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South African Journal of Animal Science

versão On-line ISSN 2221-4062
versão impressa ISSN 0375-1589

S. Afr. j. anim. sci. vol.43 no.1 Pretoria Jan. 2013

 

Megasphaera elsdenii (M.e.) NCIMB 41125 is a robust lactate utilizing strain of M.e. that is effective in minimizing the risk of ruminal acidosis in feedlot cattle. When dosed orally, cattle adapt smoothly to increasing concentrates in the diet, the incidence of digestive disturbances, morbidity and mortality is reduced, and carcass yield improves. One could therefore expect that the smooth transition should benefit overall performance. Dosing with the organism also provides the opportunity of a reduction in the time necessary for adaptation, rendering a further decrease in the cost of feeding. These two objectives were tested with 80 yearling crossbred steers blocked by weight before allotment to the respective treatments. The trial design was a randomized 2 χ 2 factorial of two drench treatments (M. e. vs. placebo) and two adaptation periods (17 vs. 8 days). In the M.e. treatment, 40 steers were dosed orally on day 1 of the trial with 200 mL inoculum containing 1011 cells. In the placebo treatment, the other 40 steers were dosed orally with only the 200 mL inoculum. In the 17-day transition period, five diets (5-transition) were used, which increased progressively in concentrate percentage, whereas in the 8-day transition period only three of the five diets were fed (3-transition). The steers were fed individually for 63 days before being transferred to group pens and fed until day 95, when they were slaughtered. Dry matter intake was not affected by dose or transition treatment. Body weight at 28 days and 63 days did not differ between dose and transition treatments; neither did ADG and FCR. Hot carcass weight was higher in M.e. steers than in placebo steers. None of the parameters differed significantly between the 5-transition and the 3 -transition treatments. It was concluded that dosing with M.e. NCIMB 41125 should provide a small benefit to performance of feedlot cattle, with a further benefit in cost savings as dosing with the organism should allow a shorter adaptation period.

Keywords: Steers, transition diets, feed intake, body weight, growth, growth, carcass characteristics


 

 

Ruminal acidosis in feedlot cattle manifests when lactic acid accumulates rapidly and pH declines as a result. The condition usually occurs during transition from an all-roughage to a high-concentrate diet over a short time when the microbial population and the rumen environment have not adapted properly to the fermentation medium of primarily starches and sugars. To prevent the condition, lactic acid needs to be removed rapidly, in which the robust strain NCIMB 41125 of the lactic acid utilizing species Megasphaera elsdenii excels (Henning et al., 2010a; Meissner et al., 2010). The strain (hereafter referred to as M.e.) is dosed at the commencement of the adaptation phase and supports a smooth transition as observed, with a reduction in the number of digestive disturbances, treatments, morbidities and mortalities (Leeuw et al. , 2009; Meissner etal., 2010). One would naturally expect that the health benefits would carry over to the total feeding period and reflect in improved performance. However, although milk production of the high-producing cow appears to benefit (Meissner et al., 2010; Aikman et al., 2011; Henning et al., 2011), the results on feedlot performance remain equivocal. Neither commercial (Meissner et al., 2010) nor controlled trials (Leeuw et al. , 2009) have shown consistent benefits to average daily gain (ADG) and feed conversion ratio (FCR), but probability tests suggest a 2.2% advantage in carcass yield (Meissner et al., 2010). The objective of the present study was to investigate this further and, in addition, to establish whether the adaptation phase on typical US feedlot diets, as on typical South African diets (Henning et al., 2009), can be shortened ifM. e. is administered, which, if successful, should be beneficial to the cost of production.

The experiment was conducted in a manner consistent with the applicable laws and regulations governing the humane care of animals, and consistent with the KSU Institutional Animal Care and Use (IACUC) protocol No. 1977, as well as the Centre for Veterinary Medicine under the Investigation Food Additive application No. 11-171. Megasphaera elsdenii NCIMB 41125 was cleared for experimental use in the USA by the USDA Animal-Plant Health Inspection Service (permit numbers SOU-855 and TRN-855).

