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Department of Animal Science, Michigan State University, East Lansing 48824-1225
Corresponding author:
M. S. Allen; e-mail:
allenm{at}pilot.msu.edu.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Key Words: conservation method of corn • feeding behavior • intake • nutrient utilization
Abbreviation key: DG = dry ground corn, HM = high-moisture corn, TRDOM = true ruminally degraded organic matter
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Although effects of starch fermentability on DMI were extensively studied, few experiments evaluated effects of starch fermentability on feeding behavior. DMI is a function of meal size and intermeal interval, which are determined by satiety and hunger, respectively. Evaluation of feeding behavior is needed to elucidate regulation mechanisms for feed intake when cows are fed fermentable diets. Corn grain is the major source of dietary starch for lactating dairy cows in the United States. Ruminal starch digestibility of corn grain can be altered by fineness of grinding and by conservation method. Ying et al. (1998) reported that ruminal starch digestibility was reduced more than 19% when dry ground corn (DG; mean particle size = 0.8 mm) was substituted for ground high-moisture corn (HM; mean particle size = 2.0 mm), with no difference in total tract starch digestibility. We hypothesized that effects of starch digestibility of corn grain on productivity of dairy cows differ by concentration of starch in diets. Greater ruminally degraded starch from HM is expected to increase the productivity of lactating dairy cows compared to DG when cows are fed low-starch diets, but to decrease DMI and productivity when cows are fed high-starch diets because of excess starch fermentation in the rumen.
The objective of this experiment was to evaluate effects of high-moisture and dry conservation methods of corn grain on feeding behavior, DMI, and productivity for lactating dairy cows fed two dietary starch concentrations.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Treatments and Cows
Eight multiparous Holstein cows (55 ± 15.9 DIM; mean ± SD) from the Michigan State University Dairy Cattle Teaching and Research Center were assigned randomly to duplicated 4 x 4 Latin squares balanced for carry-over effects with a 2 x 2 factorial arrangement of treatments. Cows were cannulated ruminally and duodenally prior to calving, and assigned randomly to a treatment sequence within a square. At the beginning of the experiment, empty BW (ruminal digesta removed) of cows was 565.5 ± 58.5 kg (mean ± SD). Treatments were dietary starch concentration (21 vs 32%) and conservation method of corn grain (HM vs DG). Treatment periods were 21 d, with the final 10 d used to collect samples and data.
One corn hybrid (Pioneer 3730; Pioneer Hi-bred International, Inc., Johnston, IA) was grown in 1998, and half of the field was harvested as HM at a DM concentration of 63.2%. High-moisture corn was ground to a mean particle size of 1863 µm and ensiled in a 2.4 x 9.0-m silage bag (Ag Bagger, Ag Bag Corp., Blair, NE). The remaining half of the field was harvested as dry corn at 89.7% DM. Dry corn was finely ground to a mean particle size of 885 µm. Nutrient composition for corn grain treatments is shown in Table 1
. Experimental diets contained either HM or DG, corn silage (50% of forage DM), alfalfa silage (50% of forage DM), a premix of protein supplements (soybean meal, distillers grains, and blood meal), and a premix of minerals and vitamins (Table 2). All diets were formulated for 18% dietary CP concentration, and fed as TMR.
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Feeding behavior and ruminal pH were monitored from d 16 to 19 (96 h) of each period by a computerized data acquisition system (Dado and Allen, 1993). Data on chewing activities, feed disappearance, water consumption, and ruminal pH were recorded for each cow every 5 s. Chewing activity for 24-h periods from feeding to feeding were deleted when chewing halters were out of adjustment. Electrodes for ruminal pH determination were checked daily and calibrated as needed, and ruminal pH data were deleted for the entire day if pH drifted more than 0.1 unit at pH 7 and 4. The system successfully collected 92.2% of the total chewing behavior data (4 d observations per cow per period), and 90.6% of the total ruminal pH data (69,120 observations per cow per period). Daily means were calculated for number of meal bouts per day, interval between meals, meal size, eating time, ruminating time, and total chewing time. These response variables were calculated as daily means, and then averaged over the 4 d for each period. Blood samples and ruminal fluid samples were collected every 20 min for 24 h by an automated sample collection system (Allen et al., 2000), starting at 1200 h on d 16. Blood was sampled from a jugular vein through a catheter inserted 1 d prior to sample collection. This system successfully collected 99.5 and 97.9% of the total samples (4608 each) for blood and ruminal fluid, respectively.
