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Department of Animal Sciences, The Ohio State University, Columbus, 43210
Corresponding author:
J. L. Firkins; e-mail:
firkins.1{at}osu.edu.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Key Words: corn processing • starch digestion • fiber digestion • whole cottonseed
Abbreviation key: ADIN = acid detergent insoluble N, FCG = forage control with ground corn, FNDF = forage NDF, G = ground corn, HG = high cottonseed with ground corn, LG = low cottonseed with ground corn, LSF = low cottonseed with steam-flaked corn, MG = medium cottonseed with ground corn, MSF = medium cottonseed with steam-flaked corn, NDIN = neutral detergent insoluble N, NFFS = nonforage fiber source, SF = steam-flaked corn, WCS = whole cottonseed
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The previous NRC (1989) suggested that 75% of the dietary fiber should be provided as forage, which is equivalent to 21% of DM as forage NDF (FNDF). Although the current NRC (2001) suggests that minimum FNDF can be decreased when dietary NSC concentration is decreased, improved quantification of the effectiveness of forage and nonforage fiber sources (NFFS) would reduce variation in ration formulations. Replacement of NDF from NFFS for FNDF has reduced DMI when FNDF decreased below 14 to 16% (Firkins, 1997), but lactation performance was maintained in diets with 11 to 12% FNDF when whole cottonseed (WCS) replaced forage and when nonfiber carbohydrate was simultaneously reduced by WCS and other NFFS (Clark and Armentano, 1997; Slater et al., 2000). Replacing starch sources with NFFS increased ruminal pH (Swain and Armentano, 1994; Firkins, 1997), but some NFFS had low NDF digestibility when substituted for forage, apparently because of faster passage rates and slower digestion rates (Firkins, 1997). Despite the common usage of WCS as a forage substitute, site and extent of WCS digestion have not been extensively evaluated when WCS was fed as a forage replacement.
Although soybean hulls (Weidner and Grant, 1994) and wet corn gluten feed (Allen and Grant, 2000) interacted with forage to alter passage rate, NDF effectiveness, and digestibility, the potential interaction of WCS and forage have not been researched adequately. Because WCS appears to be the most effective NFFS source but its effectiveness varied inversely with the effectiveness of the forage (Mooney and Allen, 1997), high levels of WCS may not be effective enough to maintain ruminal pH and digestibility in diets higher in ruminal starch availability. Thus, our objectives were to evaluate potential interactions in site of nutrient digestion and lactation performance as WCS replaced forage and when steam-flaked (SF) or ground (G) corn was fed.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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All six experimental diets (Table 1
) were alfalfa silage-based TMR included as treatments in a 6 x 6 Latin square. Diets were balanced to be equivalent in nutrient composition on a DM basis (28% NDF, 5.6% lipid, 19% CP, 12% RDP, and 1.67 Mcal/kg of NEL based on reference values). The RDP concentration was balanced using estimates from Spartan Ration Evaluator (VandeHaar et al., 1992). The forage control diet (FCG) with no WCS addition contained the NRC (1989) recommendation of 21% FNDF. The WCS diets contained linted WCS at low, medium, or high concentrations (5, 10, and 15% of dietary DM, respectively). In a review, Arieli (1998) recommended a maximal WCS inclusion rate of 15% of dietary DM. In diets with G corn, alfalfa silage was replaced with WCS on an NDF basis to achieve diets with 18% FNDF (LG), 15% FNDF (MG), and 12% FNDF (HG). Linted cottonseed hulls were added to the diets in slightly different amounts during each period to balance NDF from forage and WCS and to enable the direct substitution of alfalfa silage NDF with cotton products (WCS plus cottonseed hulls) NDF, allowing NFC to be equivalent and reducing the potential effect of a grain dilution response (Armentano and Pereira, 1997). To evaluate the potential interaction of starch availability and level of forage substitution, the G in the LG and MG diets was replaced with SF (LSF and MSF diets). Steam-flaked corn (7.94% CP, 9.96% NDF, and 2.72% FA on DM bases) was provided by Pennfield Feeds, Lancaster, Pennsylvania. The density according to the manufacturing specifications (0.360 to 0.386 kg/L of 85% DM corn) was slightly lower than our measurement (0.393 kg/L on DM basis) probably because of corn fracture during handling and storage. Ground corn (7.56% CP, 13.1% NDF, and 4.66% FA on DM bases) had a mean particle size of 1.05 mm. Mean particle size was determined according to log-normal distribution procedures after dry sieving (Kerley et al., 1985). Cotton Incorporated, Cary, North Carolina, provided the WCS (23.6% CP, 48.7% NDF, and 18.4% FA) and cottonseed hulls (6.31% CP, 84.2% NDF, and 2.88% FA). Roasted soybeans were heated as described by Tice et al. (1993) and cracked. Alfalfa silage (19.2% CP and 40.6% NDF) had a mean particle size of 5.64 mm, with 38.5, 53.9, 66.5, and 72.8% retained on screens with pore sizes of 9.5, 4.75, 2.36, and 1.18 mm, respectively, after wet sieving (Kerley et al., 1985).
