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Production Papers

Intercropping Berseem Clover with Barley and Oat Cultivars for Forage

^a Dep. of Renewable Resour., 751 GSB, Univ. of Alberta, Edmonton, AB, Canada T6G 2H1
^b Dep. of Agric., Food and Nutritional Sci., 4-10 Agric.-Forestry Cent., Univ. of Alberta, Edmonton, AB, Canada T6G 2P5
^c Northern Agric. Res. Cent., Agric. and Agri-Food Canada, Beaverlodge, AB, Canada T0H 0C0

^* Corresponding author (shirley.ross{at}ualberta.ca)

Received for publication January 6, 2004.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Intercrops of berseem clover (Trifolium alexandrinum L.) and silage cereals offer potential for high quality forage and partitioning of yield between silage harvest and fall grazing. Forage yield and quality of cereal–berseem clover intercrops may differ among oat (Avena sativa L.) and barley (Hordeum vulgare L.) cultivars and with semidwarf or early maturing cereal cultivars. Berseem clover was intercropped with five oat and four barley cultivars at Edmonton, Alberta, in 2000 and 2001 on orthic black chernozem (Typic Cryoboroll) soil. Forage yield, composition, and quality were measured with a two-cut harvest. Silage-stage yield (Cut 1) averaged 9.9 Mg ha^–1 of dry matter (DM) with 18% berseem clover by dry weight. Berseem clover regrowth (Cut 2) averaged 2.8 Mg ha^–1 DM with crude protein (CP) of 215 g kg^–1. Intercrops with oat cultivars had greater Cut 1 DM yield, and intercrops with barley had greater yields of Cut 2 DM and total CP. The earlier maturity of barley provided for longer periods of berseem clover regrowth. Intercrops with semidwarf barley had equal yields of total DM and CP and greater Cut 2 DM yield than those with conventional-stature barley. Intercrops with early maturing oat cultivars had equal total DM yield and greater yields of Cut 2 DM and total CP than those with late-maturing oat cultivars. Forage quality indicators suggested that intercrops with barley were superior to those with oat. To maximize fall forage and increase the legume component of silage harvest, early maturing and shorter-stature cultivars of oat and barley are recommended for cereal–berseem clover intercrops.

Abbreviations: ADF, acid detergent fiber • CP, crude protein • DM, dry matter • NDF, neutral detergent fiber • PAR, photosynthetically active radiation

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
A SUBSTANTIAL AMOUNT of barley and oat production in western Canada is used for forage. Forage production from annual crops in Alberta is typically about 0.6 million hectares (1.5 million acres), with barley and oat as the predominant crops for greenfeed and silage (AAFRD, 2003). In 2003, approximately 10% of the area seeded to annual grain, oilseed, and pulse crops in Alberta was harvested for forage.

Potential benefits of intercropping berseem clover with cereal crops include increased total DM yields, improved forage quality, reduced fertilizer needs, and increased subsequent crop yield (Reynolds et al., 1994; Ghaffarzadeh, 1997; Stout et al., 1997). Intercropping berseem clover with silage cereals offers potential to partition forage yield between silage harvest and fall regrowth (Ross et al., 2004). There is considerable interest in extending the fall grazing period in Alberta, and the potential of intercropping winter cereals with spring cereals has been explored as a means to extend forage production (Baron et al., 1992; Jedel and Salmon, 1995).

Choice of cereal species affects the performance of intercrops grown for forage. Jedel and Helm (1993) found that intercropping oat with pulse crops produced greater DM yield than intercropping barley or triticale (x Triticosecale rimpaui Wittm.) with pulse crops, but intercrops with barley or triticale gave a better combination of quality and protein content than intercrops with oat. In a study of berseem clover intercropped with one cultivar each of oat, barley, or triticale, biomass yields, species composition, and forage quality were affected by cereal species (Ross et al., 2004). Berseem clover intercrops with triticale and oat had greater Cut 1 silage-stage yields and a greater percentage of berseem clover in Cut 1 than intercrops with barley. However, intercrops with barley had greater yields of Cut 2 berseem clover regrowth and total CP.