Ruminal fluid was centrifuged at 2000 x g for 15 min immediately after collection, and supernatants were frozen at -20°C until analysis. Blood samples were collected into two tubes, one with lithium heparin and the other with potassium oxalate and sodium fluoride as a glycolytic inhibitor. Both were centrifuged at 2000 x g for 15 min immediately after sample collection, and plasma was harvested and frozen at -20°C until analysis.
Sample and Statistical Analysis
Diet ingredients and orts were dried in a 55°C forced-air oven for 72 h and analyzed for DM concentration. All samples were ground with a Wiley mill (1-mm screen; Arthur H. Thomas, Philadelphia, PA). Samples were analyzed for ash, NDF, ADF, lignin, indigestible NDF, CP, and starch. Ash concentration was determined after 5 h of oxidation at 500°C in a muffle furnace. Concentrations of NDF and ADF were determined (Van Soest et al., 1991; method A for NDF). Crude protein was analyzed according to Hach et al. (1985). Starch was measured by an enzymatic method (Karkalas, 1985) after samples were gelatinized with sodium hydroxide; glucose concentration was measured with a glucose oxidase method (Glucose kit #510; Sigma Chemical Co., St. Louis, MO); and absorbance was determined with a microplate reader (SpectraMax 190, Molecular Devices Corp., Sunnyvale, CA). Indigestible NDF was estimated as NDF residue after 120 h of in vitro fermentation (Goering and Van Soest, 1990). Ruminal fluid for the in vitro incubations was collected from a nonpregnant dry cow fed alfalfa hay only. Concentrations of all nutrients except for DM were expressed as percentages of DM determined by drying at 105°C in a forced-air oven for more than 8 h. Corn grain was dry sieved (Sieve apertures: 4750, 2360, 1180, 600, 300, 150, 75 µm and bottom pan) using a sieve shaker (model RX-86, W.S. Tyler Inc., Gastonia, NC) for approximately 20 min until the bottom pan weight was constant, and the mean particle size of the corn grain was calculated (ASAE, 1968). True ruminally degraded OM (TRDOM) was calculated as described by Oba and Allen (2002a).
Ruminal fluid was analyzed for VFA and lactate concentrations. Samples were centrifuged at 26,000 x g for 15 min, and supernatant (600 µl) was mixed with 600 µl Ca(OH)2 and 300 µl of CuSO4 containing crotonic acid as an internal marker in 1.7 ml microcentrifuge tubes. Samples were centrifuged at 12,000 x g for 10 min, and supernatant (1000 µl) was taken and mixed with 28 µl of H2SO4 in 1.5-ml microcentrifuge tubes. Samples were frozen and thawed twice, and centrifuged at 12,000 x g for 10 min to precipitate and remove protein thoroughly. Supernatant was transferred to HPLC vials. Concentrations of VFA and lactate of the supernatant were determined by HPLC as described by Dado and Allen (1995). Rate of VFA production (moles/d) was estimated from the measured TRDOM (Oba and Allen, 2002a) and microbial efficiency (Oba and Allen, 2002b) according to Allen (1997). However, rate of VFA production might be underestimated for this experiment; microbial efficiency observed in this experiment was relatively high (Oba and Allen, 2002b) and overestimation of microbial efficiency could result in the underestimation of calculated rate of VFA production.
Plasma samples were analyzed for concentrations of acetate, glucose, NEFA, insulin, and growth hormone. Plasma was processed to quantify acetate concentration as described for ruminal fluid. Due to greater protein concentration for plasma samples, the first stage of sample processing was duplicated to obtain enough supernatant (1000 µl) to be mixed with 28 µl of H2SO4 in 1.5-ml microcentrifuge tubes. Plasma growth hormone concentration was determined by radioimmunoassay (Gaynor et al., 1995). Commercial kits were used to determine the plasma concentration of glucose (Glucose kit #510; Sigma Chemical Co., St. Louis, MO), NEFA (NEFA C-kit; Wako Chemicals USA, Richmond, VA), and insulin (Coat-A-Count, Diagnostic Products Corporation, Los Angeles, CA). Frequency and amplitude of insulin peaks were quantified according to Merriam and Wachter (1982).