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Cows were housed in a conventional tie-stall barn with mattresses. They were allowed access to a concrete lot before milking, except on days when chewing activity was being monitored (Harvatine et al., 2002). Cows were milked at 0530 and 1600 h daily, with milk yield recorded at each milking. Milk samples were taken at a.m. and p.m. milkings on d 14 to 16 of each period and analyzed by DHI Cooperative Inc. (Powell, OH) for milk fat and CP by infrared spectroscopy and for milk urea N using a Skalar SAN Plus segmented flow analyzer (Skalar, Inc., Norcross, GA). Daily milk fat and protein yields were calculated and averaged by cow per period. Cows were weighed prior to the p.m. milking on d 7, 14, and 21 of each period; BW were averaged per period.
Core samples (approximately 600 ml) of ruminal contents from three different sites in the rumen were removed at 3, 6, 9, and 12 h after the a.m. feeding on d 14 of each period. Contents were strained through two layers of cheesecloth, and pH of the fluid was measured immediately. A 50-ml aliquot of the filtered ruminal fluid was acidified with 3 ml of 6 N HCl to stop fermentation and frozen. After thawing, the acidified ruminal fluid was mixed, centrifuged at 15,000 x g at 4°C for 15 min, and then filtered through Whatman number 1 filter paper (Whatman, Clifton, NJ). The supernatant was analyzed for VFA concentrations by GLC (Pantoja et al., 1994) and for NH3N (Chaney and Marbach, 1962).
Digestibility was measured on d 14 to 17 of each period. A pellet (equivalent to 1.1% of daily DMI) containing 5% Cr2O3 and 95% soybean hulls was dosed through the rumen cannula twice daily on d 8 through 17. Fecal grab samples were taken on d 14 to 17 of each period so that every 3 h in a 24-h period were represented (eight samples total). Samples were dried in a forced-air oven at 40°C for 48 to 60 h. Samples were composited on an air-dry basis by cow within period. Duodenal samples (250 ml) were taken on d 14 to 17 of each period so that every 90 min in a 24-h period were represented (16 samples total), combined by cow within period, and frozen immediately. After thawing, a 1000-ml subsample of the composited duodenal contents was lyophilized.
The samples of feed offered, orts, individual dietary components, and duodenal contents were ground to pass through a 2-mm screen (Wiley Mill; Arthur H. Thomas, Philadelphia, PA). The chromic oxide pellets were composited by period and ground manually using a mortar and pestle. Samples of feed, orts, chromic oxide pellets, duodenal contents, and fecal contents were analyzed for DM, OM, and N (AOAC, 1990). The NDF concentration of feed, orts, chromic oxide pellets, duodenal samples, and fecal samples was determined according to Van Soest et al. (1991) using hot ethanol, 8 M urea, and heat stable 
-amylase (Sigma A3306; Sigma Chemical Co., St. Louis, MO). The ADF and acid-detergent lignin concentrations of feed offered and chromic oxide pellets were determined sequentially; 72% H2SO4 was used for lignin quantification (Van Soest et al., 1991). Feed, orts, chromic oxide pellets, duodenal, and fecal samples were analyzed for total fatty acids by GLC (Sukhija and Palmquist, 1988). Chromic oxide pellets and duodenal and fecal samples were analyzed for Cr by atomic absorption spectroscopy (Williams et al., 1962). Duodenal samples were analyzed for NH3N content (Bremner and Keeney, 1965) and total purine concentration (Zinn and Owens, 1986) as modified by Ushida et al. (1985) for use as a microbial flow marker. Neutral detergent insoluble N (NDIN) and ADIN concentrations of feed offered were determined by the NDF or ADF procedure described previously (without urea addition) followed by Kjeldahl N analysis.