Cultivars of oat and barley have been studied to determine the effects of companion crop growth habit on the establishment of alfalfa (Medicago sativa L.) (Brink and Marten, 1986a, 1986b; Nickel et al., 1990; Chapko et al., 1991; Simmons et al., 1995). A few studies have assessed oat or barley cultivar effects in intercrops with annual forage crops (Thompson et al., 1992; Moynihan et al., 1996; Holland and Brummer, 1999). Holland and Brummer (1999) evaluated eight cultivars of oat and seven cultivars of berseem clover in intercrops in Iowa, with harvest at oat grain maturity and subsequent forage regrowth. ‘Bigbee’ berseem clover performed better than the other clover cultivars in intercrops. Oat cultivars varied for oat traits, effects on clover stands and yield, and total intercrop biomass. It was concluded that monoculture evaluation of oat cultivars can be used to predict the traits of oat in berseem clover–oat intercrops but cannot reliably predict the effects on berseem clover growth.

Semidwarf cereal cultivars may provide advantages over conventional-stature cultivars in forage intercrops. Simmons et al. (1995) recommended semidwarf cultivars as companion crops because of less potential for lodging. Semidwarf cultivars of oat and barley were less competitive than conventional-stature cultivars as companion crops for alfalfa (Nickel et al., 1990; Simmons et al., 1995). Thompson et al. (1992) found that semidwarf barley caused less suppression of annual ryegrass (Lolium multiflorum Lam.) than did conventional-stature barley in barley–ryegrass intercrops.

Holland and Brummer (1999) suggested that oat heading date may be more strongly associated with competitiveness than oat height in oat–berseem clover intercrops. Based on relative grain yields, Juskiw et al. (2000c) concluded that barley cultivars with early maturity were less competitive than late-maturing barley cultivars in barley mixtures. However, the use of early maturing cereal cultivars may decrease initial forage yields of intercrops (Thompson et al., 1992; Juskiw et al., 2000b). Juskiw et al. (2000b) concluded that per-plant biomass yields of barley, oat, and triticale were affected by genotype, production practices, and time of harvest, with the latter having the greatest effect.

The effects of barley and oat cultivars in cereal–berseem clover intercrops grown for forage have not been studied. The objectives of this study were to (i) determine the effects of barley and oat cultivars on forage yield, berseem clover component of yield, and forage quality of cereal–berseem clover intercrops grown in a short-season growing environment and (ii) determine the effects of oat and barley cultivar stature and maturity, including semidwarf and early maturing cultivars, on the performance of cereal–berseem clover intercrops.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Intercrops of berseem clover with oat and barley were grown at Edmonton (53° 25' N, 113° 33' W), Alberta, Canada, on a Malmo silty clay loam [orthic Black Chernozem (Typic Cryoboroll)] in 2000 and 2001. Experiments followed tilled fallow, and fields were disked and harrowed before seeding. Soil pH was 6.6 to 7.0, with soil levels of 48 to 56 mg nitrate kg^–1, 14 to 37 mg P kg^–1, 200 to 270 mg K kg^–1, and 14 to 37 mg sulfate kg^–1, at a depth of 0 to 30 cm. To meet soil nutrient recommendations for legume–cereal forage, triple superphosphate 0–45–0 at approximately 28 kg ha^–1 of P₂O₅ was added in 2000. Initial soil N levels were more than adequate for cereal silage production, but no N fertilizer would have been added had soil N been inadequate because adding N would promote cereal domination of the clover.