All data were analyzed using the fit model procedure of JMP according to the following model:
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where µ = overall mean, Ci = random effect of cow (i = 1 to 8), Pj = fixed effect of period (j = 1 to 4), Tk = fixed effect of treatment (k = 1 to 4), eijkl = residual, assumed to be normally distributed.
Period x treatment interaction was originally evaluated, but it was removed from the statistical model because interaction was not significant for response variables of primary interest. Orthogonal contrasts were performed for effects of dietary starch concentration, conservation method of corn grain, and interaction of dietary starch concentration and conservation method. Treatment effects and their interactions were declared significant at P< 0.05 and P < 0.10, respectively, and tendencies for treatment effects were declared atP < 0.10. When interactions of main effects were significant, treatment means were compared using Student’st-test, and differences were declared significant atP < 0.05.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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). This could be attributed to greater distension by physical fill in the rumen for low-starch diets. Low-starch diets contained more forage NDF compared to high-starch diets (25.3 vs 16.5% DM), and forage NDF is a primary factor reducing DMI by physical fill in the rumen (Allen, 2000). Interaction of dietary starch concentration and conservation method of corn grain was significant for DMI (P < 0.07). The HM treatment decreased DMI by 1.7 kg (20.8 vs 22.5 kg/d) compared to the DG treatment when fed in high-starch diets, but had no effect (19.7 vs 19.6 kg/d) when fed in low-starch diets. Consistent with treatment effects on DMI, meal size was greater for the DG treatment compared to the HM treatment (2.3 vs 1.9 kg) when cows were fed high-starch diets, but not when cows were fed low-starch diets (2.0 vs 2.1 kg).
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The reduction in DMI and meal size for HM treatment in high-starch diets can likely be attributed to greater ruminal fermentation. Because dietary forage NDF concentration was similar between HM and DG treatment within the same dietary starch concentration, differences in physical fill were not likely responsible for differences in DMI. The TRDOM was greater for high-starch diets compared to low-starch diets (P < 0.001; Table 4) and for HM compared to DG treatment (P < 0.03), and calculated rate of total VFA production was greater for cows fed HM compared to DG treatment (P < 0.01). A reduction in DMI with increased ruminal fermentation has been reported previously. McCarthy et al. (1989) increased starch digestibility in the rumen by replacing ground-shelled corn with steam-rolled barley in diets containing more than 40% starch on a DM basis, and reported an approximately 3-kg depression in DMI. Overton et al. (1995) also showed that increased substitution of barley for dry corn linearly decreased DMI for lactating dairy cows.
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). Propionate has greater hypophagic effects than acetate (Farningham and Whyte, 1993; Sheperd and Combs, 1998). Intraruminal infusion of propionate decreased DMI compared to acetate on both an isomolar (Farningham and Whyte, 1993) and isoenergetic basis (Sheperd and Combs, 1998). Although greater fermentation in the rumen is often associated with low ruminal pH and greater osmolarity of ruminal fluid, the specific effects of pH and osmolarity on feed intake have not been shown. Choi and Allen (1999) showed that hypophagic effects from ruminal infusions of VFA did not differ when the VFA were infused as acids or salts. They also found that infusion of hyperosmotic solutions of NaCl decreased meal size, but did not affect DMI because cows compensated for smaller meal size by increasing meal frequency. When cows were fed low-starch diets, HM treatment did not decrease DMI compared to DG treatment. Greater starch digestibility in the rumen does not necessarily decrease DMI (Grings et al., 1992; Knowlton et al., 1998; Callison et al., 2001). Grings et al. (1992) compared diets containing ground corn or barley at approximately 30% of dietary DM, but found that DMI was not affected by treatments. Knowlton et al. (1998) and Callison et al. (2001) fed lactating dairy cows corn grains differing in processing method or particle size at 42 and 37% of dietary DM, respectively. In both experiments, corn grain treatment altered starch digestibility in the rumen, but did not affect DMI. Inconsistent effects of ruminal starch digestibility on feed intake might imply that a threshold exists for propionate to affect DMI. Feeding behavior might be controlled by a dominant mechanism related to the stimulation of tension receptors by ruminal fill until a mechanism possibly related to propionate begins to dominate on highly fermentable diets.
