| 1. |
- Cabezas Garcia, Edward Hernando, et al.
(författare)
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Between-cow variation in digestion and rumen fermentation variables associated with methane production
- 2017
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Ingår i: Journal of Dairy Science. - : American Dairy Science Association. - 0022-0302 .- 1525-3198. ; 100, s. 4409-4424
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Tidskriftsartikel (refereegranskat)abstract
- A meta-analysis based on an individual-cow data set was conducted to investigate the effects of between-cow variation and related animal variables on predicted CH4 emissions from dairy cows. Data were taken from 40 change-over studies consisting of a total of 637 cow/period observations. Animal production and rumen fermentation characteristics were measured for 154 diets in 40 studies; diet digestibility was measured for 135 diets in 34 studies, and ruminal digestion kinetics was measured for 56 diets in 15 studies. The experimental diets were based on grass silage, with cereal grains or by-products as energy supplements, and soybean or canola meal as protein supplements. Average forage:concentrate ratio across all diets on a dry matter basis was 59:41. Methane production was predicted from apparently fermented substrate using stoichiometric principles. Data were analyzed by mixed-model regression using diet and period within experiment as random effects, thereby allowing the effect of experiment, diet, and period to be excluded. Dry matter intake and milk yield were more repeatable experimental measures than rumen fermentation, nutrient outflow, diet digestibility, or estimated CH4 yield. Between-cow coefficient of variation (CV) was 0.010 for stoichiometric CH4 per mol of volatile fatty acids and 0.067 for predicted CH4 yield (CH4/dry matter intake). Organic matter digestibility (OMD) also displayed little between-cow variation (CV = 0.013), indicating that between-cow variation in diet digestibility and rumen fermentation pattern do not markedly contribute to between cow-variation in CH4 yield. Digesta passage rate was much more variable (CV = 0.08) between cows than OMD or rumen fermentation pattern. Increased digesta passage rate is associated with improved energetic efficiency of microbial N synthesis, which partitions fermented substrate from volatile fatty acids and gases to microbial cells that are more reduced than fermented carbohydrates. Positive relationships were observed between CH4 per mol of volatile fatty acids versus OMD and rumen ammonia N concentration versus OMD; and negative relationships between the efficiency of microbial N synthesis versus OMD and digesta passage rate versus OMD, suggesting that the effects of these variables on CH4 yield were additive. It can be concluded that variations in OMD and efficiency in microbial N synthesis resulting from variations in digesta passage contribute more to between-animal variation in CH4 emissions than rumen fermentation pattern.
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| 2. |
- Cabezas Garcia, Edward Hernando, et al.
(författare)
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Effects of replacement of late-harvested grass silage and barley with early-harvested silage on milk production and methane emissions
- 2017
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Ingår i: Journal of Dairy Science. - : American Dairy Science Association. - 0022-0302 .- 1525-3198. ; 100, s. 5228-5240
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Tidskriftsartikel (refereegranskat)abstract
- This study evaluated the effects of gradual replacement of a mixture of late-cut grass silage (LS) and barley with early-cut grass silage (ES) on milk production, CH4 emissions, and N utilization in Swedish Red cows. Two grass silages were prepared from the same primary growth of timothy grass sward but harvested 2 wk apart [11.0 and 9.7 MJ of metabolizable energy/kg of dry matter (DM)]. Four diets, fed as a total mixed ration, were formulated to meet the metabolizable energy and protein requirements of 35 kg of energy-corrected milk (ECM) by gradually replacing a mixture of LS and barley with ES (0, 33, 67, and 100% of the forage component of the diet), whereas the proportion of barley decreased from 47.2 to 26.6% of diet DM. Expeller canola meal was used as a protein supplement. Sixteen Swedish Red cows were used in 4 replicated 4 x 4 Latin squares. Cows were offered diets ad libitum and milked twice daily. Each period of 28 d comprised 14 d of diet adaptation followed by 14 d of data collection. Intake and milk yield were recorded daily, and milk samples were collected on d 19 to 21 and d 26 to 28 of each period. Diet digestibility was determined by grab sampling using indigestible neutral detergent fiber as an internal marker. Gas emissions were measured using the Green Feed system (C-Lock Inc., Rapid City, SD). Dry matter intake (DMI) linearly decreased from 22.6 to 19.3 kg/d as the proportion of ES increased in the diet. The ECM yield did not differ among treatments, but milk protein yield decreased with increasing proportion of ES in the diet. Because of reduced DMI with increasing ES, feed efficiency (ECM/DMI) improved with an increased proportion of ES in the diet. Nitrogen efficiency (milk N/N intake) did not change despite a linear increase in milk urea N concentration from 9.7 (LS alone) to 11.9 mg/dL (ES alone) with graded replacement of LS and barley by ES in the diet. Lower DMI responses in ES diets were partly compensated for by increased organic matter digestibility (656 g/kg of DM for LS alone; 715 g/kg of DM for ES alone) related to improved forage digestibility at early harvesting. Total CH4 emissions and CH4 intensity (CH4/ECM) were not influenced by diet, but CH4 yield (CH4/DMI) increased linearly from 19.5 to 23.0 g/kg of DMI with greater inclusion of ES in the diet. In conclusion, replacing LS and barley with ES improved the conversion of feed to milk without increasing CH4 emissions or compromising N efficiency.