Approximately 130 yearling crossbred steers of 390 - 450 kg body weight were obtained, weighed upon arrival, vaccinated against relevant viral and clostridial diseases, treated for internal and external parasites, and identified with individually numbered ear tags. They were then placed in pens accommodating 15 to 30 head each, and fed lucerne hay plus a vitamin-mineral-salt supplement for a number of weeks to acclimate and to limit variation in gastrointestinal fill. Ten days before commencement of the experiment, the steers were again weighed, stratified by weight and a subset of 80 was selected, which differed little in weight. The subset was then allocated individually to 6.5 m χ 1.6 m pens, which were housed 20 each in four identical barns. The pens were equipped with fence-line feed bunks and a water fountain shared between adjacent pens. The 80 steers were fed chopped lucerne hay and the vitamin-mineral-salt supplement for another 10 days before being weighed, stratified by weight, and allotted randomly within strata to the four barns and the 20 individual pens per barn. The pens within the barns were assigned randomly to one of two drench treatments, consisting of an oral dose of 200 mL of liquid culture media containing 1011 viable Megasphaera elsdenii NCIMB 41125 cells (M.e. treatment) or a placebo consisting of an equal volume of the culture media containing no organism (placebo treatment). Care was taken in the assignment that M.e. and placebo steers did not share a common water fountain to limit the opportunity of cross-inoculation.

Within each barn, steers were again stratified by weight and assigned randomly to each of four groups consisting of two adaptation periods (17 or 8 days) with and without the organism (M.e. or placebo). In the 17-day adaptation regimen, five diets (5-transition) were used: 45% roughage/55% concentrate fed on days 1 - 4; 35% roughage/65% concentrate fed on days 5 - 8; 25% roughage/75% concentrate fed on days 9 - 12; 15% roughage/85% concentrate fed on days 13 - 16, and 6% roughage/94% concentrate (final diet) fed on days 17 - 95. In the 8-day adaptation regimen (3-transition) only three of the five diets were used: 45% roughage/55% concentrate fed on days 1 - 3; 25% roughage/75% concentrate fed on days 4 - 7, and 6% roughage/94% concentrate (final diet) fed on days 8 - 95. The dietary compositions are shown in Table 1.

Steers were fed individually for 63 days. The amount of feed offered to each steer was determined at about 12:00 and the entire daily amount was delivered into the feed bunk by 14:00. Residual feed was removed daily, dried and weighed in order to calculate daily feed dry matter (DM) intake per animal. At the end of the 63-day period, the five steers representing each treatment in a particular barn were combined with the five steers of the same treatment in a second barn and placed in a group pen. Between days 64 and 95, the trial steers were therefore fed in pens of 10 head each, and intake was not monitored.

Steers were weighed individually on days 1, 28, 63 and 95, prior to feeding. During the trial they were observed continuously for clinical signs of digestive and/or metabolic disorders and other diseases. These were minimal and did not affect trial integrity. On day 95 the 80 steers were transported to a commercial abattoir for slaughter. Measurements include daily DM intake until day 63, steer weights and ADG for days 1 - 28, 1 - 63 and 1 - 95, feed efficiency (FCR) for days 1 - 28 and 1 - 63, liver abscesses, hot carcass weights, fat thickness over the 12th rib, percentage kidney, pelvic and heart fat, and intramuscular fat deposition as a measure of marbling.

The study was designed as a randomized complete block of a 2 χ 2 factorial arrangement with 20 observations per treatment. Individual animal constituted the experimental unit and factors consisted of transition regimen (5-transition vs. 3-transition) and dose treatment (M.e. vs. placebo). The data were analysed using the generalized linear models procedure of the Statistical Analysis System. The statistical model included fixed effects of barn (four barns were used), transition regimen, dose treatment and the two-way interaction between transition regimen and dose treatment. Initial weight and pre-trial rate of gain were included as covariates. Since degrees of freedom were limited and between-animal variation large, even after careful blocking for weight and other measures, statistical significance was accepted at the 10% level of probability and trends at the 20% level of probability.

