Effect of concentrate type on rumen fermentation
and milk production of cows at pasture
H. Khalili
a,*, A. Sairanen
baAgricultural Research Centre of Finland, Animal Production Research, FIN-31600 Jokioinen, Finland bAgricultural Research Centre of Finland, North Savo Research Station, FIN-71750 Maaninka, Finland
Received 11 May 1999; received in revised form 31 January 2000; accepted 10 February 2000
Abstract
The experiment was conducted with 15 Autumn-calving Holstein±Friesian cows, three of which were ®tted with rumen cannula, that were allowed to access a perennial timothy and meadow fescue sward between 10 June and 12 August 1996. The control treatment (C) consisted of pasture alone. For the other two treatments, cows were fed 4 kg per day of either a concentrate supplement consisting of barley (B) or a treatment (M) containing (g/kg) barley (200), oats (200), wheat bran (220), wheat (110), wheat syrup (60) and molassed sugar beet pulp (200). The experiment was carried out as a replicated (n5) 33 Latin square design. The grazing area was divided into 15 paddocks and cows were grazed on a 1 to 2-day rotation. There was no treatment effect on rumen pH but the molar proportion of acetic acid in rumen ¯uid was higher (p<0.05) in cows grazing pasture alone (C). Ammonia N concentration was lower for M than B (p<0.01). Concentrate supplementation increased milk yield (p<0.001), the extent of which was greater for M than B (2.6 versus 1.3 kg per day;p<0.05). Protein yields increased (p<0.001) due to treatments B (51 g per day) and M (115 g per day). These results indicate that barley is a less suitable concentrate supplement than a mixture, formulated from a range of ingredients since energy corrected milk yield (ECM 20.5 versus 19.2 kg per day), protein (729 versus 665 g per day) and lactose (1000 versus 933 g per day) yields were greater (p<0.01) in treatment M than B.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Barley; Concentrate; Milk production; Pasture; Rumen fermentation 84 (2000) 199±212
*Corresponding author. Tel.:358-3-41883663; fax:358-3-41883661.
E-mail address: hannele.khalili@mtt.® (H. Khalili)
1. Introduction
Grassland is generally considered to be a low cost agricultural resource and therefore milk production is often based on grazing in many countries. In Finland, grazed grass which is also less expensive than other feeds typically accounts for 25% of the energy intake derived from forages in lactating dairy cows. Despite the grazing season being much shorter than indoor feeding, an ef®cient grazing strategy is one way to reduce production costs and increase milk production per hectare. Currently, a signi®cant amount of milk from grazed grass is produced by Autumn-calving cows in mid or late lactation. This is in contrast to earlier years when grazing cows were Spring-calving. Ettala et al. (1986) reported results from several Finnish grazing experiments (1969± 1979) concerning milk production and composition of Spring-calving cows that grazed pasture alone or supplemented with barley. They concluded that Spring-calving cows grazing good quality pasture without supplementary feeding could produce 22±25 kg milk per day. Furthermore, for cows producing in excess of 10 kg milk per day, feeding barley at proportionately 0.66 of energy requirements was reported to elicit a mean milk yield of 1.6 kg per day.
However, in many situations, current on-farm grazing feeding practices could be more ef®cient and there is scope to improve the utilization of pasture-based feeding systems for Autumn-calving cows. In order to achieve this target, it is necessary to predict marginal milk production and composition associated responses with concentrate feeding. Use of milled cereals are, for example, cheaper than commercial compound feeds, but the overall pro®tability of feeding either concentrate is dependent on their effects on milk production, and to a lesser extent milk composition. The effects of different types of supplements on milk yield and composition are often mediated via their in¯uence on rumen fermentation. According to Thomas and Chamberlain (1984) milk fat yield has consistently been increased by rumen acetate and butyrate production, but reduced by that of propionate. In Finland, winter feeding systems are most commonly based on restrictively fermented grass silage supplemented with starch rich (barley/oats) concentrates. This type of feeding generally produces a rumen fermentation pattern characterized by high molar proportions of butyrate and low proportions of propionate (Huhtanen, 1988; Jaakkola and Huhtanen, 1993). Low production of propionate is often associated with a milk production constrained by the supply of glucose. Replacing barley with unmolassed sugar beet pulp (Huhtanen, 1988) or barley ®bre (Huhtanen, 1992) has decreased the molar proportions of butyrate and increased that of propionate. In addition, a mixture of by-products has been reported to improve milk and protein yields relative to that of barley (Huhtanen, 1987). There were, however, no differences in milk yield (Sloan et al., 1988) or milk constituent yields of cows fed with grass silage supplemented with different concentrate energy sources (Huhtanen et al., 1995).