Five oat cultivars (‘AC Juniper’, ‘Jasper’, ‘AC Mustang’, ‘Waldern’, and ‘Murphy’) and four barley cultivars (‘Kasota’, ‘AC Lacombe’, ‘Niska’, and ‘Seebe’) were seeded at 60 viable seeds m^–2 in 2000. In 2001, the cereal seeding rate was increased to 69 viable seeds m^–2 to better ensure a cereal plant density of approximately 60 plants m^–2 in plots. The decision to seed the cereals at 25% of the recommended full rate for cereal monocrops of 240 plants m^–2 was based on seeding rate experiments with cereal–berseem clover intercrops (Ross et al., 2003, 2004). Cereal cultivars are described in Table 1. Cereals were seeded with a four-row disc drill at 23-cm row spacing. Bigbee berseem clover was inoculated with the appropriate Rhizobium trifolii and seeded at 15 kg ha^–1. Berseem clover was cross-seeded at a depth of approximately 1.5 to 2 cm with a six-row disc drill at 18-cm row spacing. Plot size was 1.8 by 6 m, with eight rows of cereal in each plot. Berseem clover sole crop plots were also grown. Plots were seeded on 18 May in 2000 and on 23 May in 2001. In 2001, plots were irrigated before and after seeding to counter dry soil conditions.

View this table: [in this window] [in a new window]   Table 1. Description of oat and barley cultivars and dates for Cut 1 silage-stage harvest, with number of days after planting in parentheses, for cereal–berseem clover intercrops and berseem clover sole crops at Edmonton, AB, in 2000 and 2001.

Data were subjected to analyses of variance (ANOVA) using the General Linear Model (GLM) procedure of SAS with all effects considered fixed, except replication, which was considered random (SAS Inst., 2000). Year effect was tested against the year x block error term. The total sum of squares of each model effect was thereafter partitioned into percentages of the total sum of squares for an estimation of relative importance of each effect to the total experimental variation. Data from each year are presented separately because year x treatment interactions were significant for the majority of variables. A priori contrasts were used to compare least square means of barley with oat treatments, early maturing with late-maturing oat treatments, semidwarf with conventional-stature barley treatments, and berseem clover sole crops with intercrop treatments. A significance level of P 0.05 was used for all statistical tests. Pearson correlation coefficients were calculated on treatment means by the CORR procedure of SAS to determine the relationship between berseem clover DM yield and harvest date, cereal canopy height, cereal DM yield, or cereal tiller density (SAS Inst., 2000).

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The treatment effects of oat and barley cultivars in intercrops were significant for all variables of cereal characteristics, yields, species composition, and forage quality indicators (Table 2). The effects of year x treatment interactions represented over 10% of the total sum of squares for Cut 1 cereal DM yield, percentage of berseem clover in Cut 1, total DM yield, total CP yield, percentage of berseem clover in total yield, and forage quality indicators (CP, ADF, and NDF) of Cut 1. There were some differences in moisture availability and crop emergence between years. Rainfall for May to September was near the 30-yr average (326 mm) in 2000 (330 mm) but was lower than normal in 2001 (267 mm). Seasonal 5-mo mean temperatures for May to September were near the 30-yr average of 14.5°C in both years. In 2000, timely rainfall and early emergence of berseem clover, relative to cereal emergence, may have provided more favorable conditions for berseem clover establishment and decreased the early competitive effects of cereals. In 2001, irrigation of the plots promoted rapid emergence of the cereals, but soil crusting appeared to cause some inhibition of berseem clover emergence. Fukai and Trenbath (1993) suggested that the relative performance of crops in intercrops can be greatly affected by small changes in the growth environment.

Other intercrop studies in Alberta have reported greater forage yields for oat intercrops than barley intercrops (Berkenkamp and Meeres, 1987; Jedel and Helm, 1993; Jedel and Salmon, 1994). The 21 to 25% greater Cut 1 DM yield for oat–berseem clover intercrops than for barley–berseem clover intercrops was largely due to the cereal component (Table 3). Barley cultivars reached soft dough stage earlier than oat cultivars, with silage-stage harvest (Cut 1) of barley intercrops averaging 7 to 8 d earlier than oat intercrops (Table 1). Mean canopy height of oat cultivars at Cut 1 was 50 to 56 cm higher than that of barley cultivars (Table 4). Mean tiller weight of barley cultivars at Cut 1 was only 40 to 47% of that of oat cultivars, but mean tiller production (tillers plant^–1) of oat cultivars was 55 to 58% of that of barley cultivars.