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| 3. |
- Cabezas Garcia, Edward Hernando, et al.
(författare)
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Effects of replacement of late-harvested grass silage and barley with early-harvested silage on ruminal digestion efficiency in lactating dairy cows
- 2018
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Ingår i: Journal of Dairy Science. - : American Dairy Science Association. - 0022-0302 .- 1525-3198. ; 101, s. 1177-1189
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Tidskriftsartikel (refereegranskat)abstract
- The objective of this experiment was to quantify the effects of graded replacement of late-harvested grass silage and barley with early-harvested silage on nutrient digestion and rumen fermentation. Four experimental diets were fed to 4 multiparous rumen-cannulated Nordic Red cows in 4 x 4 Latin square design with 21-d periods. Dietary treatments consisted of late-cut grass silage (LS) and rolled barley, which was gradually replaced with early-cut grass silage [ES; 0, 33, 67, and 100% of the forage component (ES + LS) of the diet]. With increased proportion of ES in the diet, the proportion of barley decreased from 47.2 to 26.6% on a dry matter basis. Early-and late-cut silages were harvested at 2-wk intervals (predicted concentrations of metabolizable energy 11.0 and 9.7 MJ/kg of dry matter). The 4 diets were formulated to support the same milk production. Nutrient flows were quantified using omasal sampling technique applying the triple-marker method (Cr, Yb, and indigestible neutral detergent fiber) and N-15 as a microbial marker. Feed intake decreased with graded replacement of LS and barley with ES, but milk production was not influenced by diet. Digestibility of nutrients improved with graded addition of ES in the diet with the greatest difference observed in digestibility of neutral detergent fiber (NDF) and potentially digestible NDF (pdNDF). The results suggested that improved cell wall digestibility with graded level of ES in the diet was partly related to higher intrinsic digestibility of ES than LS, and partly due to negative associative effects with an increased proportion of LS and barley in the diet. Efficiency of microbial N synthesis was not influenced by the diet, but ruminal protein degradability increased with ES in the diet. Rumen fermentation pattern was not affected by the diet despite large difference in the profile of dietary carbohydrates. Rumen pool size of NDF and pdNDF, and ruminal turnover time of NDF decreased with graded addition of ES in the diet, whereas digestion rate of pdNDF improved. The results of this study indicate that increased CH4 yield in a parallel production study with graded addition of ES in the diet were more related to greater ruminal and total digestibility of organic matter than to the changes in rumen fermentation pattern.
