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Real-time tumor-tracking volumetric modulated arc therapy (RT-VMAT) enabling beam-gating based on continuous X-ray tracking of the three-dimensional position of internal markers is relevant for moving tumors. Dose-volume characteristics and treatment time were evaluated in ten consecutive patients who underwent liver stereotactic body radiation therapy with RT-VMAT. Target dose conformity and sparing of the stomach and the intestine were improved comparing RT-VMAT with RT-3D conformal radiotherapy. The mean treatment time for each fraction was less than 10 min. RT-VMAT could be effective, especially for targets located adjacent to organs at risk.

Context: Cattle and sheep emit methane, a potent greenhouse gas, as part of the fermentation process of feed digestion in their gut; however, the mechanisms explaining differences among animals in enteric methane production are not fully understood. Aim: To investigate whether variation among animals in their ability to digest their test ration was associated with phenotypic and genetic variation in methane production. Methods: The experiment used 135 Angus beef cattle measured for their phenotypic and genetic merit for methane production. The extent of digestion of the dry matter (DMD) in the test ration by individual cattle was determined using silica as a naturally present indigestible marker. Its concentration in feed consumed and faeces was determined using rapid portable X-ray fluorescence spectroscopy, from which DMD was calculated. Key results: Higher daily methane-production rate (MPR), higher methane produced per unit of feed consumed (methane yield; MY) and higher methane produced than the predicted daily production (residual MPR; RMP) by animals was accompanied by higher DMD. Higher genetic merit for MPR was also accompanied by higher DMD, but DMD had no detectable association with genetic variation in the other two methane emission traits. The regression coefficients for change in MPR (g/day), MY (g/kg DMI), RMP (g/day) with change in DMD (%) were 2.6 ± 1.1 (s.e.; P < 0.05), 0.14 ± 0.07 (P < 0.1) and 0.68 ± 0.38 (P < 0.1) respectively. Conclusions: Differences among animals in their DMD were found and were associated with phenotypic variation in the three methane emission traits studied, and with genetic variation in daily methane production. The results support the caution that feeding and breeding interventions seeking to reduce methane emissions can also reduce the extent of digestion of feed by cattle. Implications: Feeding and breeding interventions that seek to reduce methane emissions may change rumen physiology and reduce the extent of digestion of feed by cattle, which may be undesirable. Cattle and sheep emit methane, a potent greenhouse gas, as part of the fermentation process of feed digestion. Differences among animals in their ability to digest feed were found and were associated with variation in the three methane emission traits studied. Feeding and breeding interventions to reduce methane emissions may change rumen physiology and reduce the extent of digestion of feed by cattle, which may be undesirable. [ABSTRACT FROM AUTHOR]

Undigested neutral detergent fibre (uNDF) is commonly used as an internal marker for the estimation of diet digestibility in ruminants. This work aimed to verify (i) whether the in vivo method with uNDF could be used to evaluate diet digestibility in growing pigs, and (ii) whether pre-treating the samples with neutral detergent solution (NDS) and α-amylase improves the accuracy of the estimates. Samples from a previously published work of two diets with known in vivo digestibility values estimated by the total faecal collection method and 16 individual samples of faeces were used. For each sample, four Ankom F57 bags were weighed. Before the incubation, two F57 bags were pre-treated with NDS and α-amylase. All the samples were incubated for 240 h in the Ankom DaisyII incubator and then analysed for their uNDF contents. Dry matter, organic matter, and neutral detergent fibre digestibilities were estimated using the uNDF contents, and the results were compared with those of the former study. The digestibility values obtained using the uNDF method with pre-treatment were not statistically different from those determined with the total faecal collection. On the contrary, the uNDF method without the pre-treatment could not satisfactorily predict the digestibilities of pig diets.

