Abstract. Introduction. M. G. Porter and R. S. Murray Agricultural Research Institute of Northern Ireland, Large Park, Hillsborough, UK

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1 The volatility of components of grass silage on oven drying and the inter-relationship between dry-matter content estimated by different analytical methods M. G. Porter and R. S. Murray Agricultural Research Institute of Northern Ireland, Large Park, Hillsborough, UK Abstract This study reviews the volatility coef cients used to convert the oven dry-matter (DM) content of grass silage to an accepted true DM base, volatile-corrected oven dry matter (VCODM). The revised coef cients quoted for DM determination at 60 C, 85 C and 100 C are based on 18 grass silages with DM contents in the range 153±365 g kg ±1. The volatility coef cients for drying at 60 C, 85 C and 100 C were 0á090, 0á224 and 0á375 for lactic and 0á554, 0á716 and 0á892 for total volatile fatty s respectively. The volatilities of ammonia and total alcohols remained unchanged from previous work and showed no temperature dependences in the range 60 C to 100 C. These revised coef cients were validated using 36 grass silages from three harvests in 1996 and 1997, and no signi cant differences were found among absolute dry matter (GCDM), alcohol-corrected toluene dry matter (ATDM) and VCODM contents based on the three drying temperatures (VCODM 100, VCODM 85 and VCODM 60 ). A series of regression equations relating absolute DM content to oven DM content determined at different temperatures gave coef cients of 1á024, 1á013 and 1á000 and constants of 12á67, 11á43 and 11á16 for oven drying at 60 C, 85 C and 100 C respectively. Mathematical manipulation of these equations enables interconversion of DM contents at the three drying temperatures. A new method is described for the analysis of volatile fatty, lactic and alcohol concentrations in grass silage by gas±liquid chromatography using a single injection in an automated procedure that makes the Correspondence to: M. G. Porter, The Agricultural Research Institute of Northern Ireland, Large Park, Hillsborough, Co. Down BT26 6DR, UK. mike.porter@dardni.gov.uk Received 26 February 2001; revised 2 May 2001 routine estimation of VCODM a practical proposition to satisfy routine high-volume requirements. Finally, in a separate study over 4 years using 2381 grass silages from research and commercial farms throughout Ireland, a simple regression is described, which, for advisory purposes, allows true silage DM content to be estimated from oven dry matter content (ODM) for silages in which ODM is >200 g kg ±1. Keywords: drying temperatures, dry-matter determination, gas±liquid chromatography, grass silage, volatile coef cients Introduction It is well documented that the content of true dry matter (TrDM) in silage is greater than that determined by oven drying in forced-air ovens (McDonald and Dewar, 1960; Larsen and Jones, 1973; Porter et al., 1984; Porter and Barton, 1997). Research workers at centres involved in silage-feeding studies predict TrDM from oven dry-matter content (ODM) by adding the proportions of volatile components lost in oven drying. Alternatively, they have modi ed the standard Ministry of Agriculture, Fisheries and Food (MAFF) recommendations on silage drying in an attempt to compensate for or reduce these volatile losses. With the former technique, the formulae and coef cients most commonly used were published in 1984 (Porter et al., 1984) and covered the silage harvesting systems and ensiling practices adopted at that time, namely ail, double and precision chop, with the additives used being predominantly based on formic or mineral s. The majority of clamp silage produced in the British Isles is now precision chopped with a short eld wilt, and additives, when used, are mainly biological types. (T. W. J. Keady, personal communication). These changes in oven drying practices and additive use necessitate a review of the volatility coef cients reported previously Ó 2001 Blackwell Science Ltd. Grass and Forage Science, 56, 405±

