Effects of grass clover-pellets and whole
plant maize-pellets on the feed intake
and performance of dairy cows
W. Knaus
a,*, K. Luger
b, W. Zollitsch
a, H. Gufler
c,
L. Gruber
d, C. Murauer
a, F. Lettner
aaDepartment of Animal Science, University of Agricultural Sciences, Gregor Mendel-Str. 33,
A-1180 Vienna, Austria
bFederal Research Institute for Agriculture, Rottenhauser Str. 32, A-3250 Wieselburg, Austria cUniversity of Veterinary Medicine, VeterinaÈrplatz 1, A-1210 Vienna, Austria dFederal Research Institute for Alpine Agriculture Gumpenstein , A-8952 Irdning, Austria
Received 14 July 1998; received in revised form 5 May 1999; accepted 15 June 1999
Abstract
Twenty dual-purpose Simmental dairy cows were used in a 22 cross-over design to evaluate the effects of feeding, both pelleted vs. ensiled grass clover and whole plant maize on feed intake, milk production and composition, as well as on blood metabolites and some parameters of rumen fermentation. In addition to the pelleted/ensiled forages, all animals were offered grass hay, and, based on the individual milk performance prior to the beginning of the experiment, 1 kg of concentrate for every 2.35 kg energy-corrected milk (ECM) exceeding a daily production of 15 kg ECM. In both groups, the forage was offered so that grass hay, grass clover silage or pellets and whole plant maize silage or pellets would contribute 100, 450 and 450 g kgÿ1to total forage dry matter intake (DMI), respectively. Both the treatment periods lasted for seven weeks. Average daily forage DMI was increased from 12.8 up to 18.6 kg when grass clover and whole plant maize were fed in the pelleted form. ECM production was elevated from 18.3 up to 20.3 kg (p< 0.001). Also, a significant increase was observed for milk protein and lactose level, while content of milk fat and concentration of milk urea were reduced. Rumen pH-values were significantly lower in the pellet-fed cows than in the silage-pellet-fed cows. The acetate to propionate ratio was narrowed down from 3.80 : 1 to 3.54 : 1. Concentration of blood serum urea was significantly reduced (0.8 mmol lÿ1) when pellets were fed. Except for glutamate dehydrogenase (GlDH), all the liver specific enzymes were within normal ranges. The analysis of glucose, protein and ketone bodies in the urine was
81 (1999) 265±277
*Corresponding author. Tel.: +43-1-47654/3285; fax: +43-1-47654/3254
E-mail address: knaus@edv1.boku.ac.at (W. Knaus)
negative for both the dietary treatments. Pelleted grass clover and whole plant maize proved to be superior to their ensiled counterparts in regard to DMI and milk performance, without having a negative impact on the health status of the animals, but extra milk output in relation to the additional feed intake was very low.#1999 Elsevier Science B.V. All rights reserved.
Keywords: Pellets; Feed conservation; Feed intake; Dairy cows; Performance
1. Introduction
The use of pelleted forages in cattle nutrition has never played an important role in Austrian agricultural production. There are several reasons for this: capital investment needed for the dehydration and pelleting facilities far exceeds the costs of other forage preservation methods and, in addition, harvesting with the original water content, artificially drying and, finally, pelleting the forage is highly energy intensive.
Nevertheless, pelleted forages are advantageous feedstuffs from a nutritionist's point of view. It is well established that dry matter intake (DMI) is the cornerstone upon which productive dairy rations are built. One possible way of achieving a high intake of forage dry matter (DM) is the alteration of the physical form of the feedstuffs. Pelleting is one of the most drastic physical changes to which forages can be subjected in commercial practice. The artificial drying of the freshly cut forage and the reduction of the forage bulk through the mechanical treatment of pelleting causes a radical reduction in particle size, promoting a higher voluntary intake and increasing animal efficiency while decreasing the effective fibre content (Van Soest, 1994). The greatest improvement in intake after pelleting is observed in bulky forages of good digestibility (Van Soest, 1994). According to the definition by Moore (1964), pellets usually consist of finely ground forage compressed into small cylinders 0.6±1.9 cm in diameter and 0.6±3.8 cm in length. Modern forage harvesters allow the forage to be chopped into material of 2±3 cm in length and make sure that every kernel is opened when maize plants are chopped. This substrate is fine enough to be formed into a firm pellet without grinding after the process of dehydration.