Feed, orts, chromic oxide pellets, and duodenal and fecal samples were reground to pass through a 1-mm screen and analyzed for total NSC concentration by GLC (Callison et al., 2001). Area under the curve was used to calculate individual sugar concentrations, which were corrected for area of the myo-inositol internal standard. Individual sugar concentrations were summed to yield total hydrolyzed sugar content. Pure corn starch was used to check recovery, which averaged 103% (SD = 10).
Core samples of ruminal concentration (approximately 1000 ml) were taken on four consecutive days (d 14 through 17) at 3, 6, 9, or 12 h after the a.m. feeding for harvesting and isolating particulate-associated bacteria by blending (Callison et al., 2001). The bacterial cells were lyophilized and analyzed for DM, OM, N, FA, and purines as described previously.
Calculations
The NEL concentration of the diet was calculated using the equation of Weiss (1993). The NFC concentration of the diet was determined by difference [OM – NDF – CP – (FA+1) + (NDIN * 6.25)]. The NFC component of the TDN equation used to calculate NEL was adjusted for NSC digestibility in the SF diets. An adjustment factor for each SF diet (LSF and MSF) was calculated by dividing the apparent total tract NSC digestibility for the SF diet by the NSC digestibility for the FCG diet.
The average amount of Cr dosed daily was divided by the concentration of Cr in the duodenal and fecal samples to determine daily flow of DM to the duodenum and rectum. Digestibility coefficients were then calculated based on the intake (adjusted for orts) and the duodenal and fecal flows of each nutrient (OM, NDF, NSC, and N). Microbial N, OM, or NSC flows were calculated by multiplying the bacterial N:purine, OM:purine, or NSC:purine ratios by total purine flow at the duodenum. The NSC content of the bacterial sample was determined by difference [NSC = OM – (N * 6.25) – (FA + 1)]. True OM and NSC flows to the duodenum were corrected for bacterial contributions. Efficiency of microbial protein synthesis was expressed per unit of OM or unit of carbohydrate (NDF plus NSC) digestion on apparent and true bases.
Statistical Analyses
Data were analyzed using the MIXED procedure of SAS (1999) with the following model:
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where µ = overall population mean, Ti = fixed effect of ith treatment, Cj = random effect of jth cow, Pk = fixed effect of kth period, and eijk = residual error. Preplanned contrasts compared the linear, quadratic, and cubic effects of increasing WCS in the diet when G corn was used as the corn source (FCG, LG, MG, and HG). An additional contrast compared the effect of corn source (LG and MG vs. LSF and MSF). The final contrast evaluated the interaction of corn source and level of WCS for LG, MG, LSF, and MSF treatments. Significant differences were declared at P < 0.05 unless otherwise stated. When the MIXED procedure would not converge for a few datasets (ruminal FA disappearance expressed as percentage of total FA intake, percentage of total C18 intake, and percentage of unsaturated FA intake), data were analyzed using Proc GLM (SAS, 1999). Individual pH, VFA, and NH3N concentrations were analyzed over time using the repeated measures analysis of the MIXED procedure of SAS (1999). Cow was designated as the only random effect. First-order autoregressive [AR(1)] was used as the covariance structure for pH, acetate:propionate, total VFA concentration, and most individual VFA concentrations. Heterogeneous compound symmetry (CSH) was used as the covariance structure for valerate. Heterogeneous first-order autoregressive [ARH(1)] was used as the covariance structure for ruminal NH3N concentration. When interactions for treatment x time were not deemed to be significant (P 
0.10), averages over all times were reported.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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. Total NDF concentrations were higher than the formulated values of 28% for each diet due to variations in NDF in WCS and the grain mix. The FNDF concentration was determined and adjusted before each collection period. Cottonseed hulls were added to the diets in slightly different amounts during each period to balance NDF and enable the direct substitution of alfalfa silage NDF with cotton product (WCS plus linted cottonseed hulls) NDF on a one-to-one basis as alfalfa silage NDF varied. The ratio of WCS to cottonseed hulls was, on average, 10:1.
