The effect of urea addition on the preservation of low dry matter oat-vetch silages

C M Guedes, M A M Rodrigues, L F Ala, L M Ferreira, S R Silva and V P Carnide*

CECAV, Department of Animal Science, UTAD, P.O. Box 1013, 5001-801, Vila Real, Portugal
cvmguedes@sapo.pt
*CGB, Department of Genetics, UTAD, P.O. Box 1013, 5001-801, Vila Real, Portugal

Abstract

In the Mediterranean region vetch is usually grown in mixtures with oat. In Portugal oat-vetch forages cut in mature stages are conserved mainly as hay with low feeding value. As alternative to haymaking, these forages are often ensiled with an earlier cut with low dry matter (DM) causing conservation problems and a decrease of the nutritive value of the ensiled crop. The addition of chemicals is a strategie used to overcome these conservation problems. Urea has been successfully used in the preservation of high moisture forages. It is also commonly used to increase nitrogen content and digestibility of fibrous feeds. Thus, the objective of this study was to evaluate the preservative and upgrading potential of urea (60 g/kg dry matter) added to high moisture oat-vetch silages in a laboratory scale trial.

Urea was extensively hydrolysed (more than 90% of added urea). Microbial activity measured by pH and volatile fatty acids (acetate, propionate and butyrate) and lactate production was reduced (P<0.001) by the urea addition. On average, the pH of silages without urea was 5.1 and increased to 8.2 after urea addition while the volatile fatty acids concentration decreased. Relative proportions of fermentation acids changed after urea addition, increasing the acetate and butyrate and decreasing the propionate and lactate proportions. The addition of urea to oat-vetch silages increased (P<0.001) water soluble and ammonium nitrogen of the forage. These nitrogen fractions represented more than 40% of total nitrogen. After urea addition, total nitrogen content of oat-vetch silages increased from 19.0 g/kg DM to 26.5 g/kg DM. Application of urea at a rate of 60 g/kg DM significantly increased (P<0.001) the in situ degradation of neutral-detergent fibre after 48h of rumen incubation (NDFsitu). The NDFsitu was 202 g/kg NDF higher on oat-vetch forages ensiled with urea than on oat-vetch forages ensiled without urea.

Oat-vetch silages can be effectively preserved and upgraded by ensiling with 60 g urea/kg dry matter. Further studies are required to evaluate voluntary intake of these forages.

Key words: cell wall degradation, oat-vetch, silage, urea

Introduction

Vetch, an annual legume with climbing growth habit and high levels of protein, is an extensively sown annual forage legume in the Mediterranean region. Due to its prostate growth it is usually grown in mixtures with small grain cereals facilitating harvesting as small grain cereals provide a support for the climbing ability of vetch (Moreira 1989; Caballero et al 1995). Under marginal environments, with low soil fertility, one of the cereals most used in intercropping with vetch is oat (Moreira 1989; Caballero et al 1995).

In Portugal oat-vetch forages, conserved mainly as hay, are often utilized as a basal diet for beef cattle production. As climate conditions often make haymaking impossible in the latter spring, it is common practice to cut this forages in mature stages. Although high yields of dry matter (DM) might be obtained, the resulting material has low feeding value which is unsatisfactory to cover the maintenance requirements of ruminants. One of the possible approaches to overcome this problem would be the use of these forages for ensiling, with an earlier cut with or without wilting. However, with an early cut forages are often ensiled with low DM causing problems during the conservation of these silages (Bolsen et al 1996) leading to losses of DM and decline of nutritive value of the ensiled crop. Various strategies have been employed to overcome the conservation problems of these forages. One of these is the manipulation of the pattern and extent of fermentation by the addition of chemical or bacterial inoculants.

Urea is a relatively inexpensive and safe product and has been successively used in the preservation of high moisture forages (moisture hays, silages), preventing microbial activity and mould during storage (Tetlow 1983; Bélanger et al 1987; Moore and Lechtenberg 1987). Whole-crop barley (Ørskov et al 1983), whole-crop wheat (Deschard et al 1987; Hill and Leaver 1999) and whole-crop triticale (Guedes et al 2006) were well preserved with urea at levels that varied from 20 to 60 g/kg DM. According to Hill and Leaver (2002), the presence of ammonia nitrogen (NH₃-N), in high concentrations in conserved feeds, inhibits microbial activity during ensiling and subsequent aerobic deterioration.