Beardsley (1964) compiled data in a review of the effects of pelleting on intake, live weight gains and feed efficiency for several different forages fed to calves or yearlings and showed that, calculated on an individual comparison basis and averaged, pelleting a long hay increased the daily feed intake by 25%, increased daily gain by 98% and decreased feed required per unit of gain by 36%. Vandersall and Douglass (1974) found that DMI increased when a combination of whole plant maize-pellets and whole plant maize silage was fed to Holstein cows, but observed no effect on milk production. As a consequence of altering particle size distribution of forage by grinding and pelleting, Thomson and Beever (1979) noted a depression in overall apparent digestibility of organic matter, which was generally greater for grasses (up to 15 percentage units) than legumes (3±6 percentage units).
The purpose of this work was to elucidate the effects of feeding pelleted grass clover and whole plant maize roughages as compared to ensiled on DMI, milk production and composition, as well as on blood metabolites and some parameters of fermentation in the
rumen of dual purpose Simmental dairy cows. Based on the available literature and on practical experience gathered at federal research institutes in Austria, it was hypothesized that dairy cows' voluntary feed consumption would increase when forages were preserved and fed as pellets rather than as silages, which would result in an elevated milk yield and body condition due to an improved level of nutrient supply to the animals. It was also assumed that feeding some long hay in addition to the pelleted forages would help counteract possible detrimental effects on ruminal digestion, therefore allowing for a greater total nutrient uptake from the digestive tract, improving the animals' overall performance.
2. Materials and methods
2.1. Experimental design and diet formulation
Twenty dual-purpose Simmental dairy cows were selected out of a herd of 70 according to their body weight (BW), days in milk (DIM), number of lactations and current milk performance, and assigned to treatments of 10 animals each, arranged factorially in a 22 Latin square design.
At the beginning of the experiment, cows ranged from 39 to 134 days postpartum, and in each group they were balanced as nearly as possible with respect to BW, DIM, number of lactations and milk production, averaging 615 kg, 75, 1.7 and 25.3 kg energy-corrected milk (ECM), respectively. The animals were kept on rubber mats in a tie stall using sawdust as a bedding material.
The ingredients of the diet for the control group were whole plant maize silage, grass clover silage, grass hay and concentrate. Animals from the experimental group were offered feedstuffs of the same origin which were harvested at the same stage of maturity, but the whole plant maize and grass clover were artificially dehydrated and pelleted instead of ensiled.
Grass clover containing about 50% alfalfa was cut, and swaths were either wilted on the field for a few hours and then ensiled as bales, or chopped to lengths of approximately 25 mm using a forage harvester and dehydrated with hot air in a rotary drum drier. Optimal DM content in the silage bales was not obtained because of inclement weather while hauling the grass clover from the field. Maize plants were chopped the same way and either dehydrated in the same drier or ensiled. The chopped and artificially dried whole plant maize and grass clover were compressed without the addition of steam or water into pellets with a matrix press (KAHL, Mod. 34-600). The diameter of the pellets was 15 mm and their length varied between 20 and 45 mm. The bulk density of the grass clover-pellets and the whole plant maize-pellets was 420 and 700 kg mÿ3, respectively.
The concentrate consisted of 250 g kgÿ1maize, 192 g kgÿ1barley, 188 g kgÿ1wheat, 125 g kgÿ1faba bean, 125 g kgÿ1oats, 62 g kgÿ1peas, 50 g kgÿ1soybean meal, 3 g kgÿ1 sodium chloride and 5 g kgÿ1vitamin premix. An extra 100 g of mineral mixture was given each cow daily. In both groups the forage was offered in such a manner that grass hay, grass clover silage/pellets and whole plant maize silage/pellets would contribute 10, 45 and 45% to total forage DMI, respectively.
The average nutrient content of the feedstuffs is shown in Table 1. Chemical composition of feeds and orts was determined as described by Naumann and Bassler (1976). In addition, silages were analyzed for lactic, acetic and butyric acids by gas chromatography (80/120 Carbopack B-DA/4% CARBOWAX1 20 M, Supelco Belle-fonte, PA), and potentiometric determination was carried out for ammonia concentration (type 15 230 3000 ammonia electrode, Mettler-Toledo AG, Greifensee, Switzerland) and pH-value. The DM content and pH-value of the whole plant maize silage and the grass clover silage were 305 g kgÿ1, 4.1 and 257 g kgÿ1, 5.2, respectively. In both silages the proportion of ammonia nitrogen in total nitrogen content was less than 100 g kgÿ1. Based on DM, concentrations of lactic, acetic and butyric acids in the whole plant maize silage and in the grass clover silage were 18.3, 3.5, 0.1 g kgÿ1 and 19.3, 9.0, 0.6 g kgÿ1, respectively. The energy content of the ingredients was estimated based on the gas test (Menke and Steingass, 1987). The grass clover and the grass hay were harvested quite late due to the weather conditions in spring/summer 1995, which led to the high fibre contents.
