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* Department of Animal Sciences University of Illinois, Urbana 61801
Monsanto Company St. Louis, MO 63198
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Key Words: glyphosate-tolerant corn • production by dairy cattle • genetically enhanced crops
Abbreviation key: CP4 EPSPS = modified enzyme 5- enolpyruvylshikimate-3-phosphate synthase, DON = deoxynivalenol, EPSPS = enzyme 5-enolpyruvylshikimate-3-phosphate synthase
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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In 1998, a variety of corn tolerant to glyphosate (Roundup Ready event GA21) was marketed for the first time in the United States (Sidhu et al., 2000). Glyphosate, the active ingredient of the herbicide Roundup, kills susceptible plants by restraining the biosynthesis of aromatic amino acids through the inhibition of the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS;Steinrüken and Amrhein, 1980). Tolerance to this herbicide was achieved by the stable insertion of a gene that codifies for a modified corn EPSPS (CP4 EPSPS; Sidhu et al., 2000). Because CP4 EPSPS is not inhibited by glyphosate, the receptor corn line can successfully synthesize and meet its needs for aromatic amino acids even in the presence of this herbicide (Sidhu et al., 2000). Provided that the genome of corn hybrids tolerant to glyphosate was only subjected to this modification (i.e., expression of CP4 EPSPS), it is expected that their chemical composition and nutritional value for livestock will remain similar to that of the nontransgenic lines with a similar genetic background. In fact, results from experiments in which corn hybrids tolerant to glyphosate that are currently available on the market and that were evaluated as feed for livestock confirm this hypothesis (Clark and Ipharraguerre, 2001). For instance, Sidhu et al. (2000) found similar contents of moisture, protein, fat, ash, carbohydrates, ADF, NDF, Ca, and P in grain and whole plant green chop from a glyphosate-tolerant hybrid (GA21), its nontransgenic line (DK626), and five commercial corn hybrids. Moreover, the concentration of amino acids and fatty acids in grain from those seven hybrids were not significantly different. Additionally, Donkin (personal communication, 2000) observed that the DMI, milk yield, and milk composition of lactating dairy cows fed whole plant silage and grain produced from either a glyphosate-tolerant hybrid (GA21) or the parental corn line (DK626) were not different. More recently, data supporting these findings from experiments with chickens (Gaines et al., 2001a; Taylor et al., 2001a, 2001b) and pigs (Gaines et al., 2001b;Stainisiewski et al., 2001) have become available. Nevertheless, published data from experiments conducted to compare the performance of lactating dairy cows fed either glyphosate-tolerant or nontransgenic corn are not available. In addition, as glyphosate tolerance is conferred to novel corn lines with desirable agronomic characteristics, more experiments aiming to explore the nutritional value of these hybrids as feed for lactating dairy cows are needed. Therefore, the objectives of this study were to compare the effects on DMI, milk production, and milk composition of lactating dairy cows fed whole plant silage and grain produced from a glyphosate-tolerant corn hybrid (event NK603), its nontransgenic control of a similar genetic background, and two commercial nontransgenic hybrids.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Animals, Experimental Design, and Diets
Sixteen multiparous Holstein cows, averaging 41.4 kg of milk/d, 623 kg of BW, and 74 DIM at the onset of the experiment, were used in four 4 x 4 Latin squares conducted concurrently according to procedures approved by the University of Illinois Laboratory Animal Care Advisory Committee. Initially, cows were assigned to squares according to DIM at the beginning of the experiment and milk production during the previous 2 wk. Thereafter, cows within each square were randomly assigned to one of the four dietary treatment sequences. Each experimental period consisted of 28 d; the first 14 d were used to adapt cows to treatments and the remaining 14 d were used for data collection.
Four experimental diets containing the same forage to concentrate ratio (52 to 48% on DM basis) were prepared with different sources of corn supplied as silage and grain (Table 1
). Specifically, diets contained 30% corn silage and 27.34% corn grain (DM basis) produced either from NK603, nontransgenic control, DK647, or RX740 hybrids (Table 1). The remaining dietary ingredients were provided from the same source and in the amounts needed to meet the NRC (2001) nutrient requirements (Table 1). Throughout the entire experiment, NK603 diet was prepared and fed with different equipment in order to avoid contamination of the other diets with silage or grain of transgenic origin. Diets were fed as TMR twice daily at 1000 and 1700 h in amounts to ensure 10% orts. Additionally, TMR were adjusted weekly to reflect changes in DM content of forages and concentrate mixtures by drying the forages and concentrates overnight at 105°C.