Urea at levels below 30 g/kg DM has also been used to improve crude protein (CP) concentration of high energy silages (Huber et al 1979; Huber et al 1980).  However, no improvement in digestibility occurs at such low treatment rates (Huber et al 1979; Lomas et al 1982). Results reported by Hill and Leaver (1999) with whole-crop wheat and Guedes et al (2006) with whole-crop triticale indicated that nutritive value after urea treatment was strongly influenced by urea level and moisture content (stage of growth) at harvest. The upgrading effect of urea was inversely related with the plant stage of growth. However, whole-crop plants harvested with low levels of DM (approximately 300 g/kg) treated with higher levels of urea (60 g/kg DM) can be upgraded and preserved. In fact, results reported by Guedes et al (2006) showed that urea applied to triticale whole-crop at beginning of flowering (324 g/kg DM) at a rate of 60 g/kg DM significantly increased the CP content as well as the in vitro degradation of neutral-detergent fibre (NDF). These results agree with those previously obtained with whole-crop wheat by Hill and Leaver (2002).

The objective of this experiment was to analyse the potential preservative and upgrading effect of urea added to mixed oat (Avena sativa L.) and vetch (Vicea villosa L.) high moisture silages.

Materials and methods 

Crop management

The forages were grown on a experimental field at Quinta de Prados, University of Trás-os-Montes and Alto Douro, Vila Real (Lat. 41º19’N, Long. 7º44’W) in 2003/2004. The experimental site is located in the Northeast of Portugal at an altitude of 479 m. The soil had a medium texture (silty-loam soil), a pH of 5.1, and the proportion of organic matter was 2.1%. Average temperature, during the trial (between September and April), was 11ºC, with an average minimum temperature of 3.0ºC in February, and a maximum medium temperature of 27ºC in September. Average precipitation during the trial was 660 mm, being essentially concentrated in the period ranging from October to November.

The crop was drilled into a prepared seedbed by hand-broadcasting at a seed rate of 200 plants/m² and 250 plants/m² for legume and cereal, respectively, on 16th September of 2003. The oat-vetch mixtures were grown in a randomized experimental design with 6 replications of 16 m² each. The row spacing was 12.5 cm and replications were separated by 2 m buffer zone. Fertilizer applications were based on soil tests results and recommendations for grass-legume forage production with a mixed NPK fertilizer (7:14:14) applied prior to harrowing at a rate of 400 kg/ha. At top-dressed, a fertilizer containing 20.5% of N was applied at a rate of 300 kg/ha.

The whole-crop oat-vetch was harvested at beginning of flowering stage of vetch (approximately milk stage of oat) varying from 23rd and 30th of April. Samples of plant material from each oat-vetch replication were harvested by hand from randomly selected 1 m² areas.

Forage treatments 

At harvest, approximately 4 kg (fresh weight) of each replication were taken and were chopped into 1-cm lengths with a guillotine. Before treatment, samples of each replication (control samples; C) were frozen at -15ºC and kept for in situ incubations and chemical analysis.

Each oat-vetch replication was treated with 0 (S samples) or 60 g/kg of DM of fine ground urea (U samples). The urea was carefully spread on top of the forage before being thoroughly mixed with the forage. Two sub-samples of 750 g of each treatment were placed in plastic bags, which were squeezed to expel excess air and then sealed with tape, and stored at room conditions (22-25ºC) for 60 days.

At the opening of the bags, the silages were divided in three samples. On the first one pH was immediately measured and the sample was then frozen at -15ºC for volatile fatty acids (VFA), lactate and un-hydrolysed urea determinations. The second sample was exposed to ambient air (22–25ºC) for 24 h for the determination of nitrogen fractions. Aerated samples were ground to pass 1-mm screen. The third sample was dried for DM determination. Dried samples were ground to pass a 4-mm screen (Retsch, SKM100, Haan, Germany) for the in situ incubations and to pass a 1-mm screen for chemical analysis.

Determination of the in situ degradation

Three rumen-canulated rams fed, twice a day (8.00 h and 20.00 h), 1.2 kg medium-quality meadow hay (108 g CP/kg DM) and 350 g concentrate (140 g CP/kg DM) were used for the incubation of the nylon bags (Ørskov et al 1980). All animals were kept in well-ventilated facilities and had free access to water.