Information concerning the rumen fermentation pattern of grazing dairy cows is limited. There is also a need to establish the effects of different energy supplements that may result in a propionate rich rumen fermentation compared to that of barley in grazing cows. This study was conducted to evaluate the effects of grazing alone, or pasture and additional barley and concentrate mixture supplements on rumen fermentation and milk production.
2. Materials and methods
2.1. Animals, treatments and experimental design
The experiment was carried out using a perennial timothy (Phleum pratense) and meadow fescue (Festuca pratensis) sward from 10 June to 12 August 1996. The sward received a total of 200 kg N haÿ1, 23 kg P haÿ1and 40 kg K haÿ1per year as a compound fertilizer (NPK). Fertilizers were applied as three dressings on 14th May before the grazing season started, from 20th to 30th June and from 2nd to 8th August post grazing. The rest period before subsequent grazing was 3 weeks in Spring and 4 weeks in Autumn. Pastures were mechanically clipped to a height of 10 cm in June. Fifteen mid and late lactating Holstein±Friesian cows, three of which were rumen cannulated, were used. At the beginning of the experiment, cows were 171 days into milking (S.D. 25). Cows grazed rotationally as a single herd and were inside only twice during daily milking (07:00 and 16:00 hours). The grazing area was divided into 15 paddocks. During the sample collection periods cows were allocated an average daily area of 0.024 ha supplying an excess of 40 kg DM per day. All paddocks were grazed on an average of three occasions (1±5) during Summer. Sward heights were estimated to be 44 cm before grazing. This might appear to be a high value, but timothy and meadow fescue are tall grasses with highly digestible stems when grown under Nordic conditions. The control treatment (C) consisted of pasture alone. For the other two treatments, cows were also given a concentrate supplement (4 kg per day), fed as two equal meals during milking at 07:00 and 16:00 hours, respectively. Treatment B consisted of rolled barley while treatment M consisted of a pelleted concentrate mixture (Raisio Feed Ltd) formulated (g/kg) from barley (200), oats (200), wheat bran (220), wheat (110), wheat syrup (60), molassed sugar beet pulp (200) and NaHCO3(10). All cows were given ad libitum access to a mineral mixture containing (g/kg) Ca (160), P (64) and Mg (80). The experiment was conducted as a replicated (n5) 33 Latin square design, with a 21-day experimental period, comprised of a 14-day adjustment period and a 7-day sampling period. Cows were allocated to squares according to milk yield and treatments were randomly assigned to each cow within a square.