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) were similar to the values used in ration balancing. The enzymatic procedure used to determine NSC concentration was based on total hydrolyzed sugar concentrations. Several of the feeds used in this study (alfalfa silage, soybean meal, and soybean hulls) contain greater than 10% soluble fiber (Hall et al., 1997). The soluble fiber would be included in NFC, but not NSC, explaining the difference between values for NFC and NSC. St-Pierre and Jones (1999) suggested that, if the interest of a researcher is to use a given set of research animals to make inferences to a larger population, the effect of cow should be considered random. On this basis, data were analyzed using the MIXED procedure of SAS (1999) with cow as a random effect. The SE for data presented in this paper using Proc MIXED are approximately 15 to 25% larger than those obtained using GLM because the MIXED model considers the variance of cow in addition to the residual variance. However, the choice of procedure (MIXED vs. GLM) had little effect on the significance of the contrasts.
As the proportion of dietary WCS increased, OM intake increased quadratically (Table 3). Ruminal and total tract OM digestibilities were not affected by level of WCS substitution, despite a linear increase in apparent ruminal NSC digestibility. However, when duodenal flows were adjusted to account for bacterial NSC flows, true ruminal NSC digestibility did not differ. Additionally, WCS addition did not affect total tract NSC digestibility considerably, although there was a quadratic trend.
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). No differences were observed for apparent total tract OM digestibility between corn sources. Compensatory postruminal digestion of starch or NDF tended to minimize differences in apparent total tract OM digestibility among grain processing methods (Firkins et al., 2001). Apparent and true ruminal NSC digestibilities were higher for SF corn diets compared with G corn diets (77.6 and 96.1% for SF vs. 55.1 and 70.7 for G, respectively). Apparent ruminal NSC digestibilities were substantially lower than true ruminal NSC digestibilities (average measured values of 21% NSC in bacteria). Despite compensatory intestinal NSC digestibility in G corn diets, total tract NSC digestibility was slightly lower for G than SF corn. Callison et al. (2001) observed slightly higher apparent ruminal and total tract NSC digestibilities (70.1 and 98.0%, respectively) when finely ground corn of similar particle size (1.15 mm) was fed. Approximately 96% of the NSC in SF corn was truly digested in the rumen; consequently, appararent total tract NSC digestibility for SF corn (99.4 and 99.8% for LSF and MSF, respectively) was slightly higher than the average of 94.2% reported by Firkins et al. (2001). This may be a result of our NSC procedure or attributed to the density of the SF corn used in the current study (0.360 to 0.386 kg/L), which should be optimal (Theurer et al., 1999).
Intake of NDF increased quadratically as the proportion of WCS in the diet increased (Table 3). Because the NDF concentration of the diets was similar (Table 2), this response was a result of increased DMI. There was no effect of level of WCS substitution on ruminal NDF digestibility (Table 3). During diet formulation, the estimated FA concentration from oilseeds was limited to approximately 3% of dietary DM (Table 1) to prevent depression in ruminal fiber digestion (Jenkins, 1994; Palmquist, 1995).
The amount of postruminal NDF digestion decreased as the concentration of WCS increased (Table 3), suggesting that NDF from WCS was not as digestible in the large intestine as was NDF from alfalfa silage. Cottonseed hulls contained more digestible NDF (54.2% of original NDF) compared with alfalfa (41.3% of original NDF), but ground alfalfa had a higher rate of digestion (0.0547 h–1) than ground cottonseed hulls (0.0208 h–1) in vitro (Bhatti and Firkins, 1995). A lower digestion rate and higher proportion of digestible NDF from WCS should promote passage and compensatory digestion of WCS NDF in the large intestine. However, the opposite effect occurred in our study, apparently because of increased negative associative effects as forage decreased (Firkins, 1997).