Dry matter intake (DMI) did not differ significantly, for either the transition regimen (Trans.) or the dose treatment (Dose), whether during days 1 - 28 (steers in adaptation) or during days 1 - 63; neither was the interaction (D χ T) significant (Table 2). Higher intakes with less animal variation and day-to-day variation during the initial adaptation phases when M.e. was administered have been reported in most experimental station trials (Henning et al., 2010a; b; Meissner et al., 2010), although there are exceptions (Leeuw et al., 2009), especially in commercial trials (Meissner et al., 2010). The reasons could be associated with rate of adaptation, dietary energy level and grain processing. Similar to the result of 5 -transition vs. 3-transition treatments here, Henning et al. (2009) found no significant difference in DMI between steers adapted for 1, 5, 9, 13, 17 or 21 days when dosed with M.e.

Together with DMI, steer weights, ADGs and FCRs for the periods 1 - 28 days and 1 - 63 days are reported in Table 2. Transition period did not have an effect on body weight, ADG and FCR, supporting the results of Henning et al. (2009). Steers on M.e. treatments had higher body weights and ADGs than those on placebo, the results at 63 days approaching significance (P = 0.11). As the probability value of D χ T also approached significance (P = 0.11), the results suggest that the effect of M.e. inoculation was more pronounced in the 5-transition treatments than the 3 -transition.

When the ADG results of 1 - 95 day period weight gains were adjusted to account for variation in gastrointestinal fill (Table 3), ADG in M.e. compared with the placebo was higher (P = 0.09), and so was hot carcass weight (P = 0.10) with on average a 9.4% and a 2.3% advantage respectively in favour of the M.e. treatments. The 5-transition treatments tended to have higher carcass adjusted ADGs (P = 0.12) and hot carcass weights (P = 0.13) than the 3-transition treatments, whereas the 3-transition treatments tended to have more liver abscesses (P = 0.14) than the 5-transition treatments. Carcass characteristics were not affected by dose or transition treatments.

Leeuw et al. 's (2009) results did not show an advantage to ADG and FCR with M. e. administration, measured on a live weight or on a carcass basis. In the review by Meissner et al. (2010), no benefit was reported of M.e. inoculation to live weight ADG and FCR, which could be explained partly by variation in gastrointestinal fill, as shown here. The benefit to carcass gain reported in this review (Meissner et al., 2010) is supported by this study. Thus, the results suggest that a single dose of Megasphaera elsdenii strain

NCIMB 41125 at the initiation of the adaptation phase should provide a small benefit in carcass gain. Although there were no major differences in transition treatments, trends imply that, compared with steers on the 5-transition treatments, steers on the 3-transition treatments might have been health compromised, suggesting that feedlot operators in the US on similar diets to the diet in the present study should be cautious in shortening the transition phase, even when steers are inoculated with strain M.e. NCIMB 41125. This is in contrast with the results of Henning et al. (2009) on typical ground maize and hominy chop-based South African feedlot diets.

 

References

Aikman, P.C., Henning, P.H., Humpfries, D.J. & Horn, C.H., 2011. Rumen pH and fermentation characteristics in dairy cows supplemented with Megasphaera elsdenii NCIMB 41125 in early lactation. J. Dairy Sci. 94, 2840-2849.        [ Links ]

Henning, P.H., Campbell, A.A., Hagg, F.M. & Horn, C.H., 2009. Effect of accelerated diet step-up rate on performance of feedlot steers dosed with Megasphaera elsdenii NCIMB 41125. In: Ruminant Physiology - Digestion, Metabolism and Effects of Nutrition on Reproduction and Animal Welfare. Eds Chilliard, Y., Glasser, F., Faulconnieur, Y., Boucquier, Y., Veissier, I. & Doreau, M., Wageningen Academic Publishers. pp. 78-79.        [ Links ]

Henning, P.H., Horn, C.H., Leeuw, K-J., Meissner, H.H. & Hagg, F.M., 2010a. Effect of ruminal administration of the lactate-utilizing strain Megasphaera elsdenii (Me) NCIMB 41125 on abrupt or gradual transition from forage to concentrate diets. Anim. Feed Sci. Technol. 157, 20-29.        [ Links ]

Henning, P.H., Horn, C.H., Steyn, D.G., Meissner, H.H. & Hagg, F.M., 2010b. The potential of Megasphaera elsdenii isolates to control ruminal acidosis. Anim. Feed Sci. Technol. 157, 13-19.        [ Links ]