The objective of this study was to evaluate the apparent digestibility of diets for sheep by internal markers iDM (indigestible dry matter), iNDF (indigestible neutral detergent fiber) and iADF (indigestible acid detergent fiber) and the external indicator LIPE®, in relation to the total feces collection method. It was used eight sheep males Santa Inês, castrated, rumen fistulated, with a mean body weight of 50 kg. The animals received diets based of manioc rasp and mature elephant grass (120 days). Internal markers (iDM, iNDF and iADF) were estimated after 288 hours of in situ incubation of food and feces samples. The external LIPE® indicator was supplied as capsules at a dosage of 250 mg/animal/day. The apparent digestibility results estimated by the LIPE® external marker differed statistically (P<0.05) from those determined by the total feces collect method and the internal marker, iNDF and iADF, indicating higher digestibility coefficients for the external marker for the variables DM (dry matter), OM (organic matter), except for treatment with mature elephant grass and CP (crude protein), except for control and nutrient treatment NFC (Non-fibrous carbohydrates), TC (total carbohydrate) and TDN (total digestible nutrients) for all treatments. No statistical difference was observed (P>0.05) between the total feces collect method and the internal markers iNDF and iADF. The internal markers iNDF and iADF can be used in studies of digestibility in sheep, as they allow to estimate in a similar way to the method of total feces collect. [ABSTRACT FROM AUTHOR]

Four castrated crossbred horses were used in a randomized block design to study the use of indigestible internal markers iNDF and iADF obtained in situ (from bovines) or in vivo (from equines). Treatments consisted of determining digestibility by the direct method comprising total feces collection (TC) and by the indirect method comprising internal markers iNDF and iADF obtained by in situ incubation in bovine rumen or in vivo by the mobile nylon bag (MNB) technique with horses. iNDF-IV and iADF-IV resulted in better marker recovery rate (RR) (91.50%), similar to TC. The in situ technique resulted in lower RR values for the two indigestible markers, averaging 86.50% (p < 0.05). Estimates of the nutrient coefficient of digestibility (CD) were adequately predicted by iADF-IV, for horses fed on hay exclusively, with rates 46.41, 48.16, 47.92 and 45.51% for dry matter (DM), organic matter (OM), FDN and gross energy, respectively. Results show that MNB may be used to obtain iADF in horses fed on coast-cross hay exclusively, whereas NDFi and ADFi were selected for horses fed on mixed diets to predict the coefficient of nutrient digestibility.Foram utilizados quatro cavalos castrados sem raça definida pelo delineamento em blocos casualizados. Objetivou-se viabilizar os indicadores internos indigestíveis, FDNi e FDAi, obtidas pelas técnicas in situ nos bovinos e in vivo nos equinos por meio do saco de náilon móvel (SNM). Os tratamentos consistiram de digestibilidade determinada por método direto com a coleta total de fezes (CT) e indireto pelo uso de FDNi e FDAi obtidos pelas técnicas in situ (IS) na cavidade ruminal de bovinos e in vivo (IV) nos equinos por meio do SNM. As FDNi-IV e FDAi-IV promoveram a melhor taxa de recuperação (TR) do indicador, igualando-se ao grupo CT, em 91,50%, enquanto a técnica in situ resultou nas piores taxas, na média 86,50%, para ambos os indicadores indigestíveis (p < 0,05). As estimativas dos CD dos nutrientes foram adequadamente preditos pela FDAi-IV, para equinos alimentados com feno, no qual os valores observados foram de 46,41; 48,16; 47,92 e 45,51% para MS, MO, FDN e EB, respectivamente. Concluiu-se que o SNM em equinos pode ser usado para obter a FDAi em equinos alimentados exclusivamente com feno de coast-cross e para cavalos consumindo dieta mista, as FDNi e FDAi foram eleitas para predizer CD de nutrientes.

Four castrated crossbred horses were used in a randomized block design to study the use of indigestible internal markers iNDF and iADF obtained in situ(from bovines) or in vivo (from equines). Treatments consisted of determining digestibility by the direct method comprising total feces collection (TC) and by the indirect method comprising internal markers iNDF and iADF obtained by in situincubation in bovine rumen or in vivo by the mobile nylon bag (MNB) technique with horses. iNDF-IV and iADF-IV resulted in better marker recovery rate (RR) (91.50%), similar to TC. The in situ technique resulted in lower RR values for the two indigestible markers, averaging 86.50% (p < 0.05). Estimates of the nutrient coefficient of digestibility (CD) were adequately predicted by iADF-IV, for horses fed on hay exclusively, with rates 46.41, 48.16, 47.92 and 45.51% for dry matter (DM), organic matter (OM), FDN and gross energy, respectively. Results show that MNB may be used to obtain iADF in horses fed on coast-cross hay exclusively, whereas NDFi and ADFi were selected for horses fed on mixed diets to predict the coefficient of nutrient digestibility

