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CORN

Influence of Brown Midrib, Leafy, and Transgenic Hybrids on Corn Forage Production

^a Dep. of Crop and Soil Sci., Cornell Univ., Ithaca, NY 14853
^b Dep. of Animal Sci., Cornell Univ., Ithaca, NY 14853

* Corresponding author (wjc3{at}cornell.edu)

Received for publication July 11, 2000.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
Hybrid selection strongly influences corn (Zea mays L.) forage production. Limited forage production and quality information exists for leafy and transgenic hybrids [Bacillus thuringiensis (Bt) and herbicide resistant]. We compared dry matter (DM) yield, forage quality, and estimated milk yields of these and brown midrib hybrids with dual-purpose hybrids for 3 yr at harvest densities of 66000 and 84000 plants ha^-1. Transgenic hybrids had no effect on DM and milk yields, so we do not recommend Bt hybrids under conditions of low infestation of European corn borer (Ostrinia nubilalis Hubner). Leafy hybrids had similar DM and milk yields as hybrids of similar maturity. Brown midrib hybrids, which had DM yields that were about 20% less than those of hybrids of similar maturity, had 40 to 80 g kg^-1 greater in vitro true digestibility (IVTD) and 70 to 200 g kg^-1 greater neutral detergent fiber (NDF) digestibility. Brown midrib hybrids had the greatest or least milk yields among hybrids, depending on hybrid and year. Leafy and brown midrib hybrids mostly had low harvest index (HI) values, which had strong negative correlations with NDF in all years (-0.71, -0.44, and -0.90). Brown midrib hybrids had high NDF digestibility, which had strong positive correlations with IVTD (0.80, 0.88, and 0.86). Currently, we do not recommend brown midrib hybrids because of inconsistent milk yields and high seed costs. Development of brown midrib and leafy hybrids with greater HI values should result in excellent forage hybrids.

Abbreviations: Bt, Bacillus thuringiensis • CP, crude protein • DM, dry matter • GDD, growing degree days • HI, harvest index • IVTD, in vitro true digestibility • NDF, neutral detergent fiber • RM, relative maturity

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
BROWN MIDRIB AND LEAFY HYBRIDS have been developed primarily for corn forage production. Transgenic hybrids, such as B. thuringiensis (Bt) and herbicide-resistant hybrids, may also improve corn forage yield and quality. More information on dry matter (DM) yield and forage quality of brown midrib, leafy, Bt, and herbicide-resistant hybrids would help dairy producers select appropriate hybrids for corn forage production.

Brown midrib hybrids contain less lignin in the stalks and leaves than normal hybrids, which results in greater stover digestibility and in vitro true digestibility (IVTD) concentrations (Colenbrander et al., 1973; Lechtenberg et al., 1972). Unfortunately, brown midrib hybrids produce 10 to 15% less DM compared with their normal counterparts (Miller and Geadelmann, 1983; Allen et al., 1997b). Furthermore, feeding studies with brown midrib corn silage do not always result in greater milk yields (Rook et al., 1977; Block et al., 1981; Stallings et al., 1982). Other studies, however, showed greater milk yields for cows (Bos taurus) that were fed brown midrib corn silage (Frenchick et al., 1976; Keith et al., 1979). A recent study also showed greater milk yields for high-yielding cows that were fed brown midrib corn silage compared with silage from a normal counterpart (Oba and Allen, 1999).

Compared with normal hybrids, leafy hybrids contain additional leaves above the ear, which should increase stover digestibility because leaves are more digestible than stalks. Dwyer et al. (1998) reported that leafy hybrids had lower harvest index (HI) values but more digestible stover above the ear compared with normal hybrids. Kuehn et al. (1998) reported that a leafy hybrid had lower crude protein (CP) concentration but greater digestibility compared with a high-grain hybrid. Bal et al. (1998), however, reported that cows fed corn silage from a leafy or high-grain hybrid had similar milk yields.