The degradation in situ of NDF (NDFsitu) was determined according to the procedures described by Guedes and Dias-da-Silva (1994). The nylon bags (Nybolt PA 40/30, Zurich, Switzerland), with approximately 2 g of ground sample (4-mm grind), were incubated in the rumen 2 h after the morning feeding and withdrawn after 48 h. All the samples were incubated in each animal at the same time and incubations were repeated once in each animal, making, in total, 6 determinations per sample. After being removed from the rumen, the bags were washed with tap water in a washing machine (TS803, Balay, Zaragoza, Spain) and subjected to neutral detergent extraction as described by Van Soest et al (1991).

Laboratory analyses

The pH was measured by inserting a glass electrode into approximately 10 g of a sample wetted with distilled water (100 ml) using a digital pH meter (Wissenschaftlich-Technische Werkstätten, pH 530, Weilheim, Germany). Un-hydrolysed urea of treated samples was assayed by the procedure of the AOAC (1990). Water extracts of silages were centrifuged to extract particulate matter and were used for VFA and lactate determinations. Concentrations of VFA were determined by gas-liquid chromatography (Shimadzu GC-141 B, Kyoto, Japan) using pivalic acid as internal standard according to Czerkawski (1976). Separation of acetate, propionate and butyrate was accomplished on a capillary column (Supelco Nukol, 0.25 mm i.d. x 30 m x 0.25 µm), operated at 135ºC using helium as the carrier gas. The injection block temperature was 210ºC. Quantification of the acids was done using a flame ionization detector at 180ºC connected to an integrator (Shimadzu C-R6A Chromatopac, Kyoto, Japan). Lactate was determined using an enzymatic assay procedure (K-late 03/06, Megazyme, Ireland).

Total nitrogen was determined by the Kjeldahl method according to the AOAC (1990) procedures. The procedure used to extract water-soluble nitrogen (WSN) and NH₃-N from the silages was as described by Dryden and Kempton (1983). Samples were extracted with distilled water for 30 minutes and then analysed for total nitrogen (WSN) by the Kjeldahl method and for NH₃-N by steam distillation of the alkaline solution into boric acid. Nitrogen bound to acid-detergent fibre (ADIN) was determined as described by Goering and Van Soest (1970).

Dry matter content was determined in a forced-draught oven at 60˚C for 24 h. Dried and ground samples were analysed for ash (AOAC 1990), neutral detergent fibre (NDF) (Van Soest et al 1991) and acid-detergent fibre (ADF) and acid detergent lignin (ADL) (Robertson and Van Soest 1981).

Statistical analysis

Data were analysed in a One-way ANOVA design, with six replications, by the general linear models procedure of SAS (1990). When significant differences occurred, least significant difference Student’s multiple range t-test at the 95% confidence interval was used to compare means.

Results 

At the opening of the bags all samples appeared to be well conserved and did not show any visible signs of moulding. Silages with added urea had a strong odour of ammonia. The urea was extensively hydrolysed; more than 90% of added urea at ensiling was converted to ammonia after 60 days of storage.

Dry matter, fermentation indicators, chemical composition and the NDFsitu of oat-vetch silages are presented on Table 1.

The DM content was significantly (P<0.001) affected by treatment, being the higher (P<0.05) values found on C samples. Urea addition increased pH (P<0.001). On average, the pH of S samples was 5.1 and had increased by 3.1 units after urea addition. The VFA and lactate concentrations were affected by treatment (P<0.001) and decreased from S to U samples. Acetate was predominant on S and U samples, representing more than 60 and 70% of the fermentation acids produced, respectively. For both samples, butyrate represented the lower proportion of the fermentation acids produced. The concentration of lactate decreased (P<0.001) from S to U samples.

Total nitrogen content of the oat-vetch was significantly increased (P<0.001) after urea addition. Urea addition also increased the WSN and NH₃-N (P<0.001) and the ADIN (P<0.05) content of oat-vetch silages. When oat-vetch mixtures were ensiled without urea (S samples), total nitrogen content was not significant different (P<0.05) from the total nitrogen content observed on C samples. However, the WSN and the NH₃-N in S samples were, respectively, higher (P<0.05) and lower (P<0.05) than those observed on C samples. After ensiling, the S samples presented lower values (P<0.05) for all nitrogen fractions compared with U samples.