The objective of the study was to evaluate utilization of dietary minerals and trace elements in pregnant heifers with distinct residual feed intakes (RFI). Feed intake, body weight (BW), and body composition traits were recorded in 36 crossbred heifers over a period of 37 wk, starting shortly after weaning at 8.3 (0.10; standard deviation) mo of age with an average BW of 276 (7.8) kg. Both BW and body composition were monitored regularly throughout the study, whereas individual feed intake was assessed during the last 84 d of the trial. Data recorded were used to calculate RFI for each heifer. Heifers were ranked based on RFI and assigned to high (n = 14) or low (n = 10) RFI groups. After the RFI study, 24 selected heifers [age 18.2 (0.14) mo; 87.5 (4.74) d in gestation; 497 (8.5) kg of BW] were used in an indirect digestibility trial (lignin as internal marker). Heifers were fed a ration containing corn silage, haylage, and a mineral premix in which Ca, P, K, Na, Mg, S, Cu, Fe, Mn, Mo, Se, Zn, and Co were provided in the diet according to National Research Council requirements of pregnant replacement heifers. The digestibility trial lasted 1 wk, during which samples of feces were gathered twice daily, and blood and liver biopsy samples were collected on the last day. We noted no significant differences between low- and high-RFI heifers in dry matter digestibility. Apparent absorption of Cu, Zn, and Mn was increased in heifers with low RFI, and apparent absorption of Co tended to be greater for these animals. Concentrations of macrominerals and trace elements in serum of pregnant heifers were similar for both groups except for Se, which was increased in the serum of low-RFI heifers. Liver concentrations of Cu, Fe, Mn, Mo, Se, and Zn did not differ between lowand high-RFI heifers. In conclusion, whereas improved absorption of some trace elements (Cu, Zn, Mn, and Co) and increased Se serum concentration appear to be associated with superior feed efficiency in pregnant heifers, further studies are needed to investigate the causality of such relationships. [ABSTRACT FROM AUTHOR]

In order to surmount drawbacks of the infrared spectroscopy (IR) itself during investigating the mesophase-transition behaviours and mechanism of the thermotropic liquid crystalline polymers (TLCPs), the elemental phosphorus as an internal marker was introduced into the main-chain TLCPs. The detail mechanism of the glass transition and mesophase phase transition of the phosphorus-containing aromatic liquid crystalline copolyester (poly(-hydroxybenzate-co-DOPO-benzenediol dihydrodiphenyl ether terephthalate) [PHDDT]) was revealed through tracing the internal marker with the perturbation correlation moving window 2-dimensional correlation and 2-dimensional correlation analysis (2DCOS) correlation IR spectra. The results showed that the phosphorus-containing unit did not participate in the glass transition of the PHDDT. The results of the 2DCOS showed that the PHDDT mesophase phase transition took place through adjustment of the phosphorus-containing units. Simultaneously, the adjustment of the phosphorus-containing unit also can induce the motion of the other groups, and the sequential orders of the spectral changes were Ar–O–Ar → ester C–O → C=O. However, the sequential orders of the spectral changes were converse during the PHDDT glass transition. [ABSTRACT FROM PUBLISHER]