2.2. Measurements and analytical procedures
(1990) methods and neutral detergent ®bre (NDF) according to Robertson and Van Soest (1981). Grass samples were also analysed for water-soluble carbohydrates by the method of Nelson (1944) incorporating the modi®cations of Somogyi (1945) and for in vitro organic matter (OM) digestibility the cellulase method using the activity of 20 ®lter paper unit per sample was used (Friedel and Poppe, 1990). Milk samples were collected for over four consecutive milkings on Days 17 and 18 and analysed for fat, protein and lactose with an infrared milk analyser (Milcoscan 605) and urea by the method of McCullough (1967). Samples of rumen ¯uid were collected from three cannulated cows on pasture at 07:00, 08:30, 11:00, 13:00, 16:00, 17:30, 20:00 and 22:00 hours on Day 19. Rumen pH was measured immediately using an Orion 410A pH meter. Rumen ¯uid samples were ®rst strained through one layer of cheese cloth. Then 0.5 ml saturated mercury(II) chloride and 2 ml 1 M sodium hydroxide were added to 5 ml of rumen ¯uid. Samples were stored frozen until analysis. Before analysis, each 0.5 ml sample was acidi®ed with 0.25 ml of formic acid (Riedel-de-Haen 33015), diluted to 5 ml with distilled water and centrifuged at 2000g for 10 min. Concentrations of volatile fatty acids (VFA) were measured by a Hewlett Packard (HP) model 5890 gas chromatograph (HP, Avondale, USA) ®tted with an autosampler HP7673, a ¯ame ionization capillary column (HP-FFAP 10 m0.53 mm1mm; HP, USA). The sample (1 ml) was injected into the column, and helium was used as a carrier gas at a ¯ow rate of 9 ml/min. The column temperature was programmed from 60 to 788C at 258C/min, then isothermal for 1 min, then 7.58C/min to 1508C and ®nally at 258C/min to 1808C with a ®nal time of 3 min. The injector was operated in split mode (split vent ¯ow 45 ml/min). The injector and detector temperatures were 220 and 2608C, respectively. The results were calculated based on an external standard method. Ammonia N was analysed according to McCullough (1967). The number of protozoa of pooled (within cow) rumen samples was counted after mixing with methyl green-formalin-saline solution, using a counting chamber (Fuchs-Rosenthal, Fortuna, Germany). The effect of treatments on the rate of disappearance of DM of grass hay from nylon bags was determined with the experimental rumen cannulated cows using the nylon bag technique as described by Huhtanen (1988). Nylon bags were incubated in the rumen for 6, 12, 24, 48, 72, 96 and 120 h. Cows were weighed on two consecutive days at the beginning and end of each period.
2.3. Calculation of results and statistical methods
Disappearance of DM from nylon bags was explained by the equation of McDonald (1981):Pab(1ÿeÿct), wherePis the proportionate amount of DM disappearing from bags after time t (h) and a, b and c are constants that represent the rapidly soluble fraction, the potentially degradable fraction and the rate of degradation, respectively. Animal production data were analysed by Statistical Analyses System (Littell et al., 1992). The model used was
yijklmSiA SijPkDlSPikSDilfijkl
wherem is the overall mean, S,A(S), Pand SP and SDare the random effects of square, animal within square, period and squareperiod and squaretreatment interactions,Dis the ®xed effect of treatment andfijlkis the random error term.
A mixed model was used for rumen fermentation data
yijkl mAiPjDkfijkTlPTjlDTklATileijkl
wheremis the overall mean,AandPare the random effects of animal and period, andD is the ®xed effect of treatment;fijk is the random error term for whole-plot error (mean square of the whole-plot) andTis the ®xed effect of time,PTandATare the random effects of periodtime and periodanimal and DT is the ®xed effect of treatmenttime, eijkl is the random error term for sub-plot error (mean square of the sub-plot). The random variables,A,P,fijk,PT,ATandeijklare all assumed independent and normally distributed with zero means and variances s2A, s2P s2f s2PT s2AT and s2e, respectively. When testing the dietary effectsDon mean values of rumen characteristics the whole-plot error mean square was used as an error variance in the F-test. When testing the interactions betweenDand sampling time on post-prandial values of rumen characteristics the sub-plot error mean square was used as an error variance in the F-test. Rumen ¯uid (pH, ammonia, VFA) data were analysed using analysis of variance for repeated measurements. The analysis used the split-plot approach with the Greenhouse± Geiser approximate (conservative) signi®cance tests (Littell et al., 1992). Statistical signi®cance of treatments were further separated into single degrees of freedom using two orthogonal contrasts: (1) effect of supplement (C versus BM) and (2) effect of the type of energy supplement (B versus M).
3. Results
The chemical composition of grass during each collection period and the mean composition of supplements are presented in Table 1. Values of NDF and in vitro OM digestibility were similar between periods, but crude protein content was lowest during period one. Water soluble carbohydrate concentration varied between periods (opposite trend to crude protein) but this was not re¯ected in the in vitro digestibility measurement. There were no treatment effects on mean rumen pH and total VFA concentrations (Table 2). Grazing alone increased the molar proportion of acetate (p<0.05) compared to concentrate treatments. The type of supplement had no effect on rumen fermentation pattern, but ammonia N concentration was higher with treatment B. Supplementation increased the numbers of Holotricha (p<0.01) compared with grazing alone. The potential degradability (ab) of hay incubated in the rumen (Table 2) was not affected by treatments. There were no signi®cant treatment effects on the rate constant (c) for degradation of componentb.