Henning, P.H., Erasmus, L.J., Meissner, H.H. & Horn, C.H., 2011. The effect of dosing Megasphaera elsdenii NCIMB 41125 (Me) on lactation performance of multiparous Holstein cows. S. Afr. J. Anim. Sci. 41, 156-160.        [ Links ]

Leeuw, K-J., Siebrits, F.K., Henning, P.H. & Meissner, H.H., 2009. Effect of Megasphaera elsdenii NCIMB 41125 drenching on health and performance of steers fed low and high roughage diets in the feedlot. S. Afr. J. Anim. Sci. 39, 337-348.        [ Links ]

Meissner, H.H., Henning, P.H., Horn, C.H., Leeuw, K-J., Hagg, F.M. & Fouché, G., 2010. Ruminal acidosis: A review with detailed reference to the controlling agent Megasphaera elsdenii NCIMB 41125. S. Afr. J. Anim. Sci. 40, 79-100.        [ Links ]

 

 

Copyright resides with the authors in terms of the Creative Commons Attribution 2.5 South African Licence. See: http://creativecommons.org/licenses/by/2.5/za Condition of use: The user may copy, distribute, transmit and adapt the work, but must recognise the authors and the South African Journal of Animal Science.
# Corresponding author: Kleeuw@arc.agric.za

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Effect of dietary starch source on milk production and composition of lactating Holstein cows

 

 

GH.R. MosaviI; F. FatahniaI, #; H.R. Mirzaei AlamoutiII; A.A. MehrabiI; H. Darmani KohiIII

IDepartment of Animal Science, University of Ilam, Ilam, Iran
IIDepartment of Animal Science, University of Zanjan, Zanjan, Iran
IIIDepartment of Animal Science, University of Guilan, Rasht, Iran

 

 


ABSTRACT

The objective of this study was to evaluate the effects of four sources of starch on milk production and composition, nutrient digestion and blood metabolites of lactating Holstein cows. Four multiparous Holstein cows (708 ± 70 kg of body weight; 83 ± 9 days in milk) were used in a 4 χ 4 Latin square design with 21-d periods. The concentrate portion of the diet contained wheat, barley, maize or potato as the primary source of starch. Intake of dry matter (DM) ranged from 18.7 kg/d to 19 kg/d, and was similar among treatments. Milk production was higher in cows fed the wheat-based diet compared with other diets while the milk fat concentration of the cows fed the maize-based diet was the highest. Milk protein concentration was unaffected by the source of dietary starch. Cows fed the potato-based diet had a lower milk protein, lactose and solid-non-fat yield. Milk yield/kg of DM intake and net energy for lactation (NEL)/NEL intake were higher in cows fed wheat-, barley- or maize-based diets compared with those fed the potato-based diet. Feed nitrogen efficiency was higher in cows fed the maize-based diet compared with the other experimental diets. Total tract apparent digestibility of organic matter, crude protein and ether extract were higher in cows fed the wheat- or maize-based diets compared with those fed barley- or potato-based diets; however, total tract apparent digestibility of neutral detergent fibre and acid detergent fibre were higher in cows fed the wheat-based diet compared with those fed the potato-based diet. These results showed that improved production performance in cows fed the wheat-based diet appeared to be because of greater nutrient digestibility and greater nutrient utilization efficiency. Furthermore, potato starch is not superior to grain starch as a readily available energy source for lactating dairy cows.

Keywords: Nutrient digestibility, nutrient efficiency, plasma metabolites, wheat, barley, potato, maize


 

 