Hybrid selection is one of the most important management decisions influencing corn forage production and subsequent milk yields (Allen et al., 1997a). Unfortunately, limited or no forage yield or quality information exists for leafy, Bt, and herbicide-resistant hybrids. The objective of this study was to compare DM yield, forage quality, and estimated milk yield of brown midrib, leafy, Bt, and herbicide-resistant hybrids with dual-purpose hybrids that are produced for grain or forage. A second objective was to evaluate these hybrids at harvest densities of 66000 and 84000 plants ha^-1 because seed companies have recommended harvest densities for the brown midrib and leafy hybrids at about 65000 plants ha^-1 instead of the recommended 75000 to 80000 plants ha^-1 on silt loam soils in the northeastern USA (CCES, 2000). The current high cost of brown midrib and Bt hybrid seed increases the importance of identifying optimum plant densities for these hybrids.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
Field experiments were conducted in 1997, 1998, and 1999 on a Honeoye silt loam soil (fine-loamy, mixed, mesic Glossoboric Hapludalf) at a Cornell University Research Farm near Aurora, NY (46°26'N, 76°26'W). The experimental site is tile drained, and soil test values indicated a pH of 7.8 and were high for P and K in each year. The experimental site has been in a corn–soybean [Glycine max (L.) Merr.] rotation since 1990, so corn followed soybean in all 3 yr.

The experimental site was plowed and then harrow-cultipacked in April of each year. Twelve, 23, and 27 hybrids were planted in late April of 1997, 1998, and 1999, respectively, at 0.76-m row spacing and about 92000 and 74000 plants ha^-1. Hybrids ranged in relative maturity (RM) from 97 to 118 d in each year (Table 1). The experimental design was a randomized complete block in a split-plot arrangement with hybrids representing main plots (15 by 3 m) and plant densities representing subplots (7.5 by 3 m). Subplots were thinned, if necessary, to final densities of 84000 and 66000 plants ha^-1 at about the fifth leaf stage.

The center two rows of each subplot were harvested by hand for DM yield when a hybrid attained about 350 g kg^-1 DM concentration, which corresponded to the one-fourth to one-third milk line stage (Hunt et al., 1989) in 1997 and 1998 and the one-eighth to one-fourth milk line stage in 1999. Five plants were selected randomly at harvest from each subplot to estimate DM concentration, grain concentration, and forage quality. The five-plant subsample was divided into stover and ear fractions and dried at 60°C in a forced-air dryer to constant moisture. The kernels were shelled from the ears after drying to determine grain concentration, which allowed for estimation of HI, expressed as kilograms of grain per kilogram of total DM. Stover, cob, and grain were then reassembled into a whole-plant sample and ground sequentially through hammer and Wiley mills. Samples were then passed through a splitter, reduced to 50 g, and further ground through a cyclone mill (Udy Corp., Ft. Collins, CO) that was fitted with a 1-mm screen. Samples (0.5 g) were analyzed by wet chemistry for whole-plant neutral detergent fiber (NDF) according to procedures by Van Soest et al. (1991) and Kjeldahl N (AOAC, 1990). Samples (0.25 g) were also analyzed for IVTD according to Stage 1 of the procedure described by Marten and Barnes (1980). Samples were incubated for 48 h at 39°C in 5 mL of buffered rumen fluid containing 20 mL of the Kansas State buffer supplemented with 0.5 g L^-1 urea. Following fermentation, residues were analyzed for NDF to determine NDF digestibility. The NDF digestibility was calculated as

Milk 95, a spreadsheet that combines DM yield and forage quality characteristics into a single term, was used to calculate milk yield produced per hectare of corn forage (Undersander et al., 1993).

All data were analyzed with analysis of variance (ANOVA) and General Linear Model (GLM) procedures using SAS (SAS Inst., 1990). A separate analysis is presented for each year because the evaluated hybrids differed across years. Means among hybrids and between plant densities for all measurements were separated by Fisher's protected LSD. Simple correlations among all measured variables were also determined. Effects were considered significant in all statistical calculations at 0.05 level.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
Weather conditions differed markedly across growing seasons (Table 2). May through August in 1997 was cool and dry with about 100 growing degree days (GDD) and 70 mm of precipitation below the 30-yr mean. May through August in 1998 was warm with about 125 GDD above the 30-yr mean and close to average precipitation. May through August in 1999 was warm and exceedingly dry with about 75 GDD above and 165 mm of precipitation below the 30-yr mean. Visible leaf wilting was observed in August 1997 and from mid-June through harvest in 1999. Hybrids attained about 350 g kg^-1 DM concentration from 10 until 20 Sept. 1997 and from 25 August until 5 September in 1998 and 1999.