Table 1
Chemical composition (g/kg DM) of sward herbage and concentrate supplementsa
Collection period Concentrate supplement
1 2 3 Barley Mixture
Dry matter (g/kg) 194 190 175 873 873
In the dry matter (g/kg)
Ash 66 80 91 24 44
Crude protein 180 207 240 126 127
NDF 591 578 577 245 350
Ether extract 27 27 33 28 31
WSC 145 106 91 nd nd
Starch and sugars (g/kg DM) 600 366
In vitro OM digestibilityb 0.779 0.757 0.776 nd nd stocking rate (ha per cow per day) 0.02 0.02 0.03
herbage mass kg DM/ha 2630 2330 2840
and: Not determined; NDF: neutral detergent ®bre; WSC: water soluble carbohydrates; OM: organic matter. bCellulase method was used.
Table 2
Effect of different treatments on rumen fermentation parameters and degradation parameters (a,b and c) of grass hay
Treatmenta S.E.M. Significance of effect
C B M C versus BM B versus M
pH 6.13 6.17 6.01 0.114
NH4-N (mmol/l) 16.9 18.9 12.8 0.42 **
Total VFA (mmol/l) 127 127 132 3.6
Molar proportion of VFA's (mmol/mol)
Acetic acid (A) 659 641 640 3.5 *
Propionic acid (P) 190 191 205 5.4 Butyric acid (B) 111 125 116 4.4
(AB)/P 4.06 4.07 3.72 0.147
Protozoal numbers (104/ml)
Total 147 173 166 29.6
Holotricha 13 16 17 0.3 **
Entodiniomorph 134 158 149 29.9
Degradation parameters of hay
a(mg/g) 266 278 328 21.0
b(mg/g) 576 565 538 22.5
ab(mg/g) 842 843 866 10.3
c(1/h) 0.052 0.053 0.037 0.0067
*p<0.05;**p<0.01.
aC: Grazing only; B: barley; M: concentrate mixture.
Concentrate supplementation signi®cantly increased (p<0.001) milk yield, milk lactose content and yields of protein and lactose compared to pasture alone (Table 3). Concentrations of milk fat and urea were higher for treatment C. Milk yield (p<0.05) and yields of protein (p<0.01) and lactose (p<0.05) were higher for treatment M than B. Supplements had no signi®cant effect on live weight.
4. Discussion
Grazing trials are often conducted as continuous trials, but several recently published studies with grazed or freshly cut grass have been carried out as a change-over design. There were differences between periods in grass crude protein content but it should be noted that even the lowest value of 18.0% was suf®ciently high to provide adequate rumen degradable protein for microbial growth. Furthermore, Delagarde et al. (1997) reported no interactions between N application and concentrate supplementation despite a markedly different crude protein content (13.5 versus 21.2%) of grass. Rumen fermentation parameters, pH, concentration of VFA and proportions of acetate, propionate and butyrate were similar between periods. Rumen ammonia N concentration was lower during the ®rst period because of a lower herbage crude protein content. It should, however, be pointed out that the mean rumen ammonia concentration (9.3 mmol/ l) during this period was much higher than microbial requirements (Satter and Slyter, 1974). It can be argued that pasture management is a more critical factor than the effects due to experimental period. In the current study grass was offered ad libitum to cows during all periods, and was associated with a mean increase in live weight from 588 to 597 kg. It is unlikely that there were signi®cant interactions between treatments and grass
Fig. 1. (Continued).
quality since no differences in in vitro organic matter digestibility were observed between periods. Small residual errors for production parameters also tend to support this suggestion. Therefore, treatment effects were considered to be due to differences between treatments rather than due to differences between periods.