A digestibilidade aparente e os consumos de matéria seca (MS), de matéria orgânica (MO), de proteína bruta (PB), de fibra em detergente neutro (FDN) e de nutrientes digestíveis totais (NDT) foram avaliados para se determinar o efeito da substituição do milho por farelo de palma forrageira (0, 33, 66 e 100%), em dietas de ovinos em crescimento. Objetivou-se, também, a comparação da estimativa de digestibilidade aparente por intermédio dos métodos de coleta total de fezes e de indicadores internos (fibra em detergente ácido [FDAi] e fibra em detergente neutro [FDNi] indigestíveis). Foi empregado delineamento experimental em quadrado latino, constituído de quatro animais, quatro períodos e quatro níveis de substituição do milho por farelo de palma forrageira. Não houve efeito da substituição sobre os consumos de MS, MO, PB e FDN. O consumo de NDT diminuiu linearmente com a inclusão de farelo de palma nas dietas. Os coeficientes de digestibilidade aparente da MS e MO diminuíram linearmente, enquanto os de PB e FDN não foram influenciados pela substituição. Não houve diferença entre a coleta total de fezes e o indicador interno FDNi na estimativa da digestibilidade dos nutrientes.The apparent digestibility and the intakes of dry matter (DM), organic matter (OM), crude protein (CP), neutral detergent fiber (NDF) and total digestible nutrients (TDN) were estimated to evaluate the effects of replacement by corn by forage cactus meal (0, 33, 66 and 100%) in growing lambs diets. It was also aimed to compare the internal markers, indigestible neutral detergent (NFDi) and acid detergent fiber (ADFi) with total feces collection to estimate the apparent digestibility of nutrients. A 4 x 4 latin square experimental design with four lambs, four periods and four levels of replacement of corn by forage cactus meal, was used. The replacement of corn by forage cactus meal did not affect the intakes of DM, OM, CP and NDF. TDN intake decreased linearly. The apparent digestibility of DM and OM decreased linearly, but no effect was observed for CP and NDF digestibilities. There was no difference between NDFi and total feces collection in the estimate of nutrients digestibility.

The digestive capacity of the Portuguese native breed of Barrosão cattle and the Holstein- Friesian breed was measured when fed meadow hay (72-74 g CP and 641-671 g NDF/kg dry matter), offered either alone or supplemented with soya bean meal (150 g/kg dry matter), at maintenance level. Four mature cows of each breed were used. Average initial live weight ( LW) was 457 and 635 kg for the Barrosão and the Holstein-Friesian cows respectively. The organic matter digestibility ( OMD) of the meadow hay was higher in Holstein- Friesian than in Barrosão cows (p < 0.08) whether supplemented or not. The neutral detergent fibre digestibility of the hay was also higher in Holstein- Frisian when the hay was fed alone (p < 0.08). The soya bean meal supplementation increased the OMD and the NDFD digestibility of the total diet (p < 0.05), but not the OMD digestibility of the hay (p > 0.05). The results obtained in this study suggest a higher ability to digest fibre in the large dairy breed than in the small native breed. This suggestion is also supported by previous findings with sheep breeds largely differing in mature live weight. [ABSTRACT FROM AUTHOR]

Four crossbred geldings were used in a randomized blocks experimental design. The objective was to study the use of the internal markers indigestible cellulose (iCEL) and indigestible lignin (iLIG), obtained in situ (cattle) or in vivo (equine) to predict nutrient apparent digestibility in horses. Treatments consisted of different methodologies to determine digestibility: direct method with total feces collection (TC), and indirect method using internal markers iCEL and iLIG obtained either by in situ incubation in bovine rumen or in vivo (IV) using the mobile nylon bag (MNB) technique in horses. Feces production was 2.80 kg in DM, and average recovery rate (p > 0.05) was 101%. Nutrient digestibility coefficient (p > 0.05) estimates were adequately predicted by iCEL and iLIG, obtained in situ or in vivo, with average values of 52.63, 54.17, 64.90, 43.73 and 98.28% for dry matter, organic matter, crude protein, neutral detergent fiber and starch, respectively. It can be concluded that iCEL and iLIG may be obtained in vivo by MNB in horses consuming a forage-concentrate diet, to predict nutrient digestibility coefficients.

Four castrated crossbred horses were used in a randomized block design to study the use of indigestible internal markers iNDF and iADF obtained in situ(from bovines) or in vivo (from equines). Treatments consisted of determining digestibility by the direct method comprising total feces collection (TC) and by the indirect method comprising internal markers iNDF and iADF obtained by in situincubation in bovine rumen or in vivo by the mobile nylon bag (MNB) technique with horses. iNDF-IV and iADF-IV resulted in better marker recovery rate (RR) (91.50%), similar to TC. The in situ technique resulted in lower RR values for the two indigestible markers, averaging 86.50% (p < 0.05). Estimates of the nutrient coefficient of digestibility (CD) were adequately predicted by iADF-IV, for horses fed on hay exclusively, with rates 46.41, 48.16, 47.92 and 45.51% for dry matter (DM), organic matter (OM), FDN and gross energy, respectively. Results show that MNB may be used to obtain iADF in horses fed on coast-cross hay exclusively, whereas NDFi and ADFi were selected for horses fed on mixed diets to predict the coefficient of nutrient digestibility