When averaged across plant densities, leafy hybrids (TMF99 and TMF106 in 1997, 1998, and 1999; TMF108 and T286602 in 1998 and 1999; WR2108L and WR2110L in 1998; and TMF100 in 1999) mostly had similar DM yields compared with other hybrids with the same RM. For example, TMF99 had similar DM yields as the 97-d (37M81), 98-d (37R71), and 99-d hybrids (DK493, DK493Bt, DK493GR, and DK493RR) in all 3 yr. Likewise, TMF106 had similar DM yields as the 105-, 106-, and 107-d hybrids in all 3 yr, except in 1998 when TMF106 had greater yields than RX601. Also, TMF108 had similar DM yields as the 107- and 108-d hybrids (DK580, DK580Bt, and DK580RR) in 1998 and 1999. The leafy hybrids had low HI values in the dry growing seasons, especially in 1999 when their HI values ranged from 0.10 to 0.32. Ironically, leafy hybrids were developed to increase photosynthate production after anthesis, thereby theoretically increasing grain fill (Shaver, 1983). Coors et al. (1997) reported that when HI values were artificially reduced from 0.42 to 0.25 or 0.00, DM yields declined by 7 and 19%, respectively. Nevertheless, in 1999, TMF99, with a HI of only 0.18, had a similar DM yield as DK493, which had a HI of 0.42. Likewise, in 1999, TMF106, with a HI of only 0.10, had similar DM yields as 34G81, which had a HI of 0.35. Apparently, leafy hybrids, despite much lower HI values, produce similar DM yields compared with other hybrids within the same RM in the northeastern USA.

When averaged across hybrids, DM yields averaged more at 84000 vs. 66000 plants ha^-1 in 1997 and 1998, despite a slightly lower HI value at 84000 plants ha^-1 in 1997 (Tables 3 and 4). Plant densities did not affect DM yields in 1999 (Table 5). Hybrid x plant density interactions did not exist for DM yields in any year, but there was a hybrid x plant density interaction for HI in 1999. The brown midrib hybrids, which had recommended harvest densities of 65000 plants ha^-1, had average DM yields of 16.2, 16.7, and 7.3 Mg ha^-1 at 66000 plants ha^-1 compared with 17.0, 18.0, and 7.6 Mg ha^-1 at 84000 plants ha^-1 in 1997, 1998, and 1999, respectively. The leafy hybrids, which also had recommended harvest densities of 65000 plants ha^-1, had average DM yields of 16.4, 18.6, and 8.9 Mg ha^-1 at 66000 plants ha^-1 compared with 17.7, 19.8, and 8.4 Mg ha^-1 at 84000 plants ha^-1 in 1997, 1998, and 1999, respectively. Evidently, the brown midrib and leafy hybrids respond positively to an increase in harvest density from about 65000 to 85000 plants ha^-1 in the northeastern USA. In 1999, however, four leafy hybrids (TMF108, T286602, TMF106, and TMF100) and one brown midrib hybrid (F657) had a decrease in HI values >0.05 as plant densities increased from 66000 to 84000 plants ha^-1. In contrast, most other hybrids had differences in HI values <0.04 between harvest densities. Evidently, the leafy hybrids, if used for grain production, do not tolerate high-density stress as well as the other hybrids do, especially in dry years. Overall, brown midrib and leafy hybrids generally had the least HI values among hybrids in all years, which indicates that they are not adapted for dual-purpose use in the northeastern USA.