The first 7 days of each treatment period were used for partial adjustment to the diet by feeding equal amounts of ensiled and pelleted grass clover and whole plant maize, based on DM content, and data collection was done from d 8 to d 49.
All cows were weighed at the onset of the experiment and once a week thereafter. Water was freely available throughout the study except for 3 h prior to weighing. Animals were milked in their stanchions at 0515 and again at 1600 h.
2.2. Data collection
2.2.1. Dry matter intake
Animals were fed individually between 0500 and 0930 and between 1530 and 1845 h during the experiment. Feedstuffs were offered twice a day in the following order: whole plant maize silage/pellets, grass hay, concentrate and grass clover silage/pellets. Daily concentrate amounts of more than 5 kg were split in thirds and fed additionally before
Table 1
Chemical composition of feedsa
Component Whole plant maize Grass clover Grass hay Concentrate Silage Pellets Silage Pellets mixture Crude protein (g kgÿ1) 74 72 197 199 69 159
NEL (MJ kgÿ1) 5.76 5.89 4.65 4.85 4.32 8.66
NDF (g kgÿ1) 478 518 419 458 651 283
ADF (g kgÿ1) 247 239 339 318 378 81 Ether extract (g kgÿ1) 35 25 39 32 23 25 Crude ash (g kgÿ1) 42 40 111 113 69 33 Calcium (g kgÿ1) 4.16 2.87 20.37 20.50 6.52 1.96 Phosphorus (g kgÿ1) 2.87 2.46 3.30 2.75 3.17 4.40 Magnesium (g kgÿ1) 1.96 1.84 3.04 3.00 1.69 1.39 Sodium (g kgÿ1) 0.01 0.03 0.97 0.42 0.24 1.15
aDM basis.
grass hay in the morning. Each forage component was offered for about an hour, except for the grass clover silage/pellets, which remained in the feeding trough until the next feeding time. Forage amounts were gauged to ensure about 5% feed refusal of the total DM offered. The amount of forage components offered was based on the intake during the adjustment period. When there were little or no weigh-backs, forage was offered in increased amounts in accordance with the forage ratios given above. Based on the individual milk performance measured 4 days prior to the beginning of the experiment, 1 kg of concentrate was offered throughout the experiment for every 2.35 kg ECM exceeding a daily production of 15 kg ECM. All feeds offered were weighed individually for each animal, as were weigh-backs at each feeding. Samples of all ingredients, as well as individual refusals, were taken twice a week and composited for 2.5-week periods.
2.2.2. Milk performance and composition
Milk yield from each cow was recorded daily using LactoCorder (FOSS Deutschland, Hamburg, Germany), and samples taken weekly from the morning and evening milkings were analyzed for protein, fat, lactose and urea by an instrumental method employing mid-infrared specific absorption spectroscopy according to System 4000 (FOSS ELECTRIC, Hillerd, Denmark). The counting of somatic cells was performed using the Fluoro-Opto-Electronic-Cell-Counting principle as also described by System 4000. Results from these analyses were weighted by the amount of milk and daily mean values were used for the statistical analysis.
2.2.3. Physiological investigations
Rumen fluid samples (50 ml) were taken via stomach tube through the esophagus from all animals four times per treatment, equally spaced throughout the treatment period, three hours after feeding. At the same time, venous blood samples were drawn from the jugular vein in vacuum tubes containing Fluoride/EDTA (Fa. Greiner, KremsmuÈnster, Austria) for determination of plasma glucose concentration, and in vacuum tubes for serum collection (Fa. Greiner, KremsmuÈnster, Austria) and were centrifuged at 3000g for 10 min. The plasma was harvested and used for colorimetric determination (Ektachem DT 60, Kodak, USA) of glucose while calcium, inorganic phosphate, magnesium, total protein (TP), albumin, urea, total bilirubin (TBIL), aspartate amino transferase (AST), glutamate dehydrogenase (GlDH) and gamma glutamyl transferase (GGT) were analyzed colorimetrically (Ektachem DT 60, Kodak, USA) from the harvested serum within 48 h. Urine samples were also collected on these occasions and pH, glucose, protein and ketone bodies were determined (Combur10-Test1UX, Fa. Boehringer Mannheim, Germany). Rumen fluid pH was measured (glass electrode) before samples were centrifuged at 4000gfor 20 min. Supernatant was diluted with H2O 1 : 10 and analyzed for volatile
fatty acids (VFAs) by gas chromatography (80/120 Carbopack B-DA/4% CARBOWAX1 20 M, Supelco, Bellefonte, PA).