Copyright of Acta Scientiarum: Animal Sciences is the property of Universidade Estadual de Maringa and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Acta Scientiarum: Animal Sciences is the property of Universidade Estadual de Maringa and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Four 4×4 Latin square experiments were conducted with four cannulated dairy cows. Eight forages were fed ad libitum or supplemented with concentrate. Digestibility and flow of neutral detergent fibre (NDF) were evaluated using chromic oxide as a marker. Additionally, duodenal and ileal flows were determined based on faecal flow of in situ estimated indigestible NDF (iNDF) as marker, where faecal flow of iNDF was determined using chromic oxide. No significant difference was found between intake of iNDF and faecal flow of iNDF calculated using chromic oxide as a marker, whereas ileal flow of iNDF accounted for 0.85±0.02 and duodenal flow of iNDF for 0.89±0.02 of faecal flow of iNDF, respectively. Using chromic oxide as a marker underestimates intestinal flows of fibre. Average iNDF:NDF ratios were 0.22, 0.39, 0.54, 0.40 and 0.54 in feed, rumen content, duodenal chyme, ileal chyme and faeces, respectively, indicating severe problems in obtaining representative ileal samples. [ABSTRACT FROM AUTHOR]

PURPOSE: This study evaluated in a phantom model and verified in patients with lung cancer whether the use of an internal positron-emitting labeled marker could localize a critical structure by positron emission tomography (PET) imaging and verify multimodality image registration.MATERIALS AND METHODS: An initial device and method were developed to demonstrate by dedicated PET the location of the normal esophagus in a phantom and in three patients using a column of 2-deoxy-2-[18F]fluoro-D-glucose (FDG) solution between proximal and distal gas phases in polyurethane tubing. The device was assessed for possible loss of radioactivity. PET, CT and PET-CT fusion imaging followed.RESULTS: X-rays of the marker device showed a continuous fluid column. No leakage of contents was detected. The internal marker in the phantom and in patients allowed visualization by PET of the esophagus, and verified an image registration algorithm.CONCLUSIONS: A positron-emitting internal marker was constructed, demonstrated to retain tracer, and shown to be capable of verifying an image registration algorithm and identifying a critical structure, the esophagus, by PET in a phantom and in patients. [Copyright &y& Elsevier]

Background and purpose: We have developed a linear accelerator synchronized with a fluoroscopic real-time tumor-tracking system to reduce errors due to setup and organ motion. In the real-time tumor-tracking radiation therapy (RTRT) system, the accuracy of tumor tracking depends on the registration of the marker''s coordinates. The registration accuracy and possible migration of the internal fiducial gold marker implanted into prostate and liver was investigated.Materials and methods: Internal fiducial gold markers were implanted in 14 patients with prostate cancer and four patients with liver tumors. Computed tomography (CT) was carried out as a part of treatment planning in the 18 patients. A total of 72 follow-up CT scans were taken. We calculated the relative relationship between the coordinates of the center of mass (CM) of the organs and those of the marker. The discrepancy in the CM coordinates during a follow-up CT compared to those recorded during the planning CT was used to study possible marker migration.Results: The standard deviation (SD) of interobserver variations in the CM coordinates was within 2.0 and 0.4 mm for the organ and the marker, respectively, in seven observers. Assuming that organs do not shrink, grow, or rotate, the maximum SD of migration error in each direction was estimated to be less than 2.5 and 2.0 mm for liver and prostate, respectively. There was no correlation between the marker position and the time after implantation.Conclusion: The degree of possible migration of the internal fiducial marker was within the limits of accuracy of the CT measurement. Most of the marker movement can be attributed to the measurement uncertainty, which also influences registration in actual treatment planning. Thus, even with the gold marker and RTRT system, a planning target volume margin should be used to account for registration uncertainty. [Copyright &y& Elsevier]