Some studies comparing the effects of oat and barley in crop mixtures have concluded that barley caused greater suppression of a companion crop than did oat when grown with alfalfa (Brink and Marten, 1986b; Nickel et al., 1990) or with other small-grain cereals (Juskiw et al., 2000c). Our results in 2001 suggested greater suppression of berseem clover by barley than by oat cultivars. Cut 1 DM yield of berseem clover in barley intercrops was 58% less than that in oat intercrops, and the percentage of berseem clover in barley intercrops was less than in oat intercrops (Table 3). Brink and Marten (1986b) reported that alfalfa seedling DM yield (g m^–2) was less with barley than with oat cultivars. They found that barley cultivars frequently had greater leaf area than did oat cultivars and thus had greater potential for competition for light with undersown alfalfa. Simmons et al. (1995) reported that PAR available at the top of alfalfa averaged 51% of full sun with oat companion crops and 41% of full sun with barley companion crops at 40 to 48 d after crop emergence. Our light readings taken at 44 d after planting in 2001, with cereals in stages of stem elongation, indicated that more PAR was available to berseem clover in the intercrops with oat cultivars (47%) than with barley cultivars (33%) (Table 6). The greater tiller density of barley, and likely greater leaf area in early stages of growth, may explain greater shading by barley. Competition for light could greatly decrease berseem clover yield because the relative growth rates of clover plants decrease rapidly in response to shading (Kendall and Stringer, 1985).

Cereal Stature Effects
The mean canopy height of conventional-stature barley cultivars (AC Lacombe, Seebe) was 21 to 23 cm greater than semidwarf barley cultivars (Kasota, Niska) (Table 4). Cut 1 DM yields of the cereal component of intercrops were greater for conventional-stature barley cultivars than for semidwarf cultivars (Table 3). In 2001, Cut 1 intercrop yields for conventional-stature barley intercrops were greater than those of semidwarf barley intercrops, with 22% greater cereal DM yield. In 2000, conventional-stature intercrops and semidwarf intercrops had equal Cut 1 DM yields as the 28% greater cereal DM yield for conventional-stature intercrops was balanced by the 40% greater berseem clover DM yield for semidwarf intercrops.

Holland and Brummer (1999) reported that oat height at grain maturity did not correlate with total berseem clover DM yield in oat–berseem clover intercrops. Our findings were more consistent with studies indicating that shorter-stature barley or oat cultivars are less suppressive as companion crops than taller cultivars (Nickel et al., 1990; Thompson et al., 1992). In barley–berseem clover intercrops, mean percentage of berseem clover in Cut 1 was greater with semidwarf cultivars in both years, Cut 1 berseem clover DM yield was greater with semidwarf cultivars in 2000, and barley canopy height was negatively correlated with Cut 1 berseem clover DM yield (r = –0.54) in 2000 (Table 3). In oat–berseem clover intercrops, oat canopy height was negatively correlated with Cut 1 berseem clover DM yield (r = –0.60) in 2001. The shorter-stature cultivars may have caused less shading of the berseem than the taller cultivars. Simmons et al. (1995) reported that the amount of PAR available to alfalfa was greater with semidwarf barley cultivars (averaging 46% of full sun) than with conventional-stature barley cultivars (averaging 35% of full sun). Although our limited assessment of light interception in 2001 did not indicate significant contrast between semidwarf and conventional-stature barley intercrops, greater levels (P 0.06) of PAR were available at the top of berseem clover with Kasota than with Seebe barley (Table 6). Kasota barley represented a combination of semidwarf stature and early maturity, and Seebe represented a combination of conventional stature and late maturity. Juskiw et al. (2000c) found that Kasota was less competitive than Seebe or AC Lacombe in barley mixtures and suggested that differences may have been due to stature or earliness.