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Sampling, Measurements, and Analyses
At harvest, a representative sample of corn grain from each of the four hybrids was obtained and sent to the Dairy One, Inc., Forage Testing Laboratory (Ithaca, NY) where samples were assayed for ash, Kjeldahl N, ether extract (AOAC, 1990); ADF, ADICP, ADL, NDF with heat-stable 
-amylase and sodium sulfite (Van Soest et al., 1991); and Ca, P, Mg, and K by inductively coupled plasma spectrometry. The NFC content of the grain was determined by calculation. Before the onset of the animal experiment, samples of corn silage were taken from a core near the end of each silo bag, packed in plastic containers with dry ice, sent to Cumberland Valley Analytical Services, Inc. (Maugansville, MD), and analyzed for pH, mycotoxins by HPLC (aflatoxin B1, B2, G1, and G2) and TLC (zearalenone, deoxynivalenol (DON), 15 acetyl-DON, 3 acetyl-DON, and T-2 toxin), ammonia-N, lactic acid, and VFA by GLC.
Feed offered and refused was recorded daily. During the last 2 wk of each period, samples of orts were collected daily, and DM content was determined by drying samples from individual cows at 105°C for 24 h. Samples of forages, concentrates, and TMR were collected once weekly and divided into two representative subsamples. One subsample of feed was used to determine DM as described for orts and the other subsample was stored frozen at -20°C for later analyses. At the end of each experimental period, feed subsamples from the weekly collections were thawed, combined on an equal wet weight basis, and sent to the Dairy One, Inc., Forage Testing Laboratory (Ithaca, NY) where they were analyzed for ash, Kjeldahl N, ether extract, ADF, ADIN, ADL, NDF, NFC, Ca, P, Mg, and K using the methods described above. The DM concentration of the TMR and orts was used to calculate DMI. Subsequently, intakes of ADF, NDF, CP, and NFC were estimated using the calculated DMI and the determined chemical composition of the TMR.
Milk production was recorded automatically at each milking throughout the experiment. Milk samples were collected for four consecutive milkings during each of the last 2 wk of each period, preserved with 2-bromo-2-nitropropane-1,3-diol, and stored at 4°C. At the end of each week, samples were sent to Dairy One Cooperative, Inc., Milk Check Laboratory (Ithaca, NY) for analyses of fat, CP, true protein, lactose, total solids, MUN, and SCC by infrared procedures (Foss 4000; Foss North America, Eden Prairie, MN). Daily milk composition was estimated as an average of the a.m. and p.m. milk samples corrected by the proportion of daily production at that milking.
Body weights and BCS were recorded during two consecutive days just before the experiment was started and on d 27 and 28 of each experimental period. Over the course of the study, BCS were determined by the same two individuals using a 5.00 scale in quarter-point increments, where 1 = thin and 5 = fat (Wildman et al., 1982).
Statistical Analyses
Before analysis, data for DMI, nutrient intakes, milk production, and milk composition were reduced to period means for each cow. Thereafter, data were analyzed using the MIXED procedure of SAS (2000) for a replicated 4 x 4 Latin square, with square and cow treated as random variables. The following model was used for all dependent variables:
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where:
| µ | = | overall mean,
| Si | = | effect of square i (i = 1, 2, 3, 4),
| Cj(S)i | = | effect of cow j nested within square i (j = 1, 2, 3, 4),
| Pk | = | effect of period k (k = 1, 2, 3, 4),
| SPik | = | interaction of square i and period k,
| Tl | = | effect of treatment 1 (l = 1, 2, 3, 4),
| STil | = | interaction of square i and treatment l, and
|  ijkl | = | residual error.
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Least square means were separated into significant main effects by the PDIFF procedure of SAS (2000). Differences among treatments were considered to be significant when P < 0.05, whereas when P > 0.05 but < 0.10 differences were considered to indicate a trend towards a significant effect.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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). In addition, the composition of these two hybrids was substantially equivalent to that of conventional hybrids of similar maturity (about 2500 heat units) available on the marketplace (i.e., DK647 and RX740). Recently, Sidhu et al. (2000) reported similar results for grain from a glyphosate-tolerant hybrid (event GA21), its parental control, and five conventional corn hybrids. Furthermore, values shown in Table 2 for all hybrids are within the range reported in the literature for dry corn grain (Miller and Hoover, 1998; NRC, 2001).