Corn processing decreased NDF intake because of reduced OM intake but did not affect NDF digestibility (Table 3). The effects of corn processing on NDF digestibility have been variable; increasing ruminal starch availability has decreased (Poore et al., 1993a; Plascencia and Zinn, 1996; Joy et al., 1997), increased (Chen et al., 1994), or had no effect (Poore et al., 1993b; Knowlton et al., 1998) on NDF digestibility.
As the level of WCS substitution increased, the average ruminal pH decreased linearly from 6.28 for cows fed the FCG diet to 5.93 for cows fed the HG diet (Table 4). The decreased pH could be a result of increased OM and NSC intakes increasing ruminally fermentable substrate, decreasing salivary buffer secretion, or to decreased cation exchange capacity as alfalfa silage decreased. Because total chewing time was not affected by treatment (Harvatine et al., 2002) and cottonseed meal had a similar cation exchange capacity as alfalfa (Van Soest et al., 1991), the former reason seems more likely. The lack of pH effect for corn source explains why ruminal NDF digestibility was not decreased when SF corn replaced G corn.
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). Change in acetate:propionate is a reflection of the linear increase in propionate concentration and the linear decrease in acetate concentration with increasing level of WCS. Ruminal VFA concentrations responded in a similar manner when NDF from WCS replaced FNDF (Clark and Armentano, 1993). Cows fed the diets with G corn had a higher acetate:propionate than did cows fed the diets with SF corn, resulting from a decrease in acetate concentration and an increase in propionate concentration. Other researchers have observed a decrease in acetate:propionate and an increase in propionate concentration when rapidly degradable NSC was fed (Ekinci and Broderick, 1997; Joy et al., 1997; Dann et al., 1999). When SF corn replaced G corn, there was an increase in concentration of valerate and a decrease in concentrations of isovalerate and isobutyrate (Table 4). Additionally, there was an interaction between corn source and level of WCS for valerate and isovalerate concentrations. An explanation for these observations is not clear.
Ruminal NH3N concentrations have been used as a measure of the ruminal availability of N and the efficiency of protein utilization in the rumen. Ruminal NH3N concentrations in the current study (Table 4) all were above the minimum concentration of 6.2 to 8.0 mg/dl suggested to maintain NDF digestion (Hoover, 1986). The diets were balanced to contain higher RDP than required (Table 1).
As expected from the linear increase in ruminal NH3N concentration (Table 4), the duodenal flow of NH3N increased linearly (Table 5) as WCS addition increased. However, nonmicrobial nonammonia N flow (% of N intake) decreased linearly with increasing WCS addition (Table 5). Diets were balanced to be equivalent in estimated RDP (64% of CP and 12% of DM). As NDF from WCS replaced FNDF, the concentration of roasted soybeans was reduced to maintain similar FA concentrations from oilseeds (roasted soybeans plus WCS) (Table 1). The RDP concentration of WCS (43.0% of CP) from Spartan Ration Evaluator (VandeHaar et al., 1992), which was used in diet formulation, was higher than that previously reported (30.4% of CP) for raw WCS (Pires et al., 1997). As a result, nonmicrobial nonammonia N (% of intake) ranged from 45.7% for FCG to 30.0% for HG (Table 5). Total tract N digestibility increased linearly with increasing WCS, which also indicates that heating of roasted soybeans might have been greater than planned.