Cut 2 berseem clover DM yields were greater for semidwarf barley intercrops than for conventional-stature barley intercrops (Table 3). Greater Cut 2 yield for semidwarf intercrops may have related to less initial suppression of berseem clover by semidwarf cultivars. Also, earlier Cut 1 harvest of semidwarf intercrops allowed for more days of regrowth in some cases (Table 1).

Moynihan et al. (1996) concluded that biomass yields for intercrops of barley and annual medic species (Medicago spp.) were similar with semidwarf or conventional-stature barley. In our study, there were no differences in total DM or CP yields between semidwarf barley intercrops and conventional-stature barley intercrops (Table 3). Advantages in Cut 1 cereal DM yield with conventional-stature cultivars were balanced by advantages in Cut 2 berseem clover DM yield with semidwarf cultivar intercrops.

Early versus Late-Maturing Cultivars
The Cut 1 harvest was 3 to 6 d earlier for early maturing oat cultivars (AC Juniper and Jasper) than late-maturing oat cultivars (Murphy and Waldern) and was 1 to 7 d earlier for the early maturing barley cultivar (Kasota) than the late-maturing barley cultivar (Seebe) (Table 1). The canopy height at Cut 1 of late-maturing genotypes was greater than early maturing genotypes (Table 4). The tiller characteristics associated with late maturity differed between oat and barley. The late-maturing oat cultivars had fewer but heavier tillers than early maturing oat cultivars. The late-maturing barley had more tillers than the early maturing barley but did not differ in tiller weight.

The cereal DM yield of Cut 1 was greater for late-maturing oat intercrops in both years and was greater for the late-maturing barley in 2001 (Table 3). Greater Cut 1 cereal yield would be expected for late-maturing cultivars. Juskiw et al. (2000b) observed that per-plant biomass yields of barley, oat, and triticale harvested at the same growth stage increased with increasing maturity date. In 2000, when the harvest dates for the early maturing and late-maturing barley intercrops differed by only 1 d, there were no differences in Cut 1, Cut 2, or total yields for these intercrops. In 2000, Cut 1 DM yields did not differ between early and late-maturing oat intercrops as greater Cut 1 cereal DM yield for late-maturing intercrops was balanced by greater Cut 1 berseem clover DM yield for early maturing intercrops. In 2001, late-maturing cereal intercrops had greater Cut 1 DM yield than the early maturing cereal intercrops. Similarly, Thompson et al. (1992) reported that late-maturing barley cultivars intercropped with ryegrass had greater first-cut intercrop yields than a medium-maturing barley intercrop.

There were indications of less berseem clover suppression by early maturing cereal cultivars. Cut 1 berseem clover DM yields and percentages of berseem clover in Cut 1 were greater for early maturing oat intercrops in both years (Table 3). Cut 1 berseem clover DM yield was negatively correlated with days of growth of oat cultivars (r = –0.60 in 2000; r = –0.44 in 2001). In 2001, the early maturing barley intercrop had a greater percentage of berseem clover in Cut 1 than did the late-maturing barley intercrop.

Holland and Brummer (1999) did not measure oat tillering, but they suggested that the greater competitiveness of late-maturing oat cultivars might be associated with greater tillering in the vegetative phase. Some of our results for barley cultivars support this hypothesis. The late-maturing barley had greater tiller production, and barley tillering was negatively correlated with Cut 1 berseem clover DM yield in one year. However, oat tiller production did not correlate with Cut 1 berseem clover DM yield, and late-maturing oat cultivars had fewer tillers per plant than did early maturing oat cultivars. Thus, tiller production was not consistently associated with late maturity or with berseem clover suppression. Jedel et al. (1998) concluded that competitive ability of barley cultivars in intraspecific barley mixtures was not associated with tillering. Greater competitiveness of late-maturing cereal cultivars may be associated with greater partitioning of biomass into leaves. Juskiw et al. (2000b) found that early maturing barley cultivars had lower proportions of biomass as leaf and stem, and higher proportions as spike, compared with late-maturing cereal cultivars at stages between heading and soft dough.