4.1. Rumen fermentation
Information concerning rumen fermentation patterns of dairy cows in pastures comprised of timothy and meadow fescue swards is limited. In the present trial with cows receiving only pasture, the mean proportions of acetic (659), propionic (190) and butyric (111 mmol/mol) acids were consistent with observations from recent studies in which pasture was the sole feed (Khalili et al. unpublished), where the values of acetic, propionic and butyric acids were 648, 182 and 124 mmol/mol, respectively. In the present trial, a high herbage allowance allowed animals the opportunity to select the most digestible parts of the grass in each period. However, rumen fermentation patterns were similar between periods. Furthermore, coef®cients of variation for individual VFA's were very low (acetate cv 0.95, propionate cv 4.81 and butyrate cv 6.47) suggesting that there were no interactions between period and treatment. Rumen ecologies were also generally very similar between treatments because there were no signi®cant differences in mean rumen pH values, post-prandial changes in rumen fermentation patterns or hay degradation characteristics. There was, however, a minor decrease in the mean molar proportion of acetate between the control and concentrate treatments which may have
Table 3
Effect of different treatments on milk production and live weight
Treatmenta SEM Significance of effect
C B M C versus BM B versus M
Milk yield (kg per day) 18.4 19.7 21.0 0.38 *** *
ECMbyield (kg per day) 18.3 19.2 20.5 0.42 ** *
Milk composition
Fat (g/kg) 41.2 38.5 37.6 0.42 ***
Protein (g/kg) 34.2 34.2 34.9 0.35 Lactose (g/kg) 46.8 47.6 47.5 0.12 ***
Urea (mg/100 ml) 40.0 36.3 37.6 0.60 ***
Yield of milk constituents
Fat (g per day) 734 741 779 18.6
Protein (g per day) 614 665 729 15.2 *** **
Lactose (g per day) 846 933 1000 19.2 *** *
Live weight (kg) 590 592 594 2.5
*p<0.05;**p<0.01;***p<0.005.
been caused by a higher herbage intake for the control diet. A low level of concentrate supplementation was used in the current study to ensure that pasture would be the major source of nutrients for Autumn-calving cows. Supplementation increased the numbers of Holotricha protozoa, but not the total number of protozoa. van Vuuren et al. (1986) and Dillon et al. (1989) reported that concentrate type had little effect on VFA composition on grass-based diets. With grass silage-based diets, supplements of barley ®bre resulted in a higher molar proportion of propionate than barley (Huhtanen, 1992). In the present study with pasture-based feeding the mean ratio of (acetatebutyrate)/propionate (3.95) was much lower than a value of 4.85 reported for grass silage-based diets (Huhtanen, 1998). In agreement with data of van Vuuren et al. (1986), rumen ammonia concentration peaked in the late evening when pH was at the lowest level, which was probably the result of considerable grazing activity after evening milking. Intake and eating behaviour were not measured in the present experiment, but other studies at this centre have indicated that grazing is most intense after evening milking (Khalili et al. unpublished). Ammonia N values were generally high with all attributable treatments due to a high grass N content, and substantial degradation of herbage protein in the rumen (van Vuuren et al., 1986). Ammonia N concentration was higher with barley compared to treatment M, a ®nding in agreement with Stakelum (1993). In contrast, van Vuuren et al. (1986) did not ®nd any difference between starch and ®brous type concentrates on rumen ammonia concentra-tion. These different results may be in¯uenced by differences in starch type and feeding level, since van Vuuren et al. (1986) used tapioca and maize fed at higher levels.
Higher ammonia N concentrations with barley supplement compared to concentrate mixture tend to suggest a less ef®cient microbial N capture that implies microbial protein production was lower for barley. Consequently, it appears that the rapid fermentation of barley starch did not improve the utilization of grass N in the rumen. Chamberlain and Choung (1995) concluded that there is no convincing evidence of a need for close synchronization of energy and nitrogen release in the rumen to ensure ef®cient synthesis of microbial protein. Grass silage based diets supplemented with barley have been associated with a much larger protozoal population than grass silage based diets supplemented with barley ®bre (Huhtanen, 1992) or with sugar beet feed (Rooke et al., 1992). This increase in protozoal number has been one reason for higher rumen ammonia N concentration with barley compared to ®brous supplement (Huhtanen, 1988; Rooke et al., 1992). In the present trial, the higher rumen ammonia N concentration is unlikely to be explained by changes in protozoal population since total numbers were not signi®cantly different between supplements.