Introduction

The increase in milk yield of dairy cows that results from their genetic improvement requires the use of large amounts of concentrates that are rich in energy and crude protein (CP) to meet their nutrients requirements (Cabrita et al., 2009). Dietary carbohydrate is composed of neutral detergent fibre (NDF) and non-fibre fractions, which collectively compose 65% to 75% of the diets of lactating dairy cattle. Non-fibre carbohydrates (NFC) may provide 30 to 45% of the diet on a dry matter (DM) basis (Hall et al., 2010). Root crops such as potato have been used in dairy rations, but have been replaced by grains and maize silage because of labour costs (Eriksson et al., 2004). Dairy cow diets usually contain barley, maize and wheat as the main carbohydrate sources because they are cost-effective sources of digestible energy. Starch is the major nutrient providing energy from these cereal grains and potato. These cereal grains and potato differ in their starch content, with wheat containing (DM basis) 77% starch, maize 72%, barley 58% (Huntington, 1997) and potato 63% (Wang et al., 2009). Differences also exist among these starch sources in their rates and extents of ruminal starch degradation, with 32%/h for wheat starch, 2%/h for maize starch, 29%/h for barley starch, and 5%/h for potato starch being digested in the rumen (Monteils et al., 2002; Wang et al., 2009). The larger granules of more enzyme-resistant B-type crystalline starch in potato compared to the smaller granules of grain, generally with A-type crystalline starch (Tester et al., 2006) could explain the difference in their ruminal fermentation patterns. Raw potato starch could not be expected to be superior to grain starch as a readily available energy source, but it could reduce diurnal fluctuations in energy supply and limit the problems caused by the incorporation of a large amount of wheat or barley starch in the diet. In principle, the rate and extent of fermentation of dietary carbohydrates (especially starch) in the rumen are important parameters that determine nutrient supply to the animal (Hall, 2004). Greater dietary concentration of nonstructural carbohydrates increases the utilization of ruminal ammonia-N for microbial protein synthesis (Nocek & Tamminga, 1991). Increasing ruminally available energy concentration of diets for dairy cows has the potential to enhance milk production through increased metabolizable nutrient supply (Gozho & Mutsvangwa, 2008). Most studies on the effect of starch source have been conducted with grains (barley, maize, millet, oat, sorghum or wheat), but rarely with potato. The study of different sources of starch using in sacco technique (Monteils et al., 2002; Wang et al., 2009) showed that the ruminal degradation of potato starch is slower than that of wheat or barley, but faster than maize starch. These observations have been confirmed by enzymatic tests with ruminal fluid (Con, 1991). Owing to differences in digestion characteristics and fermentation products, the starch source of the diet has the potential to alter feed intake, milk production and milk composition. Therefore, the objective of the study was to evaluate the effect of dietary inclusion of barley, maize, wheat or potatoes as a principle source of starch on milk production and composition, nitrogen and energy efficiency, nutrient digestibility and blood metabolites of lactating dairy cows.

 

Materials and Methods

Four multiparous Holstein cows (708 ± 70 kg of BW; 83 ± 9 days in milk; 30 ± 1.3 kg of milk/d) were used in a 4 χ 4 Latin square design. Each experimental period consisted of 14 days for adaptation to the diets and 7 days for daily data collection. The cows were housed in individual tie stalls and had free access to drinking water throughout the trial. The four dietary treatments examined, consisted of different sources of starch, with the concentrate portion of the diet containing barley, maize, wheat or potato. All cereal grains were coarsely ground, and potato was chopped. The ingredient composition and chemical analysis of the experimental diets are shown in Table 1. The experimental diets were formulated according to the NRC (2001) recommendations, and fed individually twice daily at 08:00 and 16:00 as total mixed ration (TMR) for ad libitum intake and adjusted for 100 g orts/kg as fed. The cows were milked three times daily at 08:30, 16:00 and 24:00.