Urea addition significantly affected the NDF (P<0.01), ADF (P<0.001) and ADL (P<0.001) contents of oat-vetch silages. Oat-vetch mixtures stored with 60 g urea/kg DM presented significantly higher (P<0.05) NDF, ADF and ADL concentrations compared to silages stored without urea (S samples) and to forage before ensiling (C samples).

The NDF degradation (NDFsitu) of oat-vetch mixtures was significantly (P<0.001) affected by urea addition, being the NDFsitu  higher (P<0.05) on samples ensiled with urea. Ensiling without urea (S samples) resulted in non significant (P>0.05) increase in NDF degradation compared to C samples

Discussion 

Control samples

The chemical composition of oat-vetch mixtures was generally in good agreement with current literature values (CIHEAM 1990; Caballero et al 1995; Castro et al 2000; Assefa and Ledin 2001; Lithourgidis et al 2006) and with values found by Moreira (1989) in oat-vetch forages grown in the same conditions as those reported in our study. In general, CP content found in our study (122 g/kg DM) is slightly higher than those found by these authors (CP ranged from 83 to 91 g/kg DM) probably resulting from the nitrogen fertilization used. According to Moreira (1989) and Assefa and Ledin (2001) the nitrogen content of oat-vetch mixtures is largely affected by nitrogen fertilization

Silages

Dry matter losses occurred during the ensiling process as DM values found for S samples were lower than those observed in C samples. Acetate and lactate were the predominant fermentation acids, representing, respectively, 63.3 and 23.9% of total fermentation acids produced during the conservation process. However, comparing to values found in barley-vetch silages by Kung et al (1990) and Seven and Çerçi (2006), lactate is in low concentration and acetate present higher concentration.  These authors presented for lactate 72.9 and 25 g/kg DM and for acetate 12.5 and 13.4 g/kg DM, respectively. Our pH values were also higher than those reported by these authors (pH ranged from 4.29 to 4.55). This possibly indicates that our silages were not completely and adequately conserved. Despite of the low level of butyrate (less than 1% of total fermentation acids produced), indicating that, apparently, clostridia fermentation did not occur, the high nitrogen content of the oat-vetch mixtures used in our study may have promoted some secondary fermentation during the fermentation process.

Urea breakdown, pH and fermentation acids

The urea applied to the oat-vetch silages was extensively hydrolysed (more than 90% of the urea added) indicating that urease activity of the microflora present in these silages is adequate for rapid and quantitative urea degradation. This is in agreement with previous results presented by several authors (Ørskov et al 1983; Deschard et al 1988; Hill and Leaver 1999; Guedes et al 2006) on whole-crop cereals and by Moore et al (1985) on orchardgrass-white clover silages. Urea hydrolysis is dependent on the presence of bacterial ureases, the temperature during storage and the presence of water (Dias-da-Silva et al 1988; Sahnoune et al 1992). The temperature during treatment was always higher than 18ºC, which, according to Sahnoune et al (1992), is the limiting temperature for the rate of hydrolysis of urea.

The pH and the fermentation acids, namely acetate, produced on the S samples reflected that some fermentation occurred during ensiling. On the contrary, the high pH values and the low concentration of fermentation acids (VFA less than 19 g/kg DM and lactate less than 4.0 g/ kg DM) on U samples indicated that ammonia production from urea may have inhibited the microbial fermentation activity to some extent. These results confirm previous findings on the preservation effect of urea applied to high moisture hays (Bélanger et al 1987; Moore and Lechtenberg 1987; Henning et al 1990) and silages, (Ørskov et al 1983; Deschard et al 1987; Hill and Leaver 1999; Guedes et al 2006)  preventing microbial activity and mould during storage.