4.2. Animal performance
Both supplements increased milk, protein and lactose yields but there were differences between cow performance depending on the type of supplement. Earlier studies in Finland with Spring-calving cows (Ettala et al., 1986) reported 0.30 kg more fat corrected milk/kg barley when 4 kg barley per day was fed. A similar response was found in the present trial with Autumn-calving cows (0.23 kg ECM/kg barley). This low response is also in agreement with 0.40 kg ECM-yield increment/kg supplement reported for cows fed with 4 kg barley on a grass-based diet (SpoÈrndly, 1991). There was, however, a
greater response with the concentrate mixture treatment (0.55 kg ECM/kg concentrate). Barley was rolled while the concentrate mixture was pelleted, and this difference in physical form could partly explain the higher marginal responses elicited by treatment M. High marginal milk production responses have been reported in grazing cows (SyrjaÈlaÈ-Qvist et al., 1996) where increasing the level of a pelleted concentrate mixture (oats, barley, rapeseed, beet pulp, by-products) from 3 to 6 kg resulted in an increase of 0.87 kg ECM/kg concentrate. When the concentrate level was increased from 6 to 9 kg, only a marginal response of 0.17 kg/kg concentrate was obtained.
Although pasture intake was not currently measured, there may have been differences in pasture intake between supplements which may explain the inferiority of barley compared to the concentrate mixture. In addition, a lower milk protein yield suggests a lower energy availability for treatment B. A higher rumen ammonia concentration was associated with a lower milk protein yield supporting the suggestion of less ef®cient nitrogen utilization with the barley supplement. There may have been improved nitrogen supply into the small intestine with treatment M. Huhtanen (1992) reported a greater duodenal non-ammonia N ¯ow in cattle fed ®brous supplement compared to barley. The effects of ®brous concentrates on herbage intake and milk yield have been variable. Several studies have shown higher herbage intakes with ®brous compared to starchy concentrates without an apparent effect on milk yield (Kibon and Holmes, 1987; Stakelum and Dillon, 1988; Fisher et al., 1996). SpoÈrndly (1991) did not ®nd any differences between concentrate types on intake or milk yield, but Meijs (1986) reported an increase in milk yield when starch was replaced with a ®brous concentrate due to increased herbage intake. Huhtanen (1987) reported that with cows given grass silage-based diets, fat corrected milk yield was higher for a concentrate mixture (barley, sugar beet pulp, sugar beet molasses) than barley or sugar beet pulp supplements. The concentrate formulated from various ingredients may have elicited positive associative effects on digestion due to improvements in the balance of nutrient supply that may account for improved milk production responses for treatment M relative to treatment B.
Milk fat content was highest in cows allocated only pasture which was in agreement with several previous studies of Ettala et al. (1986). In contrast, barley has increased milk fat content compared with ®brous supplements in cows fed grass silage-based diets (Thomas et al., 1986; Huhtanen, 1987). This indicates that the effect of barley on the milk composition of cows on pasture is different to that for animals fed grass silage-based diets. The reasons for this are not entirely clear but may be explained by differences in rumen ecology, such as fermentation type and/or protozoal activity. According to a review by Thomas and Chamberlain (1984) milk fat yield has consistently been increased by acetate and butyrate, but reduced by a propionate rich rumen fermentation. For cows fed pasture alone, the higher proportion of acetate was associated with a higher milk fat content.
Barley has been reported to increase both the molar proportions of butyric acid compared with ®brous supplements in cattle fed grass silage-based diets (Huhtanen, 1992). In the present trial, however, there was no difference in the ratio of acetic plus butyric acid to propionic acid. This is one possible explanation for different changes in milk composition induced by barley supplementation of diets based on pasture and grass silage. This also indicates that data from silage experiments are not directly applicable to grazing based milk production systems.
5. Conclusions
The present results showed that concentrate supplementation on pasture increased milk, protein and lactose yields of Autumn-calving cows grazing pasture. Production responses to concentrate were of similar magnitude to those reported for cows in early lactation. Concentrate composition was also important since, treatments M and B elicited responses of ECM of 0.55 and 0.23 kg/kg supplement, respectively. Protein and lactose yields were also higher with a concentrate mixture compared to barley. There were only minor differences in rumen fermentation pattern between treatments but the concentrate mixture treatment had the lowest rumen ammonia N concentration indicating a more ef®cient use of feed nitrogen.