2 406 M. G. Porter and R. S. Murray with particular emphasis on volatile compounds. In addition, ODM techniques that involve drying at lower temperatures for longer times demand that a review of the volatility coef cients should include drying at the most commonly used temperatures and times, namely 60 C, 85 C and 100 C for 48 h, 18 h and 16 h respectively. There are times when it is necessary to compare the chemical constituents of silages when the analytical work has been completed on samples that have been dried at different temperatures or using different techniques. This has been particularly important over the past 4 years at the Agricultural Research Institute of Northern Ireland, as it has transferred from a DM content determined by toluene distillation with corrections for alcohols to one based on adding volatile losses to ODM concentrations. A series of simple regression equations are presented that relate DM content determined by gas±liquid chromatography (GCDM), a method shown to be the best available measure of TrDM (Porter and Barton, 1997), to those determined by toluene distillation or oven drying. Materials and methods Eighteen stable rst- and second-cut harvest silages from predominantly perennial ryegrass swards and covering a DM range from 153 to 365 g kg ±1 ODM 100 (oven dry-matter content determined at 100 C) were selected to re ect the DM range normally found in the temperate maritime climate of western Europe. The mean chemical analyses of these silages in the nominal DM ranges of <200, 200±300 and >300 g kg ±1 ODM 100 are reported in Table 1A. Before analysis, 2 kg of each silage was prepared by chopping (Porter, 1992) and analysed in quadruplicate for ph, ammonia and nitrogen concentrations, toluene dry-matter content (TDM), by the methods described by Steen (1989), and water to give an absolute dry-matter content (GCDM) according to the method of Porter and Barton (1997). Alcoholcorrected toluene dry matter (ATDM) was taken as the sum of toluene dry matter, ethanol and propanol. Determination of volatile fatty s, lactic and alcohols Volatile fatty s, lactic and lower alcohol concentrations were determined by gas±liquid chromatography on aqueous silage extracts obtained from steeping 30 g of fresh silage in 150 ml of deionized water for 16 h at 4 C in a sealed container followed by a preliminary ltering through 3 lm lter paper (Whatman International, Maidstone, Kent, UK). Deionized water (3 ml) and 1 ml of an internal standard solution (0á5 g 3-methyl-n-valeric in 1000 ml 0á15 mol l )1 oxalic ) were added to 1 ml of ltrate from the above, and the solution was ltered through a 0á45-lm polyethersulphone membrane into a chromatographic sample vial for analysis. A 0á3-ll aliquot was injected using an on-column technique with an autosampler (Varian 8200 CX, Varian, Walnut Creek, CA, USA) into a wide-bore capillary column (SGE BP21 25 m 0á53 mm internal diameter and 0á5 lm lm thickness; P/N , SGE International, Ringwood, Victoria, Australia) installed in a Varian Star 3400 CX chromatograph running in a temperature-programmed mode with automatic baseline correction. The column was held at 45 C for 2 min and then increased at 10 C min )1 to 220 C giving a run time of»20 min. The injector and ame ionization detectors were held at 220 C, and nitrogen was used as the carrier gas at a ow rate of 1 ml min )1. With automatic calibration every 12th injection, the capacity is about 40 samples day )1. A typical chromatogram is shown in Figure 1, demonstrating the quality of the separation achieved and the lack of peak tailing normally attributed to lactic in its raw form. Determination of volatile coef cients At the same time as the chemical analysis, suf cient chopped silage to yield»35 ml of distillate was mixed with 10 g of solid PVC 2 mm 50 mm rods and weighed into 1000 ml round-bottomed asks. Nine asks for each silage (three temperatures three replicates) were sealed and stored at ±20 C until required for determination. These silages were analysed for the volatility coef cients of their fermentation products at 60 C, 85 C and 100 C by a modi cation of the method of McDonald and Dewar (1960). The apparatus (Figure 2) consisted of a closed system made up with Quick t glassware (Bibby Sterilin, Stone, Staffordshire, UK) with ve main elements: a preheater and drier for the drying air, a temperature-controlled oil bath, an `Inland Revenue' condenser (C6/13/SC) with a pumped coolant system running at 4 C, a receiver ask located in an ice bath and a vacuum pump with a ow controller. Pre-dried and heated air at a rate of»500 ml min ±1 at C, 85 2 C or60 2 C was drawn through the system for 16 h, 18 h or 48 h respectively. The double-surface condenser was washed with 5 10 ml aliquots of distilled water into the receiver ask, and the combined contents were made up to 100 ml for analysis of alcohols, volatile fatty s, lactic and ammonia concentrations as described above. Silages for the validation of volatility coef cients Thirty-six grass silages in the DM range 149±380 g kg ±1 ODM 100 taken from three harvests in 1996 and 1997