2.3. Statistical analysis
One animal was removed prematurely from the experiment because of a leg condition, and data from this animal was excluded from analysis. Only data collected from day 15 to
day 49 of each treatment were included in the statistical analysis. Statistical analyses were conducted using Model 6 of the LSMLMW computer program (Harvey, 1990). Data were analyzed as a completely randomized 22 cross-over design. The model included animal and treatment effects and their interaction. The statistical model was:
YijkGiTj GTijeijk
where:Gi= group,Tj= animal, (GT)ij= interaction between group and animal, and
eijk= error.
All data are presented as least squares means. Statistical differences were considered to be significant whenP< 0.05.
3. Results
Least squares means, pooled standard errors and probabilities from ANOVA concerning feed intake, milk performance, milk composition and body weight are shown in Table 2. Average daily forage DMI was increased from 12.8 up to 18.6 kg when grass clover and whole plant maize were preserved through the process of dehydration and pelleting instead of ensiling. The same increase was observed in total DMI because the amount of concentrate offered was not adjusted according to the forage DMI. The energy intake from forage in the pellet group equaled the total energy intake in the silage group. The residual NEL intake represents the difference between the total NEL intake and the
Table 2
Effects of feeding ensiled or pelleted grass clover and whole plant maize on dry matter intake, energy and crude protein supply, milk yield, milk composition and body weight change of dairy cows
Item Silage
group
Pellet group
Pooled standard error
Significance (P) Forage DMIb(kg dayÿ1) 12.8 18.6 0.5 <0.001 Total DMIb(kg dayÿ1) 16.5 22.4 0.6 <0.001 NELcintake (MJ dayÿ1) 98.3 131.0 3.8 <0.001 Residual NELcintake (MJ dayÿ1) 1.5 17.0 24.3 <0.001 CPdintake (g dayÿ1) 2145 2945 220 <0.001
CPdbalance (g dayÿ1) 255 819 229 <0.001
Milk (kg dayÿ1) 17.5 20.2 1.0 <0.001
ECMe(kg dayÿ1) 18.3 20.3 1.0 <0.001
Milk fat (g kgÿ1) 42.1 37.9 1.9 <0.001
Milk fat (g dayÿ1) 731 762 48 0.133
Milk protein (g kgÿ1) 36.2 37.5 0.9 0.036
Milk protein (g dayÿ1) 630 755 29 <0.001
Lactose (g kgÿ1) 48.5 48.9 0.4 0.001 Lactose (g dayÿ1) 851 986 55 <0.001 Milk somatic cells (counts103mlÿ1) 101 99 36 0.905 Milk urea (mg 100 mlÿ1) 25.7 18.2 3.25 <0.001 BWachange (kg dayÿ1) 0.12 0.59 0.17 <0.001
aBW, body weight.
calculated sum of NEL requirements for maintenance, milk production and BW change, assuming that 21.5 MJ NEL are required per kg gain. The CP balance is the difference between the total CP intake and the calculated dietary CP demand, according to needs for maintenance and observed milk output. These calculations are based on energy and CP requirements of ruminants (DLG, 1991). The significantly elevated DMI in the pellet-fed group resulted in a calculated residual NEL intake of 17.0 MJ and an average daily CP balance of 819 g as compared to 1.5 MJ and 255 g for the silage-fed group, respectively.
Milk production and composition, except for the number of somatic cells, were significantly affected by the dietary treatment. Average daily ECM yield was elevated from 18.3 up to 20.3 kg. An increase was also observed for milk protein and lactose content and yield, while percentage of milk fat and concentration of milk urea were reduced. Even though milk fat content dropped from 42.1 to 37.9 g kgÿ1when pelleted forages were fed, daily milk fat yield was still increased from 731 to 762 g dayÿ1 (P= 0.133). The nutrient supply through the use of pelleted forages resulted in an average daily gain of 590 g as compared to 120 g for the control group.