High-starch diets decreased molar ratio of acetate (P < 0.001; Table 4), isobutyrate (P < 0.001), and isovalerate (P < 0.03) compared to low-starch diets. High-starch diets increased the molar ratio of propionate (P < 0.001) and valerate (P < 0.01) concentration compared to low-starch diets. VFA profile was not affected by the conservation method of corn grain. Total VFA concentration was not affected by treatments. It is noteworthy that the concentration of total VFA in ruminal fluid did not reflect the amount of OM truly fermented in the rumen. Although treatment means for TRDOM varied from 7.7 to 11.3 kg/d in this experiment, no relationship was observed between TRDOM and total VFA concentration (Figure 1). Total VFA concentration did not reflect fermentation acid production in the rumen because VFA concentration in the rumen is not solely determined by rate of production, and treatment effects on rates of absorption and passage compensated for treatment effects of production rate on VFA concentration in the rumen.
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Milk Production and Plasma Metabolites
Milk and SCM yields were greater for high-starch diets compared to low-starch diets (P < 0.001 and P < 0.02; Table 5). An interaction of dietary starch concentration and conservation method of corn grain was detected for SCM yield (P < 0.10). High-starch diets increased SCM yield by 3.3 kg (35.2 vs 31.9 kg/d) compared to low-starch diets for cows fed DG, but not for cows fed HM. Significant interactions of main effects were also observed for milk fat concentration (P < 0.06) and milk protein concentration (P < 0.07). The HM treatment decreased milk fat concentration compared to the DG treatment when fed in high-starch diets (3.05 vs 3.59%), whereas corn grain treatment did not affect milk fat concentration when fed in low-starch diets. Milk protein concentration was greater for high-starch diets compared to low-starch diets for cows fed DG (3.02 vs 2.87%), but not for cows fed HM.
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High-starch diets decreased milk fat concentration, but increased BW gain (P < 0.01) and BCS gain (P < 0.01) compared to low-starch diets. High-starch diets decreased daily mean ruminal pH, but increased daily variance for ruminal pH. We previously proposed that partitioning of absorbed fuels to milk or body reserves is influenced by variation in ruminal pH because it determines pattern of supply of metabolic fuels from the rumen to the blood circulation (Oba and Allen, 2000). Rate of fermentation acid absorption from the rumen is affected by ruminal pH. Ruminal pH with less daily fluctuation might result in a more consistent supply of metabolic fuels from the rumen to the blood circulation, whereas a greater fluctuation in ruminal pH might indicate more pulsatile energy supply. A more pulsatile energy supply may stimulate insulin secretion, increasing energy metabolite utilization in adipose tissues more than milk fat synthesis (Oba and Allen, 2000). In agreement with this theory, high-starch diets increased insulin concentration (P < 0.01; Table 6). Diurnal pattern for plasma insulin concentration (Figure 2) shows that high-starch diets consistently increased plasma insulin concentration compared to low-starch diets. Greater daily means for plasma insulin concentration for high-starch diets are attributed to a greater baseline of insulin secretion (P < 0.001) and enhanced amplitude of insulin peaks (P < 0.001) compared to low-starch diets. However, daily variance for plasma concentration of acetate was not affected by dietary starch concentration and did not appear to support this theory. Plasma acetate concentration is determined by flux from the rumen and utilization by tissues, and does not necessarily reflect the fluctuation in supply from the rumen especially for samples taken from a jugular vein. Treatment effects on acetate utilization by tissues might have compensated for the possible treatment effects on pulsatility of acetate supply from the rumen.
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High-starch diets decreased plasma concentrations of growth hormone (P < 0.01), NEFA (P < 0.07), and acetate (P < 0.04) compared to low-starch diets. Estimated rate of acetate production in the rumen was not affected by dietary starch concentration but was greater for HM treatment compared to the DG treatment (Table 4). Lower plasma acetate concentration for high-starch diets might be a result of greater acetate utilization in peripheral tissues from stimulation by insulin compared to low-starch diets. Plasma concentration of glucose was greater for high-starch diets compared to low-starch diets (P < 0.01; Figure 3), but it is noteworthy that plasma glucose concentration decreased after feeding, regardless of diet. This reduction in plasma glucose concentration is partially attributed to an increase in plasma insulin concentration after feeding. Because insulin decreases gluconeogenesis and increases glycogen synthesis in the liver, we speculate that absorbed propionate is not directly metabolized to glucose but that glucose 6 phosphate is transiently utilized for glycogen synthesis after feeding, and glucose is released from glycogen storage over time.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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FOOTNOTES |
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Received for publication March 18, 2002. Accepted for publication June 11, 2002.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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