3 Inter-relationships between estimates of silage dry matter 407 Table 1 Composition of silages. DM range ODM (100 C) ODM (85 C) ODM (60 C) ATDM GCDM ph Nitrogen Ammonia nitrogen Total alcohols Acetic Propionic n-butyric Lactic A. Composition of 18 silages (g kg )1 fresh silage) < á0 172á3 174á8 180á2 186á0 3á80 4á79 0á37 3á20 4á35 0á41 0á31 17á7 200± á6 237á4 240á7 247á7 251á7 3á91 5á06 0á38 4á29 4á24 0á36 0á44 24á7 > á5 334á6 339á0 348á7 350á1 4á23 5á52 0á49 2á36 3á60 0á33 0á27 29á5 Mean 244á0 248á1 251á5 258á9 262á4 3á98 5á12 0á42 3á26 4á07 0á37 0á34 23á9 Minimum 153á2 157á7 160á0 164á8 170á8 3á65 3á64 0á16 1á18 2á04 0á12 0á05 11á3 Maximum 364á7 369á9 374á9 385á4 385á9 4á38 6á09 0á70 5á92 6á55 0á84 0á63 36á8 s.d. 74á42 74á93 75á03 75á33 78á14 0á221 0á625 0á061 1á338 1á320 0á175 0á192 7á62 B. Composition of 36 validation silages (g kg )1 fresh silage) < á7 175á3 177á4 182á4 189á4 3á77 4á44 0á39 2á34 3á97 0á30 0á94 13á92 200± á7 251á1 255á0 264á5 266á0 3á93 4á80 0á39 2á44 3á60 0á21 0á28 25á78 > á3 322á6 326á5 335á0 337á7 4á95 5á32 0á47 2á38 3á65 0á14 0á26 25á56 Mean 260á1 264á2 267á7 276á0 279á0 4á30 4á93 0á41 2á40 3á69 0á20 0á40 23á39 Minimum 148á7 150á7 152á8 156á4 162á1 3á65 3á39 0á27 0á98 1á51 0á00 0á00 5á60 Maximum 380á1 384á7 389á7 399á2 400á5 5á89 6á92 0á70 3á57 7á81 1á97 2á49 35á49 s.d. 62á06 62á76 63á56 65á30 63á63 0á641 0á774 0á079 0á630 1á130 0á511 0á519 8á148 ODM, oven dry-matter concentration; ATDM, alcohol-corrected toluene dry matter; GCDM, dry-matter concentration determined by gas±liquid chromatography (absolute drymatter concentration).

4 408 M. G. Porter and R. S. Murray Figure 1 Typical chromatogram of silage extract. Figure 2 Silage drying apparatus. were prepared according to the method of Porter (1992), analysed for GCDM, ATDM and ODM by drying at C for 16 h (ODM 100 ), 85 2 C for 18 h (ODM 85 ) and 60 2 C for 48 h (ODM 60 ) in forced-air ovens. Chemical analysis was carried out as described previously, and the chemical compositions of the silages are given in Table 1B.