The measurements of the ruminal fluid pH in general showed unexpectedly high levels (Table 3). Rumen pH-values observed in the pellet group were significantly lower than those in the silage group. Feeding pelleted forages markedly increased the concentration of acetate and propionate in the rumen contents, but ratio of acetate to propionate was still narrowed down from 3.80 : 1 to 3.54 : 1, because concentrations of these two VFAs were not elevated in the same proportion. N-butyrate content in the rumen fluid was significantly increased, whereas the level of iso-butyrate was reduced to half when the silage components in the diet were replaced by pelleted ingredients.
Results from the blood analyses are summarized in Table 4. Concentration of serum urea was significantly reduced (0.8 mmol lÿ1) as a result of the change from ensiled to pelleted diet constituents. Cows fed forage pellets had a markedly higher content of the liver specific enzymes AST and GlDH in the serum. No effect was seen on the amount of circulating serum GGT. Serum concentrations of TBIL, TP and albumin, calcium, inorganic phosphate and magnesium were within a normal range in both groups. Plasma glucose was 0.4 mmol lÿ1higher when cows were offered pelleted forage components. The analysis of glucose, protein and ketone bodies in the urine was negative for both dietary treatments.
Table 3
Results of ruminal characteristics of dairy cows fed ensiled or pelleted grass clover and whole plant maize Item Silage group Pellet group Pooled standard
error
Significance (P)
pH 7.30 6.90 0.42 <0.001
Acetate (mmol lÿ1) 52.1 67.6 15.3 <0.001
Propionate (mmol lÿ1) 13.9 20.1 4.7 <0.001
Acetate : propionate ratio 3.80 3.54 1.06 0.245
n-butyrate (mmol lÿ1) 8.3 13.2 7.7 <0.001
Iso-butyrate (mmol lÿ1) 1.0 0.5 0.3 <0.001
4. Discussion
As expected, the method of conservation caused no noteworthy difference in chemical composition between whole plant maize silage and pellets. However, the weather conditions and the harvesting procedure resulted in a higher ADF content in the grass clover silage as compared to the pellets. There is no plausible explanation as to why the NDF was lower in the grass clover silage than in the pellets. An increase in fibre content ought to be expected in field cured silages in any case, due to higher harvest losses of finely structured plant material (leaf shatter) through the baling of the slightly wilted forage. The net energy content of the ensiled grass clover was slightly lower as compared to the pelleted.
Production responses achieved in this feeding trial confirmed the advantages of using pelleted forage components in dairy cow rations, even though the overall advantages of pelleting forages are questionable. Efficiency of marginal utilization of additional energy was very low. Even though forage DMI and consequently total DMI were elevated about 6 kg in the pellet-fed group, the extra milk output was only 2 kg ECM.
It is generally conceded that pelleted forage is consumed in larger amounts than the same forage in a long or coarsely chopped form (Moore, 1964), but it has to be assumed that the drastic difference in forage DMI between the pellet group and the silage group was, at least in part, caused by the difference in quality between the ensiled and the freshly cut grass clover and the immediately dehydrated grass clover substrate. The ensiled grass clover was not allowed to wilt sufficiently, which was partially responsible for the suboptimal fermentation of the silage bales. As a result, this silage had a relatively low DM content and high ph-value, thus reducing DMI (Van Soest, 1994).
Several authors reported that the benefit in feed consumption obtained by pelleting a high-quality roughage is less than that obtained by pelleting a low-quality roughage (Minson, 1962; Beardsley, 1964; Moore, 1964; Campling and Freer, 1966; Shaver et al., 1986). This appears logical since one would expect an animal to eat more of a highly palatable forage than of an unpalatable one (Beardsley, 1964). On the other hand, if the quality of the forage is so poor that some nutritive deficiency exists, protein for instance, it would be unlikely that pelleting would increase consumption (Moore, 1964). For the
Table 4
Effects of feeding ensiled or pelleted grass clover and whole plant maize on serum urea, ASTa, GlDHb, GGTc
and plasma glucose concentrations
Item Silage group Pellet group Pooled standard error
Significance (P)
Urea (mmol lÿ1) 4.3 3.5 0.4 0.005
ASTa(U lÿ1) 30.4 33.9 4.2 0.008
GlDHb(U lÿ1) 10.0 15.5 6.8 0.007
GGTc(U lÿ1) 20.6 20.0 4.6 0.430
Glucose (mmol lÿ1) 2.8 3.2 0.6 0.128 aAST, aspartate amino transferase.
