minus harvest time water contents to the 1.8 m depth). Runoff from plots was prevented by the berms. Total water use included soil water evaporation and plant transpiration, but these were not mea- sured independently.

Sorghum grain and stover yield samples were obtained by hand from plants on two 3-m-long row sections at two locations in each subplot. Grain-bearing pani- cles were clipped at their base, oven dried at 50°C, and threshed, and the grain was weighed. Sorghum stover was clipped at the soil surface, and subsamples were oven dried at 50°C for determining water contents for use in calculating stover yields. Panicle dry matter other than grain was included in the stover weights. Grain and stover yields are reported on an oven-dry weight basis. Other determinations made were plant populations (no. ha-m ), panicles harvested (no. ha-a), and weight per seed (mg, based on 1000 seeds).

Data were analyzed by the analysis of variance technique (Statistical Analysis Systems, 1985 ). When the F-test indicated statistically significant differences at the 0.05 probability level, means were separated by the protected least significant difference (Prot. LSD) method (Steel and Torrie, 1980).

3. Results and discussion

3. I. Precipitation and irrigation

Total precipitation amounts during the growing season and between-crop pe- riods are given in Table 1. Also given are long-term precipitation averages for the corresponding periods. Growing season precipitation was highly variable, rang- ing from 183 mm in 1991 to 364 mm in 1989. The 7-year mean growing season precipitation was similar to the long-term mean. Mean precipitation during the between-crop period was 14 mm greater than the long-term average for the cor- responding periods. It was highly variable among periods, ranging from 126 mm for the 1989-1990 period to 312 mm for the 1986-1987 period.

Precipitation during the growing seasons generally was timely and adequate for growth and development. As a result, irrigation was applied only to the 1986 and 1990 crops. In 1986, soil water contents at planting were low because of low pre- cipitation from the time of plot leveling until sorghum planting. Hence, a 102- mm irrigation was applied for crop establishment and early growth. Soil water contents at planting were also low in 1990 because of low between-crop period precipitation (Table 1 ), and a 5 l-mm irrigation was applied for seed germina-

P. IV.. Unger / Soil & Tillage Research 31 (1994) 11-22 15

Table 1 Precipitation during growing seasons and between crops of grain sorghum, 1986-1992, Bushland, Texas

Period Period dates Precipitation (mm)

Starting Ending Period Long-term a

Growing season

Between crop

17-06-1986 16-10-1986 226 269 11-06-1987 13-10-1987 184 255 20-06-1988 14-10-1988 241 230 08-06-1989 23-10-1989 364 271 19-06-1990 23-10-1990 209 266 24-06-1991 29-10-1991 183 236 11-06-1992 21-10-1992 334 236

Average 249 252

16-10-1986 11-06-1987 312 217 13-10-1987 20-06-1988 285 245 14-10-1988 08-06-1989 191 207 23-10-1989 19-06-1990 126 230 23-10-1990 24-06-1991 207 242 29-10-1991 11-06-1992 309 200

Average 238 224

aAverage from 1939 to 1991.

tion and seedling establishment. The 1990 crop was also irrigated (156 mm to- tal) at booting and grain filling because of limited precipitation.

3.2. Soil water factors

Statistical analyses of the soil water data indicated significant differences among periods (or years) and among tillage treatments in all cases (Table 2). Differ- ences between hybrids were significant for content at the start of the between- crop period and for content at planting. The tillage-period (or year) interaction was significant for the change during the between-crop period and for content at planting. Although the significant tillage-period (or year) interaction effect sug- gested that tillage-induced soil conditions affecting soil water factors may be changing with time, examination of the yearly data indicated no trends. Hence, only mean data are presented for the soil water factors in Table 3.