Acknowledgements
The authors are grateful to Raisio Feed Ltd for partial ®nancial support. We would also like to thank the staff of North Savo Research Station for technical assistance and the Animal Production Research laboratory staff for chemical analysis.
References
AOAC, 1990. Of®cial Methods of Analysis, 15th Edition. Association of Of®cial Analytical Chemists. Bircham, J.S., 1981. Herbage growth and utilisation under continuous management. Ph.D. Thesis, University of
Edinburgh, UK.
Chamberlain, D.G., Choung, J.J., 1995. The importance of rate of ruminal fermentation of energy sources in diets for dairy cows. In: Garnsworthy, P.C., Cole, D.J.A. (Eds.), Recent Advances in Animal Nutrition. Nottingham University Press, UK, pp. 3±27.
Delagarde, R., Peyraud, J.L., Delaby, L., 1997. The effect of nitrogen fertilisation level and protein supplementation on herbage intake, feeding behaviour and digestion in grazing dairy cows. Anim. Feed Sci. Technol. 66, 165±180.
Dillon, P., Crosse, S., O'Brien, B., 1997. Effect of concentrate supplementation of grazing dairy cows in early lactation on milk production and milk processing quality. Ir. J. Agric. Food Res. 36, 145±159.
Dillon, P., Stakelum, G., Murphy, J.J., 1989. The effect of level of herbage intake and concentrate type on rumen fermentation pattern, in situ herbage degradability and blood metabolite levels in lactating dairy cows. In: 16th International Grassland Congress, Nice, France, pp. 1155±1156.
Ettala, E., Rinne, K., Virtanen, E., Rissanen, H., 1986. Effect of supplemented concentrates on the milk yields of cows grazing good pasture. Ann. Agric. Fenn. 25, 111±125.
Fisher, G.E.J., Dowdeswell, A.M., Perrott, G., 1996. The effects of sward characteristics and supplement type on the herbage intake and milk production of summer-calving cows. Grass Forage Sci. 51, 121±130. Friedel, K., Poppe, S., 1990. Ein modi®ziertes Zellulase-Verfahren als Methode zur SchaÈtzung der
Verdaulichkeit von Grobfutter. G-4-Bericht, Wilhelm-Pieck UniversitaÈt Rostock, WB TierernaÈhrung, 150 pp. Grainger, C., Mathews, G.L., 1989. Positive relation between substitution rate and pasture allowance for cows
receiving concentrates. Aust. J. Exp. Agric. 29, 355±360.
Hoden, A., Peyraud, J.L., Muller, A., Delaby, L., Faverdin, P., Peccatte, J.R., Fargetton, M., 1991. Simpli®ed rotational grazing management of dairy cows: effects of rates of stocking and concentrate. J. Agric. Sci. Camb. 116, 417±428.
Huhtanen, P., 1987. The effect of dietary inclusion of barley, sugar beet pulp and molasses on milk production, digestibility and digesta passage in dairy cows given silage based diet. J. Agric. Sci. Finland 59, 101± 120.
Huhtanen, P., 1988. The effects of supplementation of silage diet with barley, unmolassed sugar beet pulp and molasses on organic matter, nitrogen and ®bre digestion in the rumen of cattle. Anim. Feed Sci. Technol. 20, 259±278.
Huhtanen, P., 1992. The effects of barley vs. Barley ®bre with or without distiller's solubles on site and extent of nutrient digestion in cattle fed grass-silage-based diet. Anim. Feed Sci. Technol. 36, 319±337.
Huhtanen, P., 1998. Supply of nutrients and productive responses in dairy cows given diets based on restrictively fermented silage. Agric. Food Sci. Finland 7, 219±250.
Huhtanen, P., Jaakkola, S., Saarisalo, E., 1995. The effects of concentrate energy source on milk production of dairy cows given a grass silage-based diet. Anim. Sci. 60, 31±40.
Jaakkola, S., Huhtanen, P., 1993. The effects of the forage preservation method and the proportion of concentrate on nitrogen digestion and rumen fermentation in cattle. Grass Forage Sci. 48, 155±165.