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), probably because OM intake increased (Table 3). However, the efficiency of microbial N synthesis (calculated per unit of carbohydrate degraded) decreased linearly as WCS addition increased (Table 5). Because branched-chain VFA concentrations (Table 4) did not differ across treatments and NH3N concentrations should have been adequate, RDP did not appear to limit microbial growth. Therefore, the difference in microbial N synthesis appears to be related to ruminal pH. As ruminal pH decreased linearly (Table 4), the efficiency of microbial N synthesis per unit of carbohydrate digested also decreased linearly (Table 5). Russell (1998) suggested that, when excess carbohydrates are available in the rumen, microorganisms expend extra nongrowth energy in a process termed energy spilling. Although not measured in this study, increased lactate concentration might have contributed to observed pH differences as WCS increased. Additionally, the decreased efficiency of microbial protein synthesis could potentially be a result of the linear decrease in the passage rate of potentially digestible NDF (Harvatine et al., 2001). Because of the extensive attachment of ruminal microbes to feed particles, Firkins (1996) explained that a slower particulate passage rate could increase the proportion of energy spent on cell maintenance.
Substituting SF corn for G corn decreased ruminal NH3N concentration (Table 4) and duodenal flow of NH3N (Table 5). Additionally, duodenal flow of microbial N increased by about 15% with the use of SF corn. In a recent review, Firkins et al. (2001) determined that microbial N flow to the duodenum was greater for cows fed SF compared with dry-rolled or G corn. Ruminally fermentable NSC intake (NSC intake x true ruminal NSC digestibility) increased by over 1 kg/d when SF corn was fed (data not shown; see Table 3), which provided more substrate for microbial growth. Increased NSC availability combined with a lack of pH differences (Table 4), probably increased NH3N utilization by ruminal microorganisms. As a result, method of corn processing did not affect efficiency of microbial protein synthesis.
Intakes of total FA, C16, C18:1, and C18:2 increased linearly and C18:3 intake decreased quadratically with increasing WCS addition (Table 6). Although diets were balanced to contain equivalent crude fat concentrations, the FA concentration of the diet increased from 4.39 to 4.97% with increasing WCS inclusion (Table 2). Alfalfa silage has fewer lipids in the form of FA than WCS (Van Soest, 1994). By design, as the dietary proportion of WCS increased, the proportion of alfalfa silage and whole roasted soybeans decreased (Table 1). Typical alfalfa silage contains 39.0% C18:3 (as a percentage of total FA), whereas soybeans and WCS contain 6.8% and less than 0.5%, respectively (Van Soest, 1994). Additionally, WCS contain 25.3% C16 (as a percentage of total FA), but soybeans contain 10.7% C16, explaining the linear increase in C16 intake as WCS increased and roasted soybeans decreased.
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). In past trials, an average of 87% of dietary FA was recovered at the duodenum; however, de novo FA synthesis by ruminal microbes can result in a net gain of FA across the rumen (Jenkins, 1994). Thus, it is likely that de novo FA synthesis contributed to the increase in FA flow at the duodenum for cows fed FCG. There were linear increases in ruminal disappearance of total FA, total C18 FA, and unsaturated FA with increasing WCS addition (Table 6). To our knowledge, the ruminal disappearance of FA from WCS has not been compared with that of roasted soybeans in a single trial. Palmquist (1995) noted that the in situ disappearance of FA was higher for whole roasted soybeans (36.6%) compared with WCS (–5.86%). He attributed differences in FA disappearance to the physical structure of the oilseed; the hull of the soybean is more digestible than the hull of WCS, so the digestion and utilization of WCS might be more dependent on rumination and mastication to provide microbial access to seed contents (Coppock et al., 1985). When WCS was replaced with a combination of whole raw soybeans and soybean hulls on a FA and NDF basis, the diet with WCS stimulated more chewing and ruminating activity than the diet containing soybean products (Abel-Caines et al., 1997). In our study, total tract FA digestibility tended to increase linearly with increasing WCS use, apparently because of increased availability of FA from WCS compared with that from roasted soybeans.
Biohydrogenation of C18 FA and duodenal flow of trans C18:1 increased linearly with increasing inclusion of WCS (Table 6), despite pH decreasing linearly to 5.93 (Table 4). Kalscheur et al. (1997) suggested that decreased ruminal pH interfered with complete biohydrogenation. When the effects of pH on lipolysis and biohydrogenation were compared in vitro, lipolysis and biohydrogenation were not significantly inhibited until pH declined below 6.0 (Van Nevel and Demeyer, 1996). As WCS replaced alfalfa silage and roasted soybeans, the proportion of C18:2 increased from 43.0 to 47.6% and C18:3 decreased from 12.6 to 4.8% of total FA (data not shown). The C18:3 is probably primarily from alfalfa in the form of glycolipids. Anaerovibrio lipolytica, a prominent lipolytic bacterium, is apparently incapable of hydrolyzing glycolipids such as galactolipids (Harfoot and Hazlewood, 1988) because the sugar group needs to be removed before lipolysis. Therefore, although these lipids are rapidly hydrolyzed, lipolysis might have been more rate limiting as a higher proportion of lipids were from alfalfa silage.