bGlDH, glutamate dehydrogenase. cGGT,
gglutamyl transferase.
study in hand it can be assumed that the palatability of the grass clover silage was negatively affected due to the harvest and conservation conditions. Van Soest (1994) stated that the degree of improvement in intake after pelleting varies depending on the nature of the feed, and is observed to be greatest in bulky forages of good digestibility (e.g., grass hay).
Although the consumption rate of pelleted compared with chopped forage is faster (Thomson and Beever, 1979), the possible disadvantage for the silage group in terms of DMI due to differences in consumption rate may have been circumvented by the particular feeding regimen in this experiment, because the grass clover silage/pellets were offered as the last diet component at each meal and refusals from this diet component were weighed before the next feeding.
It has to be pointed out that long grass hay and concentrates were fed along with the forage pellets in the experiment in question. According to Moore (1964), there is a strong suggestion in the literature that the feeding of a small proportion of long hay with a pelleted ration may be beneficial. Vandersall and Douglass (1974) showed that feeding both whole plant corn-pellets and corn silage along with concentrate resulted in a greater DM consumption than when either of the forage forms were fed separately. Weir et al. (1959) reported that pelleting was particularly advantageous in regard to feed intake when straight roughage rations were compared. Similar observations were made by Johnson et al. (1964). Montgomery and Baumgardt (1965) observed a decrease in daily DMI from a completely pelleted ration consisting of alfalfa through the inclusion of ground shelled maize fed to heifers and lambs. They concluded that ruminants will adjust voluntary food intake in relation to physiological demand for energy if fill or rumen load does not limit their consumption, which is contradictory to the calculated residual net energy intake data observed in this experiment (see Table 2).
The low efficiency of utilization of the additional energy consumed in the pellet-fed group could be attributed to a reduction in digestibility of grass clover and whole plant maize due to the alteration of particle size distribution by pelleting. Thomson and Beever (1979) reported that less organic matter and energy are digested in the rumen, and that total apparent digestibility of pelleted forage declines as a consequence of an increased rate of fermentation and fractional outflow of particulate matter from the rumen. They also stated that the depression in overall apparent digestibility of organic matter is generally greater for grasses (up to 15 percentage units) than legumes (3±6 percentage units), as the structural carbohydrate content of grasses is higher than that of legumes. Likewise, Boucque et al. (1973) found that the digestibility (determined with wethers) and the net energy content of whole plant maize-pellets were generally lower than those of whole plant maize silages, resulting in a more efficient utilization of the DM of whole plant maize silages as compared to the pelleted form.
Besides an elevation of ECM production of 2 kg, the use of grass clover-pellets and whole plant maize-pellets led to an average daily gain of 0.59 kg compared to 0.12 kg in the silage-fed group. This can be interpreted as a result of a nutrient increase that could have been partitioned toward the mammary gland and transformed into milk performance by dairy cows with a higher genetic potential.
Keith et al. (1961) and Ronning et al. (1959) achieved maximum hay consumption and highest milk production, when hay was fed in a pelleted form as the sole source of
nutrients. Even though Burgstaller and Averdunk (1972) reduced the amount of concentrate in the diet as the forage intake rose, they observed a significant increase in milk performance by offering artificially dried grass in pellet form. In contrast, the use of various forms of alfalfa hay as the sole source of roughage by Porter et al. (1953) resulted in a reduced DMI accompanied by lowered milk production during the feeding of the pellets. Shaver et al. (1986) observed that the physical form of forage had no effect on intake or milk production when cows were fed long, chopped, and pelleted prebloom alfalfa hay (60 : 40 hay : grain ratio).