Kibon, A., Holmes, W., 1987. The effect of height of pasture and concentrate composition on dairy cows grazed on continuously stocked pastures. J. Agric. Sci. Camb. 109, 293±301.
Littell, R.C., Freund, R.J., Spector, P.C., 1992. SAS System for Linear Models. SAS series in statistical applications. 3rd Edition, 329 pp.
McCullough, H., 1967. The determination of ammonia in whole blood by a direct colorimetric method. Clin. Chem. Acta 17, 297±304.
McDonald, I., 1981. A revised model for the estimation of protein degradability in the rumen. J. Agric. Sci. Camb. 96, 251±252.
Meijs, J.A.C., 1986. Concentrate supplementation of grazing dairy cows. 2. Effect of concentrate composition on herbage intake and milk production. Grass Forage Sci. 41, 229±235.
Meijs, J.A.C., Hoekstra, J.A., 1984. Concentrate supplementation of grazing dairy cows. 1. Effect of concentrate intake and herbage intake. Grass Forage Sci. 39, 59±66.
Nelson, N., 1944. A photometric adaptation of the Somogyi method for the determination of glucose. J. Biol. Chem. 153, 375±380.
Rooke, J.A., Rymer, C., Maya, F.M., Armstrong, D.G., 1992. Effect of including barley or molassed sugar beet feet in grass silage diets on their digestion by cattle and sheep. J. Sci. Food Agric. 58, 475±483. Satter, L.D., Slyter, L.L., 1974. Effect of ammonia concentration on rumen microbial protein production in vitro.
Br. J. Nutr. 32, 199±208.
Sjaunja, L.O., Baevre, L., Junkkarinen, L., Pedersen, J., SetaÈlaÈ, J., 1990. A Nordic proposal for an energy corrected milk (ECM) formula. In: 27th Session International Committee of Recording and Productivity of Milk Animal, Paris, France, pp. 156±157.
Sloan, B.K., Rowlinson, P., Armstrong, D.G., 1988. Milk production in early lactation dairy cows given grass silage and ad libitum: in¯uence of concentrate energy source, crude protein content and level of concentrate allowance. Anim. Prod. 46, 317±331.
Somogyi, M., 1945. A new reagent for the determination of sugars. Determination of blood sugars. J. Biol. Chem. 160, 61±73.
SpoÈrndly, E., 1991. Supplementation of dairy cows offered freshly cut herbage ad libitum with starchy concentrates based on barley or ®brous concentrates based on unmolassed sugar beet pulp and wheat bran. Swedish J. Agric. Res. 21, 131±139.
Stakelum, G., 1993. Supplementary feeding and herbage intake of dairy cow. Department of Agriculture, Faculty of Dairy Science, U.C.C. National University of Ireland, Diss.
Stakelum, G., Dillon, P., 1988. The effect of concentrate type on the herbage intake of high yielding dairy cows. In: Proceedings of the 12th General Meeting of the European Grassland Federation, Dublin, Ireland, pp. 143±147.
SyrjaÈlaÈ-Qvist, L., Tesfa, A.T., Tuori, M., 1996. The effects of concentrate feeding level on milk yield and composition of rotationally grazed dairy cows. In: Parente, G., et al. (Eds.), Grassland and Land Use Systems. In: Proceedings of the 16th General Meeting of the European Grassland Federation, Grado, Gorizia, Italy, 15±19 September 1996, pp. 633±636.
Thomas, C., Aston, K., Daley, S.R., Bass, J., 1986. Milk production from silage. 4. The effect of the composition of the supplement. Anim. Prod. 42, 315±325.
Thomas, P.C., Chamberlain, D.G., 1984. Manipulation of milk composition to meet market needs. In: Haresign, W., Cole, D.J.A. (Eds.), Recent Advances in Animal Nutrition. Butterworths, London, pp. 219±243. van Vuuren, A.M., van der Koelen, C.J., Vroons-de Bruin, J., 1986. In¯uence of level and composition of
concentrate supplements on rumen fermentation patterns of grazing dairy cows. Neth. J. Agric. Sci. 34, 457±467.