Dry matter intake increased quadratically as the proportion of WCS in the diet increased (Table 7), explaining similar responses in intakes of OM, NSC, and NDF (Table 3); N (Table 5); and FA (Table 6). Although with fewer treatments to detect a nonlinear response, substituting WCS for forage typically increases DMI (Clark and Armentano, 1997; Mooney and Allen, 1997; Slater et al., 2000). Lower fill should increase DMI in the short term (Allen, 2000), but increased ruminal carbohydrate availability could decrease DMI in the long term if acidosis or other metabolic disturbances develop.
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No differences were observed for yields of milk, milk fat, or 3.5% FCM (Table 7). There was a trend (P = 0.06) for a quadratic response for milk fat percentage with increasing substitution of forage with WCS (Table 7), with milk fat percentage being higher for the LG and MG diets. Milk fat percentage was not decreased despite the lower ruminal pH, which has been associated with milk fat depression (Allen, 1997). Milk fat percentages were also not highly related to trans C18:1. Although this lack of effect does not seem to coincide with current theory (Kalscheur et al., 1997), our fatty acid procedure could not separate various trans C18:1 isomers, which has recently been documented to be critical (Griinari et al., 2000). In two previous studies, replacing alfalfa silage NDF with NDF from WCS did not affect milk fat percentage (Clark and Armentano, 1993; Mooney and Allen, 1997) but increased (Clark and Armentano, 1993) or did not affect (Mooney and Allen, 1997) fat yield. However, replacing forage and concentrate with a combination of soybean hulls and WCS in 9.4% FNDF diets decreased milk fat percentage in another study (Slater et al., 2000). The ability to maintain milk fat percentage in this study supports the calculated 0.84 physical effectiveness factor for cottonseed NDF (Harvatine et al., 2002).
Milk protein percentage increased linearly and protein yield tended (P < 0.08) to increase with increasing WCS substitution (Table 7). Replacing FNDF with NDF from WCS increased milk protein percentage in one study (Clark and Armentano, 1993) but had no effect in another (Mooney and Allen, 1997). The increased supply of microbial protein N was offset by decreased nonmicrobial nonammonia N as WCS increased (Table 5), so increased intake of digestible OM and increased propionate concentration could have spared amino acids for milk protein synthesis (Theurer et al., 1999).
Processing method did not affect milk yield or milk protein (yield or percentage) in the present study (Table 7). In a review, Theurer et al. (1999) noted an increase in milk and milk protein yields when SF corn was fed. However, Firkins et al. (2001) reported that milk protein percentage and milk yield were not different for SF versus G corn. The regression procedure used by the latter authors accounted for differences in DMI or other dietary factors rather than to total effect of grain processing attributed to starch digestibility.
Milk fat percentage was significantly decreased when SF corn replaced G corn (Table 7). Steam-flaking corn linearly increased apparent ruminal NSC digestibility (Table 3), which resulted in higher ruminal propionate concentrations in cows fed SF corn compared with G corn (Table 4). Decreased milk fat percentage can be explained by the glucogenic theory of milk fat depression for which increased propionate production causes metabolic changes that lead to decreased milk fat percentage (Gaynor et al., 1995). The lack of a corn effect on duodenal trans-C18:1 flow (Table 6) does not support the theory of trans fatty acids in milk fat depression (Kalscheur et al., 1997) or is a result of our inability to differentiate trans isomers (as discussed earlier). Steam-flaking often depresses milk fat percentage (Theurer et al., 1999), although decreasing DMI associated with higher ruminal starch availability (as for SF vs. G corn diets) can help reduce the milk fat depression (Firkins et al., 2001).