5 Inter-relationships between estimates of silage dry matter 409 Silages for the prediction of TrDM from ODM without volatile analysis A total of 2381 grass silages with DM concentrations covering the range 120±550 g kg ±1, split approximately equally between research and commercial farms and taken between 1997 and 1999 from three harvests, were analysed for ODM [85 2 C for 18 h (ODM 85 )] and, toluene DM contents, ammonia, nitrogen, lower concentrations, and ph alcohols and volatile fatty with the aim of producing a new regression for the estimation of TrDM from ODM without resorting to complex volatile analysis. Results and discussion Apparatus and recovery tests The collection of distillate was achieved by a combination of a cold multisurface condenser at 4 C and a long delivery tube. The length of this tube allowed the distillate to drip into the receiver ask, but it was suf ciently short to prevent bubbling at maximum liquid recovery. Recovery tests for added alcohols, total volatile fatty s, ammonia and lactic on three reconstituted silages in triplicate at 100 C gave recoveries of 97á5% (s.e.m. ˆ 0á98, NS), 95á7% (s.e.m. ˆ 1á52, NS), 97á2% (s.e.m. ˆ 0á55, NS) and 99á4% (s.e.m. ˆ 0á47, NS) respectively. The concentrations of lactic, total volatile fatty s, total alcohols and ammonia were determined on the distillates and expressed as proportions of each analyte group in the original silage samples. The data were analysed by two-way analysis of variance for both temperature and DM range and are reported in Table 2. As there were no signi cant interactions, only main effects are reported. The effect of DM and ph on the volatility of components Figure 3 describes the observed linear relationship (r 2 adjusted ˆ 0á861) of DM content with ph for the 18 test silages superimposed on the theoretical proportions of free lactic and acetic s. The volatilities of ic components in silage are a function of both the boiling points and the pk a of the components, where the pk a describes the amount of free at any given ph, the free being the volatile component of the equilibrium mixture of and salt. Boiling points for acetic, propionic and butyric are 118 C, 141 C and 166 C, respectively, whereas their pk a s are in the range 4á7±4á9; lactic has a boiling point of 122 C and a pk a of 3á76. For any given ph, these physical properties describe why lactic, even though having a lower boiling point than some volatile fatty s, has a lower volatility. Although the volatility of lactic tended to decrease with increasing DM content, the differences were not signi cant (Table 2). For volatile fatty components, there was no trend of decreasing volatility with increasing DM content and ph over the three DM content ranges. This result re ected the low ph range covered, 4á38±3á65, a range in which between 0á70 and 0á92, respectively, of the s are in the free state. The volatilities of alcohols, which are normally present in their free states, were not found to be DM content dependent, but it was observed that the rates of evaporation were greater with drier silages. Ammonia concentrations in well-preserved silages range from negligible amounts to about 0á10 of total nitrogen and, although these concentrations are small, i.e. up to 1á0 gkg ±1 fresh silage, they are measurable and are therefore included in the revised empirical equations. The relatively constant coef cient for ammonia re ected the high solubility of the gas in the aqueous medium, which extended to include the vapour phase. Ammonia was not detected in the residual DM of any silages. The effect of temperature on the volatility of components The alcohols, ethanol and propanol, are highly volatile compounds with low boiling points that are easily lost during oven drying. Hence, there were no signi cant differences in volatilities at the three measured Table 2 Volatility coef cients for temperature and DM (g kg )1 ) ranges. Temperature DM range Signi cance 60 C 85 C 100 C < ±300 >300 s.e.m. Temperature DM range Lactic 0á090 0á224 0á375 0á236 0á230 0á224 0á0084 *** NS Total VFA 0á554 0á716 0á892 0á689 0á733 0á670 0á0211 *** NS Total alcohols 0á991 0á969 0á975 0á993 0á985 0á957 0á0127 NS NS Ammonia 1á003 0á978 0á987 0á993 1á000 0á973 0á0113 NS NS VFA, volatile fatty ; NS, not signi cant; ***P <0á001.

6 410 M. G. Porter and R. S. Murray Figure 3 Relationship of ph to alcohol-corrected toluene dry matter (ATDM) and theoretical free concentrations. temperatures (Table 2). The effect of temperature on the volatility of lactic was signi cant (P <0á001) over the three temperatures assessed, with volatilities of 0á09, 0á22 and 0á38 at 60 C, 85 C and 100 C respectively. The small change in the volatility coef cient from 0á41 to 0á38 for the 100 C drying temperature from the previously reported coef cient re ects the increased residual ph of the silages in the present study and thus the lower concentration of free. Previous work (Porter et al., 1984) has been conducted on silages that had been conserved with mineral s or with formic (pka ˆ 3á75) at»2á5 kg tonne ±1, both of which were demonstrated to lower residual ph. The volatility coef cients for total volatile fatty s at 60 C, 85 C and 100 C were 0á55, 0á72 and 0á89 respectively (P <0á001). Ammonia volatility was not found to be temperature dependent. results were compared with GCDM, an analytical measure of TrDM by difference (Porter and Barton, 1997), and ATDM, the most widely used DM estimation used in research centres over the past 15 years. DM contents were 279á0, 276á0, 276á2, 275á0 and 275á2 gkg ±1 (s.e.m. ˆ 1á494, NS) for GCDM, ATDM, VCODM 100, VCODM 85 and VCODM 60 respectively (Table 3), validating the proposed regression equations. There are times when, for analytical reasons, it is necessary to be able to convert chemical analysis data determined on one DM base to another, e.g. when comparing bre data determined on 100 C dried samples with carbohydrate data determined on 60 C dried samples. Table 3 reports ve regression equations for the prediction of GCDM from ATDM, VCODM 100 and ODM at three temperatures, which by simple algebra can be used to inter-relate DM bases. Validation of volatility coef cients A total of 36 grass silages, as described in Table 1B, were used to calculate VCODM at drying temperatures of 100 C, 85 C and 60 C (VCODM 100, VCODM 85 and VCODM 60 ) using the revised coef cients in Table 2. The Estimation of TrDM from ODM without volatile analysis Over 2381 silages tested, the sum of measured volatile concentrations was 23á06, 26á91, 33á28, 33á72, 34á87, 32á28 and 31á91 g kg ±1 fresh silage for the <175,