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| 4. |
- Cabezas Garcia, Edward Hernando
(författare)
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Effects of source and concentration of neutral detergent fiber from roughage in beef cattle diets: Comparison of methods to measure the effectiveness of fiber
- 2020
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Ingår i: Journal of Animal Science. - : Oxford University Press (OUP). - 0021-8812 .- 1525-3163. ; 98
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Tidskriftsartikel (refereegranskat)abstract
- Methods have been developed to measure the effectiveness of many roughages, but few evaluations have been conducted with tropical feeds. The objectives of this research were to determine the effectiveness of roughage sources based on bioassay and laboratory methods and identify the biological attributes of the diets that correlate with these methods. Six ruminally cannulated Nellore steers (408 +/- 12 kg of BW) were randomly assigned to a 6 x 6 Latin square design within six diets: negative control diet (NC) with aNDF as 10% from corn silage (CS); positive control diet (PC) with aNDF as 20% from CS; and four diets containing 10% aNDF from CS and 10% aNDF from each of the following sources: sugarcane (SC), sugarcane bagasse (SCB), soybean hulls (SH), or low oil cottonseed hulls (LOCH). Physical effectiveness factor (pef, related to the physical characteristics of aNDF) and effectiveness factor (ef, related to the ruminal pH) were determined based on a linear model approach that uses a bioassay method in which CS aNDF was assumed to be the standard fiber source. Laboratory methods to estimate pef of roughage sources were based on the proportion of DM of roughage retained on a 1.18-mm sieve pef(>1.18 mm) or retained on the 8.0-mm Penn State Particle Separator screen pef(>8.0 mm). The pef calculated by the bioassay method (total chewing time and ruminal mat resistance) for CS, SCB, and SC were higher values (P < 0.05) compared with SH and LOCH. The pef(rumen mat) of SC and SCB were higher (P < 0.05) than that of CS, SH, and LOCH. The pef(rumen mat) of LOCH was 61% higher than SH. The ef(rumen pH) of SC and LOCH was higher (P < 0.05) than CS and SH. The pef(chewing, min/d), pef(chewing, min/kg of DM), pef(rumen mat), and ef(rumen pH) positively correlated with rumination time, total chewing time, and ruminal mat resistance (values from transit time in seconds). No correlation was observed (P > 0.05) between pef(>8.0 mm) and rumination time, chewing time, and ruminal pH. The pef calculated using the bioassay method as well as pef (>8.0 mm) were negatively correlated with rumen pH (P > 0.05). The values of the effectiveness of fiber sources obtained in this research can be used as a guideline for nutritionists aiming to replace roughage sources from tropical regions in beef cattle finishing diets. Under our conditions, the pef using the bioassay method or laboratory methods were not adequate in predicting ruminal pH.
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| 5. |
- Cabezas Garcia, Edward Hernando
(författare)
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Effects of source and concentration of neutral detergent fiber from roughage in beef cattle diets on feed intake, ingestive behavior, and ruminal kinetics
- 2020
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Ingår i: Journal of animal science. - : Oxford University Press (OUP). - 0021-8812 .- 1525-3163. ; 98
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Tidskriftsartikel (refereegranskat)abstract
- The objectives of this research were to evaluate the effects of source and concentration of a-amylase-treated neutral detergent fiber (aNDF) from roughage on feed intake, ingestive behavior, and ruminal kinetics in beef cattle receiving high-concentrate diets. Six ruminally cannulated Nellore steers (408 +/- 12 kg of body weight) were randomly assigned to a 6 x 6 Latin square design with six diets: 10% aNDF from corn silage (10CS); 20% aNDF from corn silage (20CS); or four diets containing 10% aNDF from corn silage and 10% aNDF from one of the following sources: sugarcane (SC), sugarcane bagasse (SCB), soybean hulls (SH), or low oil cottonseed hulls (LOCH). The parameters of passage and degradation kinetics were estimated based on a two-compartmental model with gamma- and exponential-distributed residence times. The nonlinear models were fitted by nonlinear least squares, and a linear mixed-effects model was fitted to all variables measured from the Latin square design that were related to intake, digestibility, digestion kinetic parameters, and residence times. Mean particle size (MPS) between roughage sources (CS, SCB, and SC) and coproducts (SH and LOCH) was affected (P < 0.05). Dry matter intake (DMI) was not affected (P > 0.05) by 20CS, SC, SH, or LOCH. Steers fed 20CS or LOCH diets had 16% and 20% greater DMI, respectively, (P < 0.05) than steers fed 10CS diet. Steers fed SCB consumed the least dry matter (DM). The SH and LOCH diets had lower MPS values (about 8.77 mm) in comparison to 20CS, SCB, and SC diets (about 13.08 mm) and, consequently, affected (P < 0.05) rumen content, ruminal in situ disappearance, nutrient digestibility, and solid fractional passage rate. Chewing time was affected (P < 0.05) by roughage sources and concentration. Lower values of distance travel inside the rumen (min/cm) were observed (P < 0.05) for the SCB and SC diets in comparison with any other diet. Except for SCB, there was no difference (P > 0.05) in rumen fill, among other treatments. Mean daily ruminal pH was not affected (P > 0.05) by 20CS, SCB, SC, and LOCH diets, and it ranged from 6.1 to 6.23. Total short-chain fatty acids concentration was affected (P < 0.05) by roughage source and concentration. Based on our results, we recommend that under Brazilian finishing diets, replacing roughage sources, except for SCB, based on aNDF concentration of the roughage in high-concentrate diets containing finely ground flint corn does not affect DMI.