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). Apart from the content of DM, the composition of these two silages was equivalent to that of the RX740 corn silage (Table 3). As shown in Table 3, the RX740 silage contained about 5 percentage units less DM than the other three corn silages, denoting the lower DM content of the RX740 whole-cut plants at harvest (~ 6 percentage units). The DK647 corn silage provided the largest concentration of ADF and NDF and the lowest content of CP and NFC (Table 3). More important, however, is the fact that the chemical composition of the four corn silages was within published literature ranges (Miller and Hoover, 1998; NRC, 2001). In comparison with the control and commercial corn silages, the pH and contents of NH3N, lactic acid, and acetic acid were slightly higher for the NK603 silage (Table 3). Almost certainly, however, these differences were not biologically significant. Moreover, the pH and concentration of organic acids in all corn silages, particularly their large content of lactic acid (>86% of total VFA), reveal the occurrence of desirable fermentation processes (Pitt, 1990) that resulted in well-preserved forages.
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Concentrations of chemical components were equivalent among experimental diets, except for the lower CP percentage of the DK647 diet (Table 4). Because only the source of corn grain and silage differed among diets, this divergence can be attributed to the lower content of CP in grain (Table 3
) and silage (Table 4) produced from DK647. All diets, however, met or exceeded the requirements (NRC, 2001) for CP, RUP, and RDP by cows used in this experiment.
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). On average, feeding the NK603 and DK647 diets decreased DMI by 1.25 kg/d when compared with the nontransgenic control and RX740 diets. It should be noticed, however, that the DMI of cows fed the NK603 diet was only 0.9 kg/d lower than the DMI of cows fed the control diet (Table 5). Furthermore, cows consumed almost identical amounts of the NK603 and DK647 diets (Table 5). In agreement with these findings, Donkin (personal communication, 2000) reported that when lactating dairy cows ranging from 71 to 107 DIM were fed diets that contained 62% corn silage and 17% corn grain from either a glyphosate-tolerant (GA21) or a nontransgenic control (DK626) differences in DMI (21.9 and 21.5 kg/d, respectively) were not significant. Likewise, the feeding of glyphosate-tolerant or nontransgenic (i.e., control and commercial lines) corn grain to broilers (Taylor et al., 2001b) and pigs (event GA21; Stanisiewski et al., 2001) resulted in similar feed intakes.
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). However, there were some significant differences in intakes of CP, ADF, and NDF among other treatments. The intake of CP by cows fed the NK603 and nontransgenic control hybrid was intermediate compared with DK647 and RX740 hybrids. Intake of CP was lowest for cows fed the DK647 treatment because the CP content of the diet and DMI were slightly lower than for other treatments. The highest intake of CP was for cows fed the RX740 hybrid because the CP content and DMI of this diet were slightly higher than for the other treatments. Intakes of ADF and NDF were greater for cows fed the RX740 hybrid than for the other three treatments because of the slightly greater DMI and ADF and NDF contents of the diet. Cows fed all treatments consumed similar amounts of NFC. Therefore, differences that occurred in intakes of CP, ADF, and NDF arose from the small variations in both DMI and chemical composition of feed ingredients and experimental diets.
Production of Milk and Milk Composition
For all treatments, the production of milk and 3.5% FCM averaged 32 and 33 kg/d per cow, respectively, and were not significantly different among treatments (Table 6). Likewise, the concentration of fat, CP, true protein, MUN, lactose, and total solids in milk did not differ among treatments (Table 6). Consequently, the production of milk components was unaffected by experimental diets (Table 6). Additionally, SCC averaged 167 (103/ml) for all treatments and significant differences were not evident when SCC were transformed (log 10) before performing the statistical analysis (Table 6). These results agree with data of Donkin (personal communication, 2000) indicating that milk production, milk composition, and udder health of cows fed glyphosate-tolerant (GA21) corn or its nontransgenic control line (DK626) did not differ. In contrast, Grant (2003) reported that production of milk and 4% FCM was decreased when glyphosate-tolerant (event NK603) corn grain and corn silage were fed to dairy cows. This decrease in production was attributed to the glyphosate-tolerant corn silage having a higher than desired DM content (42.5%) compared with its nontransgenic control (36.6%) because of extremely fast drying conditions at harvest. The higher DM content of the glyphosate-tolerant corn silage resulted in a less desirable fermentation pattern and a 2.8 kg/d per cow lower DMI resulting, in turn, in a decrease in milk production (3.3 kg/d per cow) compared with controls.
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). Additionally, BCS averaged 2.85 and remained unaltered among cows fed NK603 or non-transgenic corns (Table 6). These findings support published data showing that the performance (Gaines et al., 2001a; Stanisiewski et al., 2001; Taylor et al., 2001a, 2001b) and meat composition (Stanisiewski et al., 2001; Taylor et al., 2001a, 2001b) of chickens and pigs is not affected by the feeding of glyphosate-tolerant corn.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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FOOTNOTES |
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Corresponding author: Jimmy H. Clark; e-mail:
jhclark{at}uiuc.edu.
Received for publication June 17, 2002. Accepted for publication July 11, 2002.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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