Soil water storage between crops Mean content of plant available water (PAW) at the start of between-crop pe-

riods (at sorghum harvest) was least with ConvT, intermediate with ConsTE, and greatest with ConsTL (Table 3). These differences suggest that PAW con- tents were greater with ConsTE and ConsTL than with ConvT during the late part of the previous growing season, and possibly contributed to the greater yields

16 P. W. Unger ~Soil& Tillage Research 31 (1994) 11-22

Table 2 Analysis of variance results showing F-ratios and significance levels for the effects of tillage treatments on soil water factors related to grain sorghum production with limited irrigation, 1986-1992, Bush- land, Texas

Source of variation Content at start Change during Content at Growing session of between-crop between-crop planting change period period

d.£ F-ratio d.£ F-ratio d.£ F-ratio d.£ F-ratio

Period (P) orYear (Y) 6 91.86"* 5 60.28*** 6 234.95*** 6 407.51"** Hybrid (H) 1 8.84" 1 NS 1 15.00"* l NS H × P (or Y) 6 NS" 5 NS 6 NS 6 NS Tillage (T) 2 10.38"* 2 5.33" 2 17.28"* 2 3.67* T × P ( o r Y ) 12 NS 10 2.58* 12 2.62" 12 NS T X H 2 NS 2 NS 2 NS 2 NS TXHXP (orY) 12 NS l0 NS 12 NS 12 NS

"Not significant. * " and ***significant at the 0.05, 0.01, and 0.001 probability levels, respectively.

with the ConsTE and ConsTL treatments (Table 4). The mean PAW content at harvest was 10 mm greater for the 'Funk' than for the 'Pioneer' hybrid, suggesting that PAW contents were greater late in the growing season for the 'Funk' hybrid. However, in this case, this difference did not result in a significant grain yield difference.

The change in (storage of) PAW during the between-crop period was greater for the ConsTL treatment than for other treatments (Table 3 ). The greater stor- age with the ConsTL treatment is attributed to the standing stubble that resulted in less soil water evaporation. Snow trapping by standing stubble may also have been involved. In contrast, shredding the stubble early (ConsTE treatment) re- suited in the residues being concentrated in the furrows, thus having little benefit with regards to reducing evaporation because most of the surface was bare. The ConvT treatment with its bare surface soil also provided no evaporation reduc- tion benefits. The effect of cultivars on water storage was not significant, even though the 'Pioneer' hybrid produced more stover than the "Funk' hybrid (Table 4).

Water storage differed among between-crop periods, ranging from 20 mm for the 1989-1990 period to 120 mm for the 1987-1988 period. Precipitation for the 1989-1990 period was about 55% of the long-term average. The treatment by between-crop period interaction effect was also significant. In general, storage differences among treatments tended to be greatest during periods with low pre- cipitation, and vice versa.

Overall, only 20% of the precipitation that occurred during the between-crop periods was stored in the root zone as PAW. It ranged from 17% for the ConvT treatment to 22% for the ConsTL treatment. Major factors contributing to the

P.W. Unger /Soil & Tillage Research 31 (1994) 11-22 17

Table 3 Soil water contents and changes for between-crop and growing season periods for grain sorghum, 1986-1992, Bushland, Texas

Hybrid Tillage treatment a

ConvT ConsTE ConsTL Mean

Contentatstartofbetween-cropperiod(mm) 'Funk' 117 122 137 'Pioneer' 107 117 122 Mean 112 120 130 Prot. LSDb(0.051evel):Tillage--8, Hybrid--7, Tillage×hybrid--NS c

Change duringbetween-cropperiod(mm) 'Funk' 70 64 87 'Pioneer' 60 77 80 Mean 65 71 84 Prot. LSD (0.051evel):Tillage--12, Hybrid--NS, Tillagexhybrid--NS

125 115

74 73

Content at planting (ram) 'Funk' 172 179 202 184 'Pioneer' 155 183 188 175 Mean 164 181 195 Prot. LSD (0.05 level): Til lage- 11, Hybr id - 5, Tillage × hybr id- NS

Growingseasonchange(mm) 'Funk' -55 -57 -65 -59 'Pioneer' -48 -65 -66 -60 Mean -52 -61 -66 Prot. LSD (0.051evel):Treatment--ll, Hybrid--NS, Tillage×hybrid--NS

aTillage treatments were: ConvT, conventional tillage; ConsTE, conservation tillage, residues shred- ded early; ConsTL, conservation tillage, residues shredded late. bProtected least significant difference. cNS, not significant.

low precipi ta t ion storage efficiency are the generally small precipi ta t ion events and high evapora t ion rates that prevai l in the SGP.