Urea clearly decreased acetate, propionate and lactate production in oat-vetch silages. A similar effect of ammonia on fermentation acids was observed by Moore et al (1985) with whole orchardgrass-white clover treated with 30g ammonia/kg^- DM and stored for 60 days. In fact, acetate and lactate decreased, respectively from 30.9 to 20.1 g/kg DM and from 11.9 to 6.8 g/kg DM after urea addition (Moore et al 1985).  A decrease in acetate and lactate production in whole-crop wheat silage (Hill and Leaver 1999) and in total VFA production in whole-crop triticale silages (Guedes et al 2006) were also reported. The relative concentration of fermentation acids in U samples, namely the increase in acetate:lactate relation from 2.7 to 4.8, suggested that not only the total amount but also the composition of the microbial population was probably affected by urea addition. Similarly changes in the relative proportion of fermentation acids of silages as the result of urea treatment were also found by others authors (Moore et al 1985; Chestnut et al 1988; Deschard et al 1988; Hill and Leaver 1999; Seven and Çerçi 2006).

Losses in DM occurred between C samples and ensiled samples (S samples and U samples) reflecting the rapid initial utilisation, by plant respiration or by the microflora, of available substrates, such as water soluble carbohydrates and VFA. However, in U samples, DM losses were slightly depressed, thus reflecting the potential effect of urea in reducing the bacterial conversion of utilizable substrates. This reduction of DM losses was also observed by Hill and Leaver (1999) in whole-crop wheat treated with urea.

Nitrogen fractions

As expected, total nitrogen retained by the U samples, after a further 24 h exposure to ambient air, was markedly increased by the release of ammonia from urea hydrolysis. Recovery of added nitrogen averaged 30% and almost all of the retained nitrogen was analysed as WSN. The nitrogen in the ammonium form in U samples represented approximately 40% of the total nitrogen. These values are similar to those reported by Moore et al (1985) (34% of total nitrogen) in orchardgrass-white clover silages. However, our values are lower than those reported by Deschard et al (1987) (75% of total nitrogen) and higher than those found by Sutton et al (2002) (25% of total nitrogen) with whole-crop wheat. Differences in the proportion of NH₃-N between authors may be related to moisture content of the forages and to aeration or drying period prior to nitrogen determinations.

The amount of ADIN retained increased after urea addition indicating that some nitrogen binding to ADF occurred during storage of U samples, as previously reported by other authors (Moore et al 1985; Guedes et al 2006).  However, the ADIN represents less than 1% of total nitrogen on oat-vetch samples studied. Since ADIN is the only nitrogen constituent considered unavailable for rumen microbes, most of the retained nitrogen following urea addition to oat-vetch silages should be readily available nitrogen for rumen microbes. Several authors (Mandell et al 1988; Chermiti et al 1989; Muñoz et al 1991; Ben Salem et al 1994) have pointed out that much of the soluble nitrogen obtained by urea treatment of straws can be used by ruminal microflora provided that energy is available for microbial growth.

Cell wall composition and degradation

Cell wall components (NDF) on U samples increased compared to C samples from 576 to 626 g/kg DM. However, comparing to DM losses during the ensiling process we may conclude that this increase, as a percentage of DM, probably represented a change in proportion and is not a true increase. In fact, the amount of NDF of the U samples decreased, compared to C, samples from 170 to 166 g. This small drop in NDF content in U samples was essentially due to the partial solubilisation of the hemicellulose as it decreased from 224 to 207 g/kg DM.

Urea addition increased the ADF and ADL content of oat-vetch mixtures. Other authors (Kernan et al 1980; Thorlacius and Robertson 1984) have also reported increases in cell wall components following urea addition to forages, mainly in ADF content. It is accepted that this increase may be due to a carbohydrate x ammonia (Maillard) reaction and the increase observed in ADIN on U samples provides further support for this statement. However, this effect may also be the result of the high drying temperature (60ºC) these samples were exposed before analysis. The high moisture of oat-vetch silages may also be responsible for the increase of ADF. After urea treatment of cereal straws, the ADF content generally increases and this increase is favoured by a high moisture level (> 400 g/kg) (Hassoun et al 1990).