Brown midrib hybrids had the greatest IVTD and NDF digestibility among hybrids in all 3 yr (Tables 6, 7, and 8). Brown midrib hybrids consistently had about 40 and 70 g kg^-1 greater IVTD and NDF digestibility, respectively, compared with hybrids with the next greatest IVTD and NDF digestibility. Furthermore, brown midrib hybrids consistently had about 80 and 200 g kg^-1 greater IVTD and NDF digestibility, respectively, compared with hybrids with the least IVTD and NDF digestibility. Allen et al. (1997b) reported about 40, 80, and 20 g kg^-1 greater IVTD, NDF digestibility, and CP, respectively, and similar NDF concentrations between brown midrib hybrids and their normal counterparts. Hybrid F867 had above-average CP concentrations (data not shown) and NDF concentrations in all 3 yr, and XB667 had above-average CP concentrations (data not shown) and about average NDF concentrations in 1999. Hybrid F657, the brown midrib hybrid with the greatest HI in 1998 and 1999, however, had low NDF concentrations in both years. As plant breeders further develop brown midrib hybrids, improvements in forage quality characteristics and DM yield potential will undoubtedly ensue.

Leafy hybrids consistently had average to somewhat below-average IVTD concentrations. For example, the leafy hybrids had average IVTD concentrations of 726 g kg^-1 compared with the hybrid mean of 725 g kg^-1 in 1997, 764 g kg^-1 compared with the hybrid mean of 774 g kg^-1 in 1998, and 841 g kg^-1 compared with the hybrid mean of 850 g kg^-1 in 1999. Furthermore, the leafy hybrids consistently had below-average CP concentrations (data not shown) and much above-average NDF concentrations compared with other hybrids. The leafy hybrids, however, did have above-average NDF digestibility. Nevertheless, the relatively high NDF digestibility of the leafy hybrids did not result in high IVTD concentrations. Coors et al. (1997) stated that effective grain fill was necessary to maximize corn forage quality. Apparently, ineffective grain fill of the leafy hybrids, as indicated by their low HI values, offset their relatively high NDF digestibility, resulting in only average forage quality.

When averaged across hybrids, IVTD concentrations were greater at 66000 compared with 84000 plants ha^-1 in 1997 and 1999 but were similar in 1998 (Tables 6, 7, and 8). Also, NDF concentrations averaged less at 66000 compared with 84000 plants ha^-1 in 1997 and 1999 but averaged the same in 1998. Cox et al. (1998) and Cusicanqui and Lauer (1999) previously reported that increased plant densities can decrease IVTD and increase NDF concentrations. Hybrid x plant density interactions did not exist for IVTD and NDF concentrations, except for NDF in 1999, because two leafy hybrids (TMF100 and TMF108) had a 35 g kg^-1 or more increase in NDF as plant densities increased. Plant density did not affect NDF digestibility, and there were no hybrid x plant density interactions for NDF digestibility.

Dairy farmers produce corn forage to maximize milk yields, which are strongly influenced by corn DM yields, IVTD, and NDF (Undersander et al., 1993). Estimated milk yields, associated with feeding a 50% corn silage ration, can thus be calculated from these three measurements (Undersander et al., 1993). Other factors, such as NDF digestibility and starch digestibility of corn silage, also influence milk production. Nevertheless, the calculation of milk yield from DM yield, IVTD, and NDF is an excellent way to evaluate corn silage hybrids (Cusicanqui and Lauer, 1999). For example, when averaged across plant densities, Bt hybrids and herbicide-resistant hybrids had similar calculated milk yields as their normal counterparts did except in 1998 when 35N05 had greater calculated milk yields than 3563 (Tables 3, 4, and 5). Consequently, we do not recommend Bt hybrids to dairy producers in the northeastern USA under typical conditions of corn borer pressure. Because herbicide-resistant hybrids did not affect calculated milk yields, we would recommend that dairy producers substitute herbicide-resistant hybrids for their normal counterparts in situations that warrant their use.

F867, the 118-d brown midrib hybrid that produced the least DM yield in 1997, produced the greatest calculated milk yield in 1997. Interestingly, F657, a 110-d hybrid with low DM yield, and 33V08, a 111-d hybrid with high DM yield, were among the hybrids that produced the greatest calculated milk yields in 1998 and 1999. The very high IVTD concentrations of F657 offset its low DM yield, which resulted in greater calculated milk yields for F657 than most other hybrids in 1998 and 1999. F867, however, had calculated milk yields close to the hybrid mean in 1998 and 1999. Furthermore, XB667 was among the hybrids with the least-calculated milk yields in 1999. Brown midrib hybrids apparently require further agronomic development before they can consistently provide high milk yields to dairy producers in the northeastern USA.