Plant available soil water content and use Mean PAW content at plant ing was greatest with ConsTL and least with C o n v T

(Table 3 ). Grea te r storage with the C o n s T L t r ea tmen t is a t t r ibuted to less evap- orat ion and possibly more snow t rapping due to the s tanding stubble as c o m p a r e d with that for the o ther t rea tments . Storage was in te rmedia te with the ConsTE t rea tment . Mean PAW content at plant ing was greater for the ' Funk ' than for the 'P ioneer ' hybrid, with the difference being s imilar to the m e a n difference at har- vest. The m e a n contents at plant ing differ f rom the sums o f contents at the start o f be tween-crop per iods and changes during those per iods because seven crops were grown, but only six be tween-crop per iods occurred. Differences in PAW contents at harves t and in be tween-crop per iod precip i ta t ion resulted in differ-

18 P.W.. Unger / Soil & Tillage Research 31 (1994) 11-22

Table 4 Plant populations, panicles harvested, seed weights, and grain and stover yields for grain sorghum, 1986-1992, Bushland, Texas

Hybrid Tillage treatment a

ConvT ConstTE ConsTL Mean

Plantpopu~tion (1000sha -9 'Funk' 94 82 82 'Pioneer' 62 57 61 Mean 78 70 72 Prot. LSDb(0.051evel) :Ti l lage--5, Hybr id - -3 , T i l l age×hybr id - -NS c

86 60

Panicles harvested (1000 s ha - 1) 'Funk' 149 134 134 'Pioneer' 147 136 141 Mean 148 135 138 Prot. LSD (0.05 level): T i l l a g e - 5, H y b r i d - NS, T i l l a g e × h y b r i d - NS

139 141

Seed weight (rag per seed) 'Funk' 21.7 22.3 22.4 22.1 'Pioneer' 27.9 28. I 27.9 28.0 Mean 24.8 25.2 25.1 Prot. LSD (0.05 level ) : T i l l a g e - NS, H y b r i d - 0.5, Tillage × h y b r i d - NS

Grain yield (Mg ha - 9 'Funk' 3.89 4.39 4.43 4.24 'Pioneer' 4.54 4.44 4.62 4.53 Mean 4.21 4.42 4.52 Prot. LSD (0.05 level): Tillage - - 0.19, Hybrid - - 0.09, Tillage × hybrid - - 0.49

Stover yield (Mg ha - 1) 'Funk' 5.11 5.44 5.21 5.25 'Pioneer' 5.65 5.64 5.74 5.68 Mean 5.38 5.54 5.48 Prot. LSD (0.05 level): Tillage - - NS, Hybrid - - 0.10, Tillage × hybrid - - NS

Water-use efficiency d (growing season water use) (kg m -3) 'Funk' 1.12 1.25 1.24 1.20 'Pioneer' 1.33 1.24 1.29 1.29 Mean 1.23 1.25 1.27

Water-use efficiency (total water use) (kg m -3) 'Funk' 0.66 0.75 0.74 0.72 'Pioneer' 0.78 0.74 0.77 0.76 Mean 0.72 0.75 0.76

aTillage treatments were: ConvT, conventional tillage; ConsTE, conservation tillage, residues shred- ded early; ConsTL, conservation tillage, residues shredded late. bprotected least significant difference. q~IS, not significant. dWater-use efficiency data were not analyzed statistically.

P. W. Unger /Soil & Tillage Research 31 (1994) 11-22 19

ences in PAW contents among years at sorghum planting. The mean content was greatest (228 mm) in 1992 and least (56 mm) in 1986. In 1990, it was 118 mm, which was about 51% of the storage capacity of 230 mm for the soil. Low water storage during between-crop periods frequently results in the Pullman soil profile not being filled with water at planting time of dryland crops (Unger, 1972; John- son and Davis, 1972; Jones, 1975).

Mean use (growing season change) of PAW was greatest with the ConsTL treatment and least with the ConvT treatment (Table 3 ). The greater use with the ConsTL treatment contributed to the greater yield with that treatment (Table 4). Results were intermediate with the ConsTE treatment. Use was similar for the two hybrids, but it varied among years. Because of low contents at planting, mean PAW increased during the growing season in 1986 and 1990 (data not shown).

3.3. Crop factors

All crop factors were significantly different due to years, and significantly dif- ferent due to hybrids and tillage treatments in some cases (Table 5 ). Some inter- action effects were also significant. However, examination of the yearly data re- vealed no trends with time. Hence, only mean data for crop factors are given in Table 4.