During the 7-d sample and data collection period, individual cow feed intake was recorded daily. Samples of diets and orts were collected daily, frozen at -20 °C, and composited per cow for each experimental period. Composite samples of orts were based on daily amounts of orts for each cow. After the experiment, pooled samples of the diets (TMR) and orts were dried at 60 °C for 48 h, ground through a 1mm screen using a Wiley mill, and analysed for DM, organic matter (OM), ether extract (EE), CP (AOAC, 1990), acid detergent fibre (ADF) (Van Soest et al., 1991) and NDF with heat-stable α-amylase and sodium sulphite (Van Soest et al., 1991). The starch concentration of the experimental diets (TMR) was analysed by an enzymatic method (Karkalas, 1985). Faecal grab samples were collected for six consecutive days at 08:00 on d 15, 10:00 on d 16, 12:00 on d 17, 14:00 on d 18, 16:00 on d 19, and 18:00 on d 20 of each experimental period. Faecal samples were dried at 55 °C in a forced draft oven for 72 h, then ground through a 1 -mm screen using a Wiley mill. Equal DM from each faecal subsample was mixed to obtain a single composite for each sampled cow during each period and analysed for OM, CP, EE, ADF and NDF, using the methods already described for pooled TMR and orts samples. Total tract nutrient digestibility was determined by acid insoluble ash (AIA) as an internal marker (Van Keulen & Young, 1977). Milk production was recorded during the 7-d data collection period. On days 16 and 17 of each experimental period, milk samples were collected from the six consecutive milking into plastic vials that contained a preservative (dichromate potassium, K2Cr2O7). Milk samples were then pooled daily, based on milk yield, and kept at room temperature for determination of protein, fat and lactose concentrations (Milko-Scan 133B Foss Electric, Denmark). On d 19 of each experimental period, blood samples were drawn before the morning feeding from the jugular vein into vacutainer tubes containing heparin. The plasma was separated by centrifugation (2,500 χ g for 20 min) and total cholesterol (kit no. 10-508; ZiestChem Diagnostics Co., Tehran, Iran), high-density lipoprotein (HDL) cholesterol (kit no. 10-507; ZiestChem Diagnostics Co., Tehran, Iran), triglycerides (kit no. 10-525; ZiestChem Diagnostics Co., Tehran, Iran ), and glucose (kit no. 10-505; ZiestChem Diagnostics Co., Tehran, Iran) concentrations were analysed by colorimetric methods. Low-density lipoprotein (LDL) cholesterol was calculated by difference between total cholesterol and high-density lipoprotein (HDL) cholesterol.

All statistical analyses were performed using PROC MIXED of SAS (1999). Data on DM intake, milk production, milk composition, nutrient digestibility, faecal excretion, nitrogen and energy efficiency, and blood metabolites were analyzed as a 4 χ 4 Latin square design using the following model:

Yijk = µ + Ti + Cj + Pk + eijk, where Yijk is the dependent variable, µ is the overall mean, Ti is the fixed effect of treatment, Cj is the random effect of cow, Pk is the fixed effect of period, and eijk is the random residual error. Treatment effects were declared significance at P <0.05.

 

Results and Discussion

Actual milk production was significantly higher (P <0.05) in cows fed the wheat-based diet compared with those fed the other diets. On average, cows fed barley-, maize-, and potato-based diets produced 1.4, 1.74, and 3.54 kg/d less milk, respectively, compared with those fed a wheat-based diet (Table 2). The potato-based diet produced significantly less milk than the other three diets, but there was no significant difference when replacing maize with barley. Fat corrected milk (FCM) and energy corrected milk (ECM) were higher in cows fed wheat-, or maize-based diets compared with those fed barley- or potato-based diets (Table 2). Similarly, Silveira et al. (2007) reported higher 4% FCM in cows fed maize-based diet compared with those fed barley-based diet, but, in a study by Gozho & Mutsvangwa (2008), 3.5% FCM was higher in cows fed barley- and maize-based diets compared with a wheat-based diet.

In studies of Cabrita et al. (2009), with maize and wheat, and Silveira et al. (2007), with maize and barley as the main sources of dietary starch, actual milk production was increased for cows fed the maize-containing diets compared with those fed wheat- or barley-containing diets. However, Gozho & Mutsvangwa (2008) with barley, maize, wheat and oats, Khorasani et al. (2001), with maize and barley, and Jurjanz et al. (1998), with wheat and potato peelings, did not observe any effect of starch source on milk production. Responses of lactating cows to different cereal grains depend on the level of dietary inclusion, the basal ration, physical processing of the cereal grains, the composition of a given batch of cereal grain, and the level of dietary intake (Khorasani et al, 2001). Although in our experiment DM intake was not affected by experimental treatments, the higher milk production in cows fed the wheat-based diet was attributed to greater total tract digestibility of DM (Table 4) and higher efficiency for milk production, defined as milk production/DMI (Table 3), although not for NEL milk/NEL intake. The reduction in milk production for cows fed potato-based diet was attributed to a lower total tract digestibility of DM and higher faecal excretion of DM (Table 4). However, in the current experiment, actual milk production was significantly higher for cows fed the wheat-based diet compared with those fed the maize-based diet, their similar 4% FCM production can be explained by higher milk fat content of cows fed maize-based diet.