The small effect of urea on cell wall constituents, namely the decrease of NDF content observed in this study and reported by several authors (Moore et al 1985; Deschard et al 1988; Sutton et al 2001), suggests that there was a small effect of ammonia on the ligninocellulose linkages on high moisture forages. Thus it seems that, in this experiment, urea showed an increased potential for silage preservation rather than an upgrading effect. The high moisture content of the studied silages may have influenced these results as the mode of action of the ammonia is determined by the moisture content (Hassoun et al 1990). With high moisture forages any ammonia produced by urea hydrolysis would rapidly dissolve forming the ammonium ion, reducing the presence and diffusion of ammonia gas across the silage. In this way, degradation of ester linkages between the sugars at the reducing end of hemicellulose chains and lignin compounds was not properly enhanced by the ammoniation process. Although several authors, including Dias-da-Silva (1988), Hassoun et al (1990) and Muñoz et al (1991) had observed the importance of humidification in the urea treatment of straw, they had also observed that excess moisture may impede the diffusion of the gas.

Despite the small effect on cell wall, urea treatment increased NDFsitu as much as 222 g/kg NDF compared to the C samples and 202 g/kg NDF compared to the S samples. In ammonia treated orchardgrass-white clover silages (30 g/kg DM), Moore et al (1985) also observed similar increases of the extent of NDF digestion measured in vitro (208 g/kg NDF). This increase of NDFsitu was probably due to the partial solubilization of a fraction of the NDF (hemicellulose) and/or due to an indirect effect through the increase of available nitrogen, favouring the action of cellulolytic bacteria inside the bags during the in situ incubation.

Conclusions

-   The addition of 60 g urea/kg DM to oat-vetch silages increased the total nitrogen and the degradation of NDF measured in situ. Urea applied at this rate inhibited the microbial activity as indicated by the high pH values and the low concentration of VFA and lactate on oat-vetch silages.

-   Further studies using animals should be performed to determine the voluntary intake of the oat-vetch ensiled with urea in order to evaluate the effects of urea on the palatability these silages.

Acknowledgements 

The authors are grateful to Professor Vicente Sousa for providing the meteorological data presented in this paper.

References 

AOAC 1990 Official methods of analysis. 15th edition, AOAC, Washington DC, USA.

Assefa G and Ledin I  2001 Effect of variety, soil type and fertiliser on the establishment, growth, forage yield, quality and voluntary intake by cattle of oats and vetches cultivated in pure stands and mixtures. Animal Feed Science and Technology 96: 95-111

Bélanger G,  St-Laurent A-M, Esau C A,  Nicholson J W G  and McQueen R E 1987 Urea for the preservation of moist hay in big round bales. Canadian Journal of Animal Science 67: 1043-1053

Ben Salem H, Nefzaoui A and Rokbani N 1994 Upgrading of sorghum stover with anhydrous ammonia or urea treatments. Animal Feed Science and Technology 8: 15-26

Bolsen K K, Bonilla D R, Huck G L, Young M A and Hart-Thakur R A 1996 Effect of a propionic acid bacterial inoculant on fermentation and stability of whole-crop maize silage. In: Proceedings of the XI International Silage Conference, Aberystwyth, UK, p 154-155

Caballero R, Goicoechea E L and Hernaiz P J 1995 Forage yield and quality of common vetch and oat sown at varying seeding ratios and seeding rates of vetch. Field Crops Research 41: 135-140

Castro M P, Sau F and Piñeiro J  2000 Effect of seeding rates of oats (Avena sativa L.), wheat (Triticum aestivum L.) and common vetch (Vicia sativa L.) on the yield, botanic composition and nutritive value of the mixture. In: Legumes for Mediterranean forage crops, pastures and alternative uses (Editor, Sulas L), CIHEAM-IAMZ, Zaragoza, p 207-211 http://ressources.ciheam.org/om/pdf/c45/00600198.pdf

Chermiti A, Nefzaoui A and Cordesse R 1989 Paramètres d’uréolyse et digestibilité de la paille traitée a l’urée. Annales de Zootechnie 38: 63-72 http://animres.edpsciences.org/index.php?option=article&access=standard&Itemid=129&url=/articles/animres/pdf/1989/02/Ann.Zootech._0003-424X_1989_38_2-3_ART0001.pdf

Chestnut A B, Berger L L and Fahey G C 1988 Effects of conservation methods and anhydrous ammonia or urea treatments on composition and digestion of tall fescue. Journal of Animal Science 66: 2044-2056 http://jas.fass.org/cgi/reprint/66/8/2044

CIHEAM 1990 Tableaux de la valeur alimentaire pour les ruminants des fourrages et sous-produits d'origine méditerranéenne. In: Tableaux de la valeur alimentaire pour les ruminants des fourrages et sous-produits d'origine méditerranéenne, CIHEAM, Paris, p 116-127 http://ressources.ciheam.org/om/pdf/b04/91605001.pdf