Plant populations and panicles harvested Even though seeding rates were the same, initial plant populations were greater

with the ConvT than with other treatments for which differences were not signif- icant. Populations also were greater for the 'Funk' than for the 'Pioneer' hybrid (Table 4) because of differences in seed size.

The panicle number per hectare at harvest was greatest with the ConvT treat- ment, which paralleled the trend for initial plant populations. However, panicles

Table 5 Analysis of variance results showing F-ratios and significance levels for the effects of tillage treatments on crop factors related to grain sorghum production with limited irrigation, 1986-1992, Bushland, Texas

Source of d.f. Plant Panicles Seed weight Grain yield Stover variation population harvested yield

Year (Y) 6 97.91"* 202.89*** 147.72"** 125.34"** 34.86*** Hybrid (H) 1 336.74*** NS a 668.73*** 45.96*** 39.15*** H × Y 6 95.69*** 6.49** 9.68*** 45.80*** NS Tillage (T) 2 6.61"* 13.41"** NS 5.73** NS T × Y 12 3.10"* 2.17* 5.49*-- 3.02** 3.99*** T X H 2 NS NS NS 5.61"* NS T × H X Y 12 NS NS NS NS NS

aNot significant. *'** and ***significant at the 0.05, 0.01, and 0.001 probability levels, respectively.

20 P. W. Unger ~Soil & Tillage Research 31 (1994) 11-22

at harvest were similar for the two hybrids (Table 4). Greater numbers of pani- cles than initial plant populations are common for sorghum planted at relatively low rates because sorghum often tillers extensively and tends to overcome low and erratic initial plant populations (Krieg and Lascano, 1990).

Seed weight, and grain and stover yields Tillage treatments did not affect mean seed weights, but mean weights were

greater for the 'Pioneer' than for the 'Funk' hybrid (Table 4). Differences among years were also significant, ranging from 18.5 mg per seed in 1991 to 29.4 mg per seed in 1988. Examination of rainfall data suggested that the seed weights were related to rainfall amounts during the grain-filling period. In 1988, rainfall to- taled 110 mm for the last 35 days before physiological maturity, when grain fill- ing occurs (Krieg and Lascano, 1990); in 1991, the total was only 6 mm. A late- season irrigation would have improved grain filling in 1991. Unfortunately, such irrigation was not applied.

Mean grain yields were greater with the ConsTE and ConsTL treatments than with the ConvT treatment (Table 4). However, the tillage-hybrid interaction indicated that the differences resulted primarily from the 'Funk' hybrid, which yielded at least 0.50 Mg ha- ~ less with the ConvT treatment than with the other treatments. Treatment effects on the 'Pioneer' hybrid were not significant. Mean yield for the 'Pioneer' hybrid was greater than for the 'Funk' hybrid, but the dif- ference was only 0.29 Mg ha- ~. Certainly, both hybrids performed well with lim- ited irrigation and ridge tillage in this study, especially with the ConsTE and ConsTL treatments for which differences between hybrids were not significant.

Mean grain yields ranged from 2.19 Mg ha- ~ in 1986 to 5.96 Mg ha- ~ in 1992, almost a three-fold difference. The greater yields in 1992 resulted from beneficial rainfall amounts (Table 1 ) and distribution throughout the growing season. In 1986, although rains occurred rather frequently, the growing season total was 108 mm less than in 1992. In addition, most rains provided less than 17 mm of water, which limited the effectiveness of the rain for crop use under the growing condi- tions of the semi-arid SGP.

The grain yield trends followed the seed weight trends, with mean grain yields being greater for the 'Pioneer' hybrid that also had the greater mean seed weight. Also, the mean grain yields were greater for the ConsTE and ConsTL treatments, for which the mean seed weights tended to be greater. However, mean grain yields and seed weights for individual crops were not closely related (data not shown).

Stover yields were not affected by tillage treatments, but mean yields were 0.43 Mg ha-~ greater for the 'Pioneer' than for the 'Funk' hybrid (Table 4). The til- lage-hybrid interaction was not significant. Stover yields were similarly low in 1986 and 1991 when grain yield were low and greatest (iderttical) in 1989 and 1992 when grain yields ranked third and first, respectively. Hence, for individual crops, grain and stover yields were not closely related (data not shown). While good plant growth is essential for grain production, timing of rain and irrigation can greatly influence stover and grain production, especially under limited-irri- gation conditions (Musick and Dusek, 1971 ), as was the case for this study.