Simple Summary: Knowledge of diet digestibility in animals is essential to reduce feed costs and the amount of undigested nutrients excreted in the manure. Diet digestibility can be evaluated via in vivo, in situ (i.e., in the rumen of cannulated animals), and in vitro methods. The in vivo by total faecal collection methods and the in situ methods are expensive and ethically questionable. The in vitro methods are cheaper but also less accurate. This work aimed to verify whether the in vivo method with undigested neutral detergent fibre (uNDF), which is commonly used in ruminants as an internal marker, could be adapted toward growing pigs. Dry matter, organic matter, and neutral detergent fibre digestibilities estimated with the uNDF were compared with in vivo values determined by total faecal collection in a previous study. The effects of pre-treating samples with the neutral detergent solution and adding α-amylase were also tested to improve the repeatability and accuracy of the results. It was concluded that the estimation of diet digestibility with pre-treated uNDF as an internal marker in growing pigs could be an alternative to the total faecal collection method. Undigested neutral detergent fibre (uNDF) is commonly used as an internal marker for the estimation of diet digestibility in ruminants. This work aimed to verify (i) whether the in vivo method with uNDF could be used to evaluate diet digestibility in growing pigs, and (ii) whether pre-treating the samples with neutral detergent solution (NDS) and α-amylase improves the accuracy of the estimates. Samples from a previously published work of two diets with known in vivo digestibility values estimated by the total faecal collection method and 16 individual samples of faeces were used. For each sample, four Ankom F57 bags were weighed. Before the incubation, two F57 bags were pre-treated with NDS and α-amylase. All the samples were incubated for 240 h in the Ankom DaisyII incubator and then analysed for their uNDF contents. Dry matter, organic matter, and neutral detergent fibre digestibilities were estimated using the uNDF contents, and the results were compared with those of the former study. The digestibility values obtained using the uNDF method with pre-treatment were not statistically different from those determined with the total faecal collection. On the contrary, the uNDF method without the pre-treatment could not satisfactorily predict the digestibilities of pig diets. [ABSTRACT FROM AUTHOR]

• Digestibility by n -alkane method was compared with the reference method. • Fecal recovery of n -alkanes increased with carbon chain length. • The n -alkanes were not completely recovered in the feces of the horse. • The n -alkane method showed low accuracy even after the fecal recovery correction. This study was conducted to determine the fecal recovery rate of n -alkanes and compare the precision and accuracy of the n -alkane technique and the conventional total fecal collection method in estimating the apparent nutrient digestibility of mixed hay diets fed to horses. Six castrated male adult horses (361 ± 12 kg body weight) were fed diets composed of different proportions of Tifton 85 bermudagrass (Cynodon dactylon) hay and alfalfa (Medicago sativa) hay in a 6 × 6 Latin square design. Treatments were as follows: T 100 , 100% Tifton hay; T 80 , 80% Tifton hay and 20% alfalfa hay; T 60 , 60% Tifton hay and 40% alfalfa hay; T 40 , 40% Tifton hay and 60% alfalfa hay; T 20 , 20% Tifton hay and 80% alfalfa hay; and T 0 , 100% alfalfa hay. The n- alkane detected at the greatest (P < 0.05) mean concentration in feed was n- hexatriacontane (C 36 H 74), followed by n- hexacosane (C 26 H 54), n- heptacosane (C 27 H 56), n- nonacosane (C 29 H 60), n- hentriacontane (C 31 H 64), n- tritriacontane (C 33 H 68), and n- pentatriacontane (C 35 H 72). A strong correlation (r = 0.91, P < 0.001) was found between carbon chain length and fecal recovery. The n- Hentriacontane, n- tritriacontane, n- pentatriacontane, and n- hexatriacontane had the greatest fecal recovery (mean = 0.91, P < 0.05) and were therefore used to estimate nutrient digestibility. None of the n- alkanes provided consistent results. Fecal recovery was incomplete, and fecal recovery rates above 100% were obtained regardless of the proportion of Tifton and alfalfa hay in diets. Dry matter, crude protein, neutral detergent fiber, acid detergent fiber, and mineral matter digestibility estimated using n- alkanes differed (P < 0.05) from digestibility values obtained by the reference method (total fecal collection). The simplest explanation for these results would be that horses preferred one type of hay over the other and had an uneven intake of each diet. Because Tifton and alfalfa hay differ greatly in composition, their uneven intake makes estimation of nutrient digestibility by fecal recovery of n -alkanes unreliable. Even after correction for fecal recovery, n- alkanes were inaccurate markers of nutrient digestibility. [ABSTRACT FROM AUTHOR]