Czerkawski J W 1976 The use of pivalic acid as a reference substance in measurements of production of VFA by rumen microorganisms in vitro. British Journal of Nutrition 36: 311-316

Deschard G, Mason V C and Tetlow R M 1988 Treatment of whole-crop cereals with alkali. 4. Voluntary intake and growth in steers given wheat ensiled with sodium hydroxide, urea or ammonia. Animal Feed Science and Technology 19: 55-66

Deschard G, Tetlow R M and Mason V C 1987 Treatment of whole-crop cereals with alkali. 3. Voluntary intake and digestibility studies in sheep given immature wheat ensiled with sodium hydroxide, urea or ammonia. Animal Feed Science and Technology 18: 283-293

Dias-da-Silva A 1988 Effects of treatments with urea and supplementation with molasses and monosodium glutamate on the digestibility and intake of rye straw by sheep. In: Proceedings of the World Association of Animal Production, Helsinki, Finland, p 197

Dias-da-Silva A, Mascarenhas-Ferreira A and Guedes C V M 1988 Effects of moisture level, treatment time and soya bean addition on the nutritive value of urea-treated maize stover.  Animal Feed Science and Technology 19: 67-77

Dryden G, McL and Kempton T J 1983 Digestion of organic matter and nitrogen in ammoniated barley straw. Animal Feed Science and Technology 10: 65-75

Goering H K and Van Soest P J 1970 Forage fiber analysis (apparatus, reagents, procedures, and some applications). Agricultural Handbook 379, Washington, DC: USDA, ARS

Guedes C V M and Dias-da-Silva A A 1994 Cinétique de la dégradation dans le rumen de la matière sèche et de l’azote des graines des légumineuses méditerranéenes. Annales de Zootechnie 45: 423-435 http://animres.edpsciences.org/index.php?option=article&access=standard&Itemid=129&url=/articles/animres/pdf/1996/05/Ann.Zootech._0003-424X_1996_45_5_ART0004.pdf

Guedes C M, Rodrigues M A M, Gomes M J, Silva S R, Ferreira L M and Mascarenhas-Ferreira A 2006 Urea treatment of whole-crop triticale at four growth stages: effects on chemical composition and on in vitro digestibility of cell wall. Journal of the Science of Food and Agriculture 86: 964-970

Hassoun P, Geoffroy F, Saminadin G, Briar and P Beramis M 1990 Studies on the ammoniation of sugar cane bagasse. Effects of moisture, urea levels, urease source and treatment period on composition, in vitro dry matter digestibility and evolution of ureolytic bacteria. Animal Feed Science and Technology 29: 113-129

Henning J C, Dougherty C T, O’Leary and J Collins M 1990 Urea for preservation of moist hay.  Animal Feed Science and Technology 31: 193-204

Hill J and Leaver J D 1999 Effect of stage of growth at harvest and level of urea application on chemical changes during storage of whole-crop wheat. Animal Feed Science and Technology 77: 281-301

Hill J and Leaver J D 2002 Changes in chemical composition and nutritive value of urea treated whole crop wheat during exposure to air. Animal Feed Science and Technology 102: 181-195

Huber J T,  Bucholz T F and Boman R L 1980 Ammonia versus urea treated silages with varying urea in concentrate. Journal of Dairy Science 63: 76-81 http://jds.fass.org/cgi/reprint/63/1/76.pdf

Huber J T, Foldager J and Smith N E 1979 Nitrogen distribution in corn silage treated: with varying levels of ammonia. Journal of Animal Science 48: 1509-1516 http://jas.fass.org/cgi/reprint/48/6/1509

Kernan J A, Coxworth E C and Moody M J A 1980 A survey of the feed value of various speciality crop residues and forages before and after chemical processing. Saskatchewan Research Council publication C-814-K-1-B-80, Saskatchewan, SRC, Canada

Kung L, Carmean S R and Tung R S 1990 Microbial inoculation or cellulase enzyme treatment of barley and vetch silage harvested at three maturities. Journal of Dairy Science 73: 1304-1311 http://jds.fass.org/cgi/reprint/73/5/1304