Cut 2 berseem clover DM yields were greater for early maturing oat intercrops than for late-maturing oat intercrops (Table 3). In 2001, the early maturing barley intercrop also had greater Cut 2 clover yield than did the late-maturing barley intercrop. Greater Cut 2 yield may be partly explained by less initial suppression and by more days of regrowth. Cut 2 berseem clover yield was negatively correlated with the date of Cut 1 among oat cultivars in both years (r = –0.73 in 2000; r = –0.54 in 2001) and among barley cultivars in 2001 (r = –0.85).

Total DM and CP yields of early maturing cereal intercrops were equal or greater than those of late-maturing intercrops. Total CP yields were greater for early maturing oat intercrops than for late-maturing oat intercrops in both years (Table 3). In 2001, total DM and CP yields were greater for the early maturing barley intercrop than the late-maturing barley intercrop due to greater berseem clover yield. Cereals with a combination of late-maturity and taller stature may be a poor choice for cereal–berseem clover forage intercrops. The tallest late-maturing oat, Murphy, had the lowest yields of Cut 2 DM, total DM, and total CP amongst the oat intercrops. Juskiw et al. (2000c) cautioned that inclusion of a highly competitive cultivar or species in a mixture may not lead to any overall yield advantage. Cereals with both early maturity and shorter stature may be the best choice for cereal–berseem clover intercrops.

Forage Quality
Based on forage quality indicators for the cereal components of intercrops, the forage quality of barley cultivars was superior to that of oat cultivars (Table 5). Barley cultivars had greater CP and less or equal ADF compared with oat cultivars at Cut 1. There were no consistent quality advantages related to cereal cultivar maturity or stature. Differences in forage quality between the early and late-maturing cultivars varied between years. There were few differences in quality between semidwarf and conventional-stature barley cultivars. Forage quality advantages of cereal cultivars may be more related to individual cultivars than to a particular growth habit. For example, the semidwarf Niska barley had higher CP and lower NDF than the semidwarf Kasota and the conventional-stature AC Lacombe barley in 2000.

The forage quality of berseem clover at Cut 1 was better than the mean quality of oat and barley cultivars, with equal or greater CP and lower NDF (Tables 5 and 7). Berseem clover sole crops harvested at the same time as Cut 1 of barley intercrops (BE1 and BE2) had greater CP and less or equal ADF and NDF compared with those harvested at the same time as late-maturing oat intercrops (BE3) (Table 7). Berseem clover regrowth in Cut 2 had high nutritional quality with mean CP of 215 g kg^–1. In 2001, the CP of Cut 2 berseem clover declined with increasing days of regrowth.

Results for intercrops with Niska barley and Waldern oat indicated that adding berseem clover to cereals generally had little effect on forage quality at silage stage (Table 7). The limited impact of berseem clover on intercrop quality was likely due to a combination of small percentage of berseem clover in some mixtures and relatively high CP levels for cereals. Mean CP levels of oat cultivars and barley cultivars were greater than in some studies of silage cereals in central Alberta: 70 to 100 g kg^–1 CP for oat and barley (Juskiw et al., 2000a) and 90 to 125 g kg^–1 CP for barley (Jedel and Salmon, 1995). The high CP values for cereals were likely related to high initial soil N levels at Edmonton. Carr et al. (1998) found that adding pea to oat or barley did not increase forage CP in high-soil-N environments but did increase CP in low-soil-N environments.