The feeding of pelleted forage components significantly reduced the fat content and -due to the marked increase in energy intake - increased the protein content of the milk. These findings are in accordance with the observed narrowing of the acetate : propionate ratio and the drop in milk and blood serum urea concentrations. The difference in the acetate : propionate ratio is rather small, and it should be taken into account that the concentrations of the VFAs were not measured over time but obtained from several single samplings three hours after feeding during the experiment. The numerically small but significant increase of the lactose percentage is presumably the result of the surplus energy supply. Milk fat depression has been described frequently in the literature when pelleted forages were used in dairy rations (Porter et al., 1953; Keith et al., 1961; King and Hemken, 1962; Moore, 1964; Eriksson et al., 1968; Burgstaller and Averdunk, 1972; Vandersall and Douglass 1974; Woodford and Murphy, 1988), and can be (among other reasons) a result of diets deficient in coarse roughage (Van Soest, 1994). Contrary to this, Putnam and Davis (1961) reported that pelleted complete rations can be fed to lactating cows without depressing the milk fat content. Ronning et al. (1959) also observed no effect on milk fat content when concentrate was offered in conjunction with pelleted or chopped alfalfa hay to dairy cows.
The pH-value of the rumen content three hours postfeeding was 7.3 in the silage-fed and 6.9 in the pellet-fed group, and was inversely proportional to the concentrations of acetate, propionate and n-butyrate. The high level of the recorded pH in both groups possibly reflects a contamination with saliva while rumen fluid samples were taken. The feeding of pelleted forage results in less mastication and therefore in less salivary net flow (Van Soest, 1994), which ultimately reduces the secretion of buffer salts into the rumen (Moore, 1964). Cullison (1961) also offers inadequate secretion of saliva as an explanation for the significantly lower rumen pH levels found among those steers receiving only pellets as compared to those that were fed approximately 0.7 kg of oat straw per day along with the pellets.
Additionally, it must be kept in mind that a more rapid ruminal fermentation might have occurred in the pellet-fed cows, resulting in a more rapid production of organic acids, therefore intensifying the consequent lowering of pH (Moore, 1964). The concentrations of the most relevant VFAs in the rumen fluid were elevated in the pellet group in this experiment. However, the relative proportion of acetate was slightly reduced and that of propionate was increased, even though the change of the acetate : propionate ratio was not significant. Similar trends were described by several authors (Montgomery and Baumgardt, 1965; Eriksson et al., 1968; Shaver et al., 1986; Woodford and Murphy, 1988; Van Soest, 1994). Van Soest (1994) stated that there is an increase in energy efficiency when the acetate : propionate ratio in the rumen is decreased, and Jenkins and
Thonney (1988) observed a positive effect on the weight gain of lambs by adding propionate even when metabolizable energy was held constant. Data from the study in question suggest a reduction in energy efficiency and/or an overestimation of the increase in energy supply when grass clover and whole plant maize were fed in a pelleted form. érskov and MacLeod (1993) concluded that the utilization of absorbed energy or metabolizable energy (ME) is unlikely to be influenced by the proportions of VFAs, and that differences in utilization of ME between roughages and concentrate can probably not be attributed to differences in the proportions of VFAs produced.
Cows fed pelleted forages had a significantly higher content of the liver specific enzymes AST and GlDH, and a significantly lower urea concentration in the serum, but except for GlDH they were all within the normal range. Albrecht and Unglaub (1992) found a highly positive correlation between GlDH concentration and milk performance in cows showing no metabolic (liver) disorders. The markedly elevated GlDH content in the serum of the pellet-fed animals might be a consequence of the increased milk yield in this experiment. Results of the analysis of glucose, protein and ketone bodies in the urine as well as AST, TBIL, GGT, and albumin in the serum suggest that the use of pelleted forages did not have any detrimental effects on the overall health of the cows.
In conclusion, the present study has demonstrated that the use of grass clover-pellets and whole plant maize-pellets, along with concentrate and about 10% unchopped grass hay, based on the forage DMI, results in a significant increase in feed intake and consequently in an elevated milk production in dual purpose Simmental dairy cows as compared to the use of ensiled grass clover and whole plant maize. However, it should be pointed out that the extra milk output in relation to the additional DMI was very low, indicating a very low efficiency of utilization of the supplementary energy. Since pelleting is also a much more expensive method of conservation than ensiling, it is unlikely that the value of the increased milk yield is higher than the cost of the additional feed and processing expenses.
Further research is needed to determine the optimal ratio of pelleted to ensiled forages necessary in the diet to improve energy efficiency and to avoid a possible milk fat depression, and at the same time minimize the use of cereal grains. Future experiments using pelleted forages should also be conducted on dairy cows with a higher potential for milk production, thus increasing the emphasis on milk synthesis rather than on weight gain.
Acknowledgements
The authors want to express their gratitude to Kathleen Knaus for her valuable contributions to this paper.
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