Lithourgidis A S, Vasilakoglou I B, Dhima K V, Dordas C A and Yiakoulaki M D 2006 Forage yield and quality of common vetch mixtures with oat and triticale in two seeding ratios. Field Crops Research 99: 106-113

Lomas L W, Fox D G and Black R J 1982 Ammonia treatment of corn silage. I Feedlot performance of growing and finishing steers. Journal of Animal Science 55: 909-923 http://jas.fass.org/cgi/reprint/55/4/909

Mandell L B, Christinson G I, Nicholson H H and Coxworth E C 1988 The effect of variation in the water content of wheat straw before ammoniation on its nutritive value for beef cattle. Animal Feed Science and Technology 20: 111-124

Moore K J and Lechtenberg V L 1987 Chemical composition and digestion in vitro of orchardgrass hay ammoniated with different techniques. Animal Feed Science and Technology 17: 109-119

Moore K J, Lechtenberg V L, Lemenager R P, Patterson J A and Hendrix K-S 1985 In vitro digestion, chemical composition and, fermentation of ammoniated grass and grass-legume silage. Agronomy Journal 77: 758-763

Moreira N 1989 The effect of seed rate and nitrogen fertilizer on the yield and nutritive value of oat-vetch mixtures. Journal of Agricultural Science 112: 54-66

Muñoz K S, Joy M, Faci R and Alibés X 1991 Treatment of lingo-cellulosic residues with urea. Influence of dosage, moisture, temperature and addition of ureases.  Annales de Zootechnie 40: 215-225 http://animres.edpsciences.org/index.php?option=article&access=standard&Itemid=129&url=/articles/animres/pdf/1991/04/Ann.Zootech._0003-424X_1991_40_4_ART0001.pdf

Ørskov E R, Deb Hovell F D and Mould F 1980 The use of the nylon bag technique for the evaluation of feedstuffs. Tropical Animal Production 5: 195-213 http://www.utafoundation.org/TAP/TAP53/53_1.pdf

Ørskov E R, Reid G W, Holland S M, Tait C A G and Lee N H 1983 The feeding value for ruminants of straw and whole-crop barley and oats treated with anhydrous or aqueous ammonia or urea. Animal Feed Science and Technology 8: 247-257

Robertson J B and Van Soest P J 1981 The detergent system of analysis and its application to human foods. In: The Analyses of Dietary Fibre in Foods (Editors, James W D T and Theander O) Marcell Dekker, New York, pp 123–158

Sahnoune S, Girard L, Besle J M and Chenost M 1992 A kinetic study of ammoniation of straw either via hydrolysis of urea or by anhydrous ammonia. Annales de Zootechnie 41: 42-53 http://animres.edpsciences.org/index.php?option=article&access=standard&Itemid=129&url=/articles/animres/pdf/1992/01/Ann.Zootech._0003-424X_1992_41_1_ART0019.pdf

SAS 1990 User’s guide. Release 6.12, SAS Institute Inc, Cary, NC, USA

Seven P T and Çerçi I H 2006 The effects on nutrient digestibility of hay and silages made in different conditions in lambs. Veterinarski Archiv 76: 111-117

Sutton J D, Phipps R H, Cammell S B and Humphries D J 2001 Attempts to improve the utilization of urea-treated whole-crop wheat by lactating dairy cows. Animal Science 73: 137-147

Sutton J D, Phipps R H, Deaville E R, Jones A K and Humphries D J 2002 Whole-crop wheat for dairy cows: effects of crop maturity, a silage inoculant and an enzyme added before feeding on food intake and digestibility and milk production. Animal Science 74: 304-318

Tetlow R M 1983 The effect of urea on the preservation and digestibility in vitro of perennial ryegrass. Animal Feed Science and Technology 10: 49-63

Thorlacius S O and Robertson J A 1984 Effectiveness of anhydrous ammonia as a preservative for high-moisture hay. Canadian Journal of Animal Science 64: 867-880

Van Soest P J, Robertson J B and Lewis B A 1991 Methods for dietary fiber, NDF and nonstarch polysaccharides in relation to animal nutrition. Journal of Dairy Science 74: 3583-3597 http://jds.fass.org/cgi/reprint/74/10/3583

Received 3 June 2008; Accepted 16 June 2008; Published 5 August 2008

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