In the current study, feeding the maize-based diet resulted in a higher milk fat concentration (P <0.05) compared with the feeding of the wheat-, barley-, or potato-based diets (Table 2). A higher fat concentration in milk from cows fed maize compared with barley or wheat diets has been reported previously (Gozho & Mutsvangwa, 2008; Cabrita et al, 2009). However, other research observed no effect of starch source on milk fat concentration (Khorasani et al., 2001; Silveira et al, 2007). Jurjanz et al. (1998) did not observe any dietary effects on milk fat concentration with diets containing potato peelings or wheat as the principal source of starch at low and medium starch concentration, but milk fat concentration was higher when potato peelings were fed at the higher starch concentration. Observed differences in milk fat concentration undoubtedly reflect differences in patterns of ruminal fermentation (Moren, 1986) and in site and the extent of digestion of these starch sources (Okine & Kennelly, 1994). Slower degradation of maize starch compared with wheat, barley or potato starches in the rumen could enhance a higher delivery of precursors (acetate and butyrate) of milk fat synthesis to the udder. The rumen is the major site of starch digestion in cattle (Taniguchi et al., 1995). In vivo studies (Sauvant et al, 1994; Sauvant, 1997) showed a decreased acetate + butyrate to propionate ratio for cows fed diets containing starch that was degraded quickly in the rumen. In situations where the diet promotes lower ruminal pH, the biohydrogenation pathway for the saturation of 18-carbon polyunsaturated FA can be perturbed, with a resultant increase in C18:1 trans-FA. Some of these trans-FA have been shown to be potent inhibitors of milk fat synthesis (Griinari et al., 1998).

Dietary treatment did not affect the protein concentration in milk (P >0.05), but wheat-, barley-, or maize-based diets increased (P <0.05) milk protein yield compared with the potato-based diet (Table 2). Similarly, previous research (Khorasani et al., 2001; Silveira et al., 2007; Cabrita et al., 2009) found no differences in milk protein concentration of cows fed diets containing rapidly degradable starch compared with slowly degradable starch. Gozho & Mutsvangwa (2008), investigating wheat, barley, maize and oats as principal sources of starch, found that milk protein concentration was higher in cows fed a maize-based diet compared with those fed an oats-based diet. However, they observed no differences in milk protein concentration in cows fed wheat-, barley-, or maize-based diets.

Milk lactose concentration did not differ (P >0.05) among dietary treatments (Table 2). Similarly, other research observed no effect of starch source on milk lactose concentration (Khorasani et al., 2001; Gozho & Mutsvangwa, 2008; Cabrita et al., 2009).

Dry matter intake was not affected (P >0.05) by dietary source of starch (Table 3). Cabrita et al. (2009) reported similar DM intakes in cows fed maize-based diets compared with wheat-based diets. Gozho & Mutsvangwa (2008) did not observe any effect of dietary starch source on DM intake in cows fed wheat-, barley-, maize- or oats- based diets. Jurjanz et al. (1998) did not observe any dietary effects on DM intake of dairy cows fed diets containing wheat or potato peelings as the principal sources of starch. In contrast, Silveira et al. (2007) reported higher DM intakes in cows fed a maize-based diet compared with those fed a barley-based diet. A review by Cabrita et al. (2006) found contradictory effects of changing starch fermentability on DM intake of lactating dairy cows. With diets containing rapidly degradable starch, a decrease in rumen pH and an increase in volatile fatty acid production may decrease DM intake by regulatory mechanisms. Inconsistent effects of starch source on DM intake can be attributed to the starch concentration of the diet (DeVisser et al., 1990), and forage particle length (Rode & Satter, 1988). Diets containing higher concentrations of forage NDF promote chewing, salivation and high rumen pH (Mertens, 1997), which mask some effects caused by differences in fermentability of diets in the rumen.