Leafy hybrids generally had similar calculated milk yields compared with hybrids of similar RM. Leafy hybrids had average DM yields, average to below-average IVTD concentrations, and much above-average NDF concentrations. Compared with hybrids of similar RM, leafy hybrids had similar calculated milk yields in 34 comparisons, greater calculated milk yields in seven comparisons, and lower calculated milk yields in 16 comparisons. Clearly, the current leafy hybrids, compared with dual-purpose hybrids, provide no additional benefit to dairy producers in the northeastern USA.

When averaged across hybrids, both plant densities produced similar calculated milk yields in 1997, despite greater DM yields at 84000 plants ha^-1 (Table 3). The 9 g kg^-1 decrease in IVTD and 18 g kg^-1 increase in NDF concentrations offset the 0.5 Mg ha^-1 DM yield advantage for 84000 plants ha^-1, resulting in similar calculated milk yields between the two harvest densities. In 1998, a year when plant densities did not affect forage quality, calculated milk yields averaged 1.4 Mg ha^-1 more at 84000 vs. 66000 plants ha^-1 (Table 4). In 1999, however, a year when IVTD concentrations decreased by 7 g kg^-1 and NDF concentrations increased by 9 g kg^-1 as harvest densities increased from 66000 to 84000 plants ha^-1, milk yields averaged 0.6 Mg ha^-1 less at 84000 plants ha^-1, despite similar DM yields between plant densities (Table 5). Cox et al. (1998) and Cusicanqui and Lauer (1999) also observed that maximum calculated milk yields occur at lower densities than do maximum DM yields because of the decrease in forage quality as plant densities increase. The high calculated milk yields of F867 in 1997 and F657 in 1998 and 1999, despite low DM yields, as well as the lower harvest density for maximum calculated milk vs. maximum DM yields in 1997 and 1999, underscore the importance of evaluating both DM yield and corn forage quality when conducting corn forage research.

	CONCLUSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
Commercial Bt, brown midrib, and leafy hybrids provided no consistent calculated milk yield increases compared with other commercial hybrids of similar RM. Consequently, we currently do not recommend these hybrids, especially the more costly Bt and brown midrib hybrids, to dairy producers in the northeastern USA. Nevertheless, further agronomic development of the brown midrib and leafy hybrids, especially an increase in their HI values, should increase their forage quality and subsequent milk yields. For example, HI, which was not correlated with DM yield and IVTD, except in 1998 with IVTD, had strong negative correlations with NDF concentrations in all 3 yr (Table 9), which is consistent with a previous study by Cox et al. (1993). In fact, HI values explained 50 and 80% of the variation in NDF concentrations in 2 of the 3 yr. The NDF digestibility, which had weak negative correlations with DM yields and positive correlations with NDF concentrations in 2 of 3 yr, had strong positive correlations with IVTD concentrations in all 3 yr (Table 9). The NDF digestibility explained about 65 to 75% of the variation in IVTD concentrations across years.

Corn forage hybrids, however, must be grown at plant densities that balance the increase in DM yield with the decrease in forage quality as plant densities increase. Harvest plant densities of 66000 vs. 84000 plants ha^-1 produced similar or greater calculated milk yields in the dry 1997 and 1999 growing seasons. Consequently, we recommend harvest densities of about 65000 plants ha^-1 on excessively drained soils in the northeastern USA (CCES, 2000). We continue to recommend, as supported by the 1998 data, harvest densities of about 85000 plants ha^-1 on soils with high water-holding capacity and about 75000 plants ha^-1 on well to moderately well-drained soils in the northeastern USA (CCES, 2000). Hybrid x plant density interactions did not exist for DM and milk yields, so we recommend these densities for all hybrids, including the brown midrib and leafy hybrids.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 

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