7 Inter-relationships between estimates of silage dry matter 411 Table 3 Summary of ANOVA and regression analysis on 36 validation silages (g kg )1 ). GCDM ODM 100 ODM 85 ODM 60 ATDM VCODM (100 C) s.e.m. Dry matter 279á0 c 260á1 264á2 ab 267á7 b 276á0 c 276á2 c 2á38 Coef cient 1á024 1á013 1á000 0á974 0á995 Error 0á0095 0á0074 0á0060 0á0065 0á0035 Constant 12á67 11á41 11á16 10á26 4á15 Error 2á53 2á02 1á65 1á85 0á993 r 2 0á997 0á998 0á999 0á998 1á000 VCODM, volatile corrected oven dry-matter content. Other abbreviations as in Table 1. Subscripts to ODM indicate drying temperature. Values with differing superscripts differ at the 5% level. 175±199, 200±249, 250±299, 300±349, 350±399 and ³ 400 DM ranges respectively. The differences in total concentrations, which included alcohols, volatile fatty s, lactic and ammonia, were found to be not signi cant at the 5% level above 200 g kg ±1 DM, giving validity to the proposal that a single prediction equation may be used to predict DM content for all silages with DM contents above this 200 g kg ±1 threshold. The distribution of DM contents was unequal in the described nominal DM ranges, and a bootstrap procedure (Efron and Tibshirani, 1993), allied to linear 1 regression analysis, was used to produce an equation for the estimate of TrDM content. True dry-matter content gkg 1 ˆ1996 s.e.ˆ ODM 85 s.e.ˆ Adjusted r 2 ˆ098 1 This equation, although developed using data from silages dried at 85 C, may be transformed using the regression equations in Table 3 by substituting ODM 85 for (1á011 ODM á24) or (0á987 ODM 60 ±0á26) for drying at 100 C and 60 C respectively. Conclusions The revised coef cients for the volatility of components in fresh silage give an increased accuracy to the existing calculated VCODM at 100 C. The additional coef cients determined at two further drying temperatures, 60 C and 85 C, allow the volatile corrected estimation technique to be used more widely. Of equal importance, the derived regression equations outlined in Table 3 allow analytical results obtained for silages prepared by different sample pretreatment techniques to be converted back to the same DM base. Finally, for those workers without the facilities for silage volatiles estimation or for advisory work, the regression equation 1 gives a useful tool for the rapid estimation of TrDM. Acknowledgments Thanks are due to Dr David Kilpatrick of the Queen's University of Belfast for his guidance and assistance with the statistical analysis, and to the laboratory staff of the Agricultural Research Institute of Northern Ireland for the many hours that they spent, often at unsociable times, overseeing the silage drying processes. References EFRON B. and TIBSHIRANI R. (1993) An Introduction to the Bootstrap. London: Chapman and Hall. LARSEN R.E. and JONES G.M. (1973) Effects of different dry-matter determination methods on chemical composition and in vitro digestibility of silages. Canadian Journal of Animal Science, 53, 753±760. MCDONALD P. and DEWAR W.A. (1960) Determination of dry-matter and volatiles in silage. Journal of the Science of Food and Agriculture, 11, 566±570. PORTER M.G. (1992) Comparison of sample preparation methods for the determination of gross energy concentration of fresh silage. Animal Feed Science Technology, 37, 201±208. PORTER M.G. and BARTON D. (1997) A comparison of methods for the determination of dry-matter in grass silage including an extraction method for water. Animal Feed Science Technology, 68, 67±76. PORTER M.G., PATTERSON D.C., STEEN R.W.J. and GORDON F.J. (1984) Determination of dry-matter and gross energy of grass silage. 7th Silage Conference Summary of Papers. Paper 45. The Queen's University of Belfast. STEEN R.W.J. (1989) A comparison of soya-bean, sun ower, and sh meals as protein supplements for yearling cattle offered grass silage based diets. Animal Production, 48, 127±132.

The analysis of organic acid content of additives, premix, feed, and water.

The analysis of organic acid content of additives, premix, feed, and water. Contents Foreword Introduction Warnings 1. Scope 2 1.1 LOD and LOQ 3 2. Normative References 3 3. Definitions 3 3.1 Feed (or