Efficiency for milk production, defined either as milk yield/DM intake or as NEL milk/NEL intake, was significantly (P >0.05) higher in cows fed wheat-, barley- or maize-based diets compared with the potato-based diet (Table 3). Contrary to the results of this study, Silveira et al. (2007) reported a higher NEL milk/NEL intake in cows fed barley-based diets than for cows fed maize-based diets, but milk yield/DM intake was not affected by grain source. It is generally recognized that the efficiency of utilization of energy from starch is greater when it is digested in the small intestine and absorbed as glucose rather than when starch is fermented in the rumen and the propionate fraction is converted to glucose in the liver (Reynolds, 2006). However, the effects of this higher energetic efficiency on production response are equivocal. Theurer et al. (1999) concluded that higher ruminal starch digestion increases milk production. In the current study, lower efficiency for milk production in cows fed potato-based diet could be attributed to lower total tract digestibility of DM and higher faecal DM excretion (Table 4). Nitrogen intake (Table 3) was similar across diets, in part because DM intake (Table 3) was not affected by starch source and also because the diets were formulated to be isonitrogenous. Feed nitrogen efficiency was lowest in cows fed the potato-based diet, intermediate in those fed the barley-, or wheat-based diets, and greatest in cows fed the maize-based diet (Table 3), reflecting the lower total tract digestibility of CP and the higher faecal nitrogen excretion (Table 4) in cows fed the potato-based diet. In the current study, the lower feed nitrogen efficiency in cows fed the potato-based diet might be partly explained by lower microbial protein synthesis and higher ammonia concentration in the rumen. Surber & Bowman (1998) speculated that increased ruminal and post ruminal digestibility of barley starch, compared with maize starch, may result in a greater energy yield and an improved feed conversion. They reported 17% greater microbial nitrogen synthesis for steers fed barley than for those fed maize. In our experiment, the lower starch content in barley-based diet (20.7% of DM) and consequently the lower available energy for microbial protein synthesis could partly explain the lower feed nitrogen efficiency when compared with the maize-based diet. Theurer et al. (1999) showed that a decrease in total starch or rumen degradable starch intake was associated with a decrease in microbial nitrogen flow in dairy cows. Considering that DM intake was not affected by dietary treatments (Table 3) and the lower starch content of barley, the barley-based diet was expected to result a lower starch intake when compared with the maize-based diet.

Total tract digestibility of DM, OM, CP and EE was higher in cows fed the wheat- or maize-based diets than those fed the barley- or potato-based diets (Table 4). Total tract ADF and NDF digestibility were higher in cows fed the wheat-based diet than cows fed the barley-, maize- or potato-based diets (Table 4). Similarly, Silveira et al. (2007) reported higher DM, OM and EE digestibility for cows fed a maize-based diet compared with those fed a barley-based diet. However, CP and NDF digestibility were not affected by experimental diets. In another comparative study with wheat, barley, maize and oats as principal sources of dietary starch, digestibility of DM, OM, and NDF was not affected by dietary treatments, but CP digestibility was higher in cows fed oats-based diet compared with those fed wheat-, barley- or maize-based diets (Gozho & Mutsvangwa, 2008). In the current study, the lower CP digestibility in cows fed the barley- or potato-based diets compared with those fed the wheat-, or maize-based diets might be related to the lower extent of ruminal starch degradation and higher undigested starch reaching the hindgut. Higher levels of undigested starch reaching the hindgut might have promoted more bacterial protein synthesis (0rskov et al., 1970). Because there is no mechanism for hindgut enzymatic digestion of the resultant bacterial protein, it is voided in the faeces, thus reducing total tract CP digestibility (0rskov et al., 1970). In addition, differences in CP digestibility of experimental diets may be due to differences in the digestibility of the cereal grains protein (McAllister et al, 1993).

There were no differences in plasma glucose, total cholesterol, HDL-cholesterol and LDL-cholesterol concentrations between dietary starch sources. However, plasma triglyceride concentrations were higher in cows fed the potato-based diets compared with the wheat-, barley-, or maize-based diets (Table 5). We expected the lower plasma triglyceride concentrations for cows fed the potato-based diet compared with the other diets (especially the wheat- and maize-based diets) because of the lower EE digestibility (Table 4). Contrary to expectations, plasma triglycerides concentration was higher in cows fed the potato-based diet. Similar plasma glucose concentrations were reported in studies of Cabrita et al. (2009) with maize and wheat and Silveira et al. (2007) with maize and barley as the main sources of dietary starch.

 

Conclusions

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