P. W. Unger ~Soil & Tillage Research 31 (1994) 11-22 21

Both stover and grain yields were greater for the 'Pioneer' hybrid, which re- sulted in greater total dry matter production (10.21 Mg ha -~ ) than the 'Funk' hybrid (9.49 Mg ha- t ) . However, harvest indexes (portions of above-ground dry matter that are grain) were similar, being 0.447 for the 'Funk' and 0.444 for the 'Pioneer' hybrid. This indicates that both hybrids were equally effective in converting the photosynthate produced into harvestable grain.

Water-use efficiency Mean water-use efficiency (WUE) (Table 4) was calculated on the basis of

grain yield both per unit of growing season water use (mean growing season pre- cipitation, soil PAW use, and irrigation) and per unit of total water use (mean between-crop and growing season precipitation, net change in PAW during the study, and irrigation). Mean WUE, based on growing season water use, differed only 0.04 kg m -3 among treatments and 0.09 kg m -3 between hybrids. The dif- ference between hybrids was greatest (0.23 kg m-3) with the ConvT treatment and 0.05 kg m - 3 o r less with the other treatments.

The WUEs for this study (Table 4) were greater than those for a dryland study in the SGP when grain sorghum was grown where a straw mulch was on the soil surface (Unger, 1978 ). For the best treatment (mulch at 12 Mg ha-1 ), the mean WUE was 1.15 kg m - 3 . The greater WUE in this study is attributed to, at least in part, the use of plots surrounded by berms. In the dryland study, runoff was pos- sible, but was not measured. Mean WUE was 1.48 kg m - 3 with the best treat- ments for irrigated grain sorghum in the SGP (Musick and Dusek, 1971 ). For limited-irrigation treatments, the WUEs were similar to those for this study.

When based on total water use, mean WUE differed 0.04 kg m-3 among treat- ments and between hybrids. The difference between hybrids again was greatest (0.12 kg m - 3 ) with the ConvT treatment and 0.03 kg m - 3 o r less for the other treatments. Overall, WUE was greater for the 'Pioneer' than for the 'Funk' hy- brid, mainly because of the good performance of'Pioneer' with the ConvT treat- ment. In this and in a previous study (Unger, 1991 ), differences in WUE be- tween the hybrids were slight under reduced (ridge) or no-tillage conditions. Thus, either hybrid seems well adapted to reduced (ridge) or no-tillage, but the 'Pi- oneer' hybrid seems better adapted than the 'Funk' hybrid to conventional tillage conditions.

4. Conclusions

( 1 ) Ridge tillage was better than conventional tillage for grain sorghum pro- duction under limited irrigation conditions in the semi-arid southern Great Plains because residues retained on the surface with ridge tillage during the between- crop periods reduced evaporation and possibly increased snow trapping.

(2) Although yield differences between hybrids were significant, the differ- ences were relatively small and any adapted hybrid should perform well under similar conditions.

22 P.W. Unger / Soil & Tillage Research 31 (1994.) 11-22

Acknowledgment

The assistance of L.J. Fulton, Biological Technician, USDA-Agricultural Re- search Service, Bushland, TX, in conducting the study and analyzing the data is gratefully acknowledged.

References

Allen, R.R., Musick, J.T. and Wiese, A.F., 1975. No-till management of furrow irrigated continuous grain sorghum. Tex. Agric. Exp. Stn. PR-3332 C, College Station, TX, pp. 68-78.

Johnson, W.C. and Davis, R.G., 1972. Research on stubble-mulch farming of winter wheat. USDA- ARS Conserv. Rpt. 16, US Govt. Printing Office, Washington, DC, 43 pp.

Jones, O.R., 1975. Yields and water-use efficiencies of dryland winter wheat and grain sorghum pro- duction systems in the Southern High Plains. Soil Sci. Soc. Am. Proc., 39: 98-103.

Jones, O.R., Wiese, A.F. and Johnson, G.L., 1988. Tillage and crop sequence effects on dryland crop production. In: P.W. Unger, T.V. Sneed, W.R. Jordan and R. Jensen (Editors), Challenges in Dryland Agriculture - - A Global Perspective. Proc. Int. Conf. on Dryland Farming, Amarillo/ Bushland, TX, 15-19 August 1988, Tex. Agric. Exp. Stn., College Station, TX, pp. 826-829.

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