The contribution of berseem clover to decreased NDF values for cereal–berseem clover silage may improve forage quality and increase intake. The NDF levels of oat and barley cultivars at Cut 1 were 550 g kg^–1 (Table 5) in half the cases. Baron et al. (1992) cited Van Soest (1965) in stating that NDF levels 550 g kg^–1 could severely reduce voluntary intake of forage. In related experiments at Edmonton, berseem clover components of about 20% in intercrops with oat, barley, or triticale reduced NDF by 25 to 45 g kg^–1 compared with cereals alone at silage stage (Ross et al., 2004).

Berseem Clover Sole Crops versus Intercrops
Berseem clover sole crops had lower Cut 1 yields and greater Cut 2 yields compared with intercrop yields (Table 3). These results were consistent with findings in related experiments with cereal–berseem clover intercrops (Ross et al., 2004). The lower midseason forage yields of berseem clover sole crops would make them less attractive for silage production than the intercrops. Sole crops of berseem clover might be preferable to producers wanting greater yield of high quality late-season forage. Total seasonal yields of berseem clover sole crops compared well with those of intercrops. Mean total DM yields from two cuts of berseem sole crops were 17% less than intercrops in 2000 and 9% greater than intercrops in 2001. However, findings in other experiments suggest that cereal–berseem clover intercrops provide greater yield stability than berseem clover sole crops (Ross et al., 2004).

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Cereal–berseem clover intercrops offered potential to partition forage yield between silage harvest and fall regrowth. Berseem clover regrowth in intercrops provided an average of 2.8 Mg ha^–1 DM of high quality forage. This late-season growth could provide extra forage at a time when forage quality and quantity is often limited. The N yield of the berseem clover regrowth biomass would be about 90 kg N ha^–1. Although the additional berseem clover growth could reduce soil water reserves, it could provide soil and environmental benefits of increased soil N, greater soil cover and weed suppression, reduced potential for soil erosion and N losses, and improved soil quality.

Juskiw et al. (2000c) cautioned against using plant height, biomass production, or a formula of traits to predict cereal cultivar competitive ability in mixtures. It would not be appropriate to draw general conclusions about oat and barley cultivars in mixtures from this study, but some comments can be made about the effects of cereal growth habit on berseem clover in intercrops at our particular site. Within cereal species, an association between increases in initial cereal DM yield and greater suppression of berseem clover was evident for Cut 1 and Cut 2 DM yields of berseem clover. This association was consistent with other experiments we conducted over several years with berseem clover intercrops on highly productive soils. An association between earlier Cut 1 and substantial increase in Cut 2 yield was also consistent with findings in other experiments. Although it might be specific to our short growing season, a few days difference in the silage-stage harvest had potential for considerable impact on berseem regrowth yield. Our finding that tiller production by cereals was not consistently associated with late maturity or with suppression of berseem clover may be of interest to crop breeders. Further research is needed on differences in competitive ability between oat and barley cultivars and between early maturing and late-maturing cultivars.

Our findings suggest that deficits in initial cereal forage yields that may occur with early maturing or shorter-stature cereal cultivars in intercrops may be balanced by gains in season-long berseem clover yields. It was true of our experiment that greater total yields sometimes occurred with oat or barley cultivars that caused less suppression of berseem clover. Advantages in total intercrop yield observed with less competitive cereal cultivars, and with reduced seeding rates of cereals, are contrary to conventional crop production approaches. In monocrop cereal production, yield advantage is often associated with greater seeding rates and more competitive cultivars. The findings in this study illustrate that the theory and approaches in intercrop production will differ from those used in monocrop production. To maximize fall forage and increase the legume component of silage harvest, early maturing and shorter-stature cultivars of oat and barley are recommended for cereal–berseem clover intercrops.

	ACKNOWLEDGMENTS

 
The technical assistance of C. Martin, G. Rawluk, J. Deeks, R. Davy, G. Su, and J. Zhao is gratefully acknowledged. Our thanks to P. Juskiw and the late S. Kibite for advice on barley and oat cultivars. We also acknowledge the financial support of the Alberta Agriculture Research Institute and the Canadian Wheat Board scholarship program.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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