As the level of WCS increased, there was a linear increase in MUN (Table 7). Gustafsson and Palmquist (1993) have shown a positive relationship between MUN and ruminal NH3 N concentration, which also increased linearly (Table 4). Similarly, the lower MUN concentration for SF corn than that for G corn paralleled a decreased ruminal NH3N concentration with SF corn. Ekinci and Broderick (1997) and Dann et al. (1999) have also shown decreased MUN concentrations when processed grains were compared with those with a lower degree of processing.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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FOOTNOTES |
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2 Additional research support was provided by the Ohio Dairy Farmers Federation Dairy Research Fund; Pennfield Feeds, Lancaster, PA; and Cotton Incorporated, Cary, NC.
3 Current Address: Agway Feed and Nutrition, 512 West King St, Shippensburg, PA 17257.
Received for publication June 8, 2001. Accepted for publication February 2, 2002.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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9-desaturase. J. Nutr. 130:2285–2291.This article has been cited by other articles:
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  K. M. Cooke, J. K. Bernard, and J. W. West Performance of Dairy Cows Fed Annual Ryegrass Silage and Corn Silage with Steam-Flaked or Ground Corn J Dairy Sci, June 1, 2008; 91(6): 2417 - 2422. [Abstract] [Full Text] [PDF]
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J. L. Firkins, B. S. Oldick, J. Pantoja, C. Reveneau, L. E. Gilligan, and L. Carver Efficacy of Liquid Feeds Varying in Concentration and Composition of Fat, Nonprotein Nitrogen, and Nonfiber Carbohydrates for Lactating Dairy Cows J Dairy Sci, May 1, 2008; 91(5): 1969 - 1984. [Abstract] [Full Text] [PDF]
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B. N. Janicek, P. J. Kononoff, A. M. Gehman, K. Karges, and M. L. Gibson Short Communication: Effect of Increasing Levels of Corn Bran on Milk Yield and Composition J Dairy Sci, September 1, 2007; 90(9): 4313 - 4316. [Abstract] [Full Text] [PDF]
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J. L. Firkins, A. N. Hristov, M. B. Hall, G. A. Varga, and N. R. St-Pierre Integration of Ruminal Metabolism in Dairy Cattle J Dairy Sci, March 1, 2006; 89(e_suppl_1): E31 - E51. [Abstract] [Full Text] [PDF]
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C. Reveneau, C. V. D. M. Ribeiro, M. L. Eastridge, N. R. St-Pierre, and J. L. Firkins Processing Whole Cottonseed Moderates Fatty Acid Metabolism and Improves Performance by Dairy Cows J Dairy Sci, December 1, 2005; 88(12): 4342 - 4355. [Abstract] [Full Text] [PDF]
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J. T. Sylvester, S. K. R. Karnati, Z. Yu, C. J. Newbold, and J. L. Firkins Evaluation of a Real-Time PCR Assay Quantifying the Ruminal Pool Size and Duodenal Flow of Protozoal Nitrogen J Dairy Sci, June 1, 2005; 88(6): 2083 - 2095. [Abstract] [Full Text] [PDF]
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V. R. Moreira, L. D. Satter, and B. Harding Comparison of Conventional Linted Cottonseed and Mechanically Delinted Cottonseed in Diets for Dairy Cows J Dairy Sci, January 1, 2004; 87(1): 131 - 138. [Abstract] [Full Text] [PDF]
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J. L. Firkins, D. I. Harvatine, J. T. Sylvester, and M. L. Eastridge Lactation Performance by Dairy Cows Fed Wet Brewers Grains or Whole Cottonseed to Replace Forage J Dairy Sci, October 1, 2002; 85(10): 2662 - 2668. [Abstract] [Full Text] [PDF]
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D. I. Harvatine, J. L. Firkins, and M. L. Eastridge Whole Linted Cottonseed as a Forage Substitute Fed with Ground or Steam-flaked Corn: Digestibility and Performance J Dairy Sci, August 1, 2002; 85(8): 1976 - 1987. [Abstract] [Full Text] [PDF]
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