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| 6. |
- Cabezas Garcia, Edward Hernando, et al.
(författare)
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Fibre digestion potential in sugarcane across the harvesting window
- 2014
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Ingår i: Grass and Forage Science. - : Wiley. - 0142-5242 .- 1365-2494. ; 69, s. 176-181
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Tidskriftsartikel (refereegranskat)abstract
- The objectives of this study were to determine the proportion of indigestible neutral detergent fibre (iNDF) in the neutral detergent fibre (NDF) fraction of sugarcane, to estimate changes in NDF digestibility (NDFD) during the harvesting window and to predict sugarcane digestibility based on its fibre fractions. Whole plants of the IAC86-2480 and IAC93-3046 varieties were collected during the harvesting windows in 2007 and 2009, respectively. The in vitro true dry matter digestibility (IVTDMD) and fibre contents (NDF, iNDF, acid detergent fibre) were determined by near infrared reflectance spectroscopy (NIRS). The NDFD and potentially digestible NDF digestibility (pdNDFD) were estimated, and the fractional digestion rate of pdNDF was calculated. There was no relationship between growing days and iNDF as a proportion of NDF (P = 0.28) or with the fractional digestion rate of pdNDF (P = 0.30). Therefore, NDFD (mean = 335 g kg(-1)) and pdNDFD (mean = 657 g kg(-1)) remained almost constant during the harvesting window (P = 0.70 and P = 0.32 respectively). Acid detergent fibre and NDF were the best predictors of sugarcane dry matter (DM) digestibility. In conclusion, NDFD seems to be unchanged during crop growth. The point at which to harvest sugarcane as a forage source should therefore be decided based on NDF concentration, which is greatly influenced by sucrose accumulation.
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| 7. |
- Cabezas Garcia, Edward Hernando, et al.
(författare)
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In Vitro Incubations Do Not Reflect In Vivo Differences Based on Ranking of Low and High Methane Emitters in Dairy Cows
- 2021
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Ingår i: Animals. - : MDPI AG. - 2076-2615. ; 11
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Tidskriftsartikel (refereegranskat)abstract
- This study evaluated if ranking dairy cows as low and high CH4 emitters using the GreenFeed system (GF) can be replicated in in vitro conditions using an automated gas system and its possible implications in terms of fermentation balance. Seven pairs of low and high emitters fed the same diet were selected on the basis of residual CH4 production, and rumen fluid taken from each pair incubated separately in the in vitro gas production system. In total, seven in vitro incubations were performed with inoculums taken from low and high CH4 emitting cows incubated in two substrates differing in forage-to-concentrate proportion, each without or with the addition of cashew nutshell liquid (CNSL) as an inhibitor of CH4 production. Except for the aimed differences in CH4 production, no statistical differences were detected among groups of low and high emitters either in in vivo animal performance or rumen fermentation profile prior to the in vitro incubations. The effect of in vivo ranking was poorly replicated in in vitro conditions after 48 h of anaerobic fermentation. Instead, the effects of diet and CNSL were more consistent. The inclusion of 50% barley in the diet (SB) increased both asymptotic gas production by 17.3% and predicted in vivo CH4 by 26.2%, when compared to 100% grass silage (S) substrate, respectively. The SB diet produced on average more propionate (+28 mmol/mol) and consequently less acetate compared to the S diet. Irrespective of CH4 emitter group, CNSL decreased predicted in vivo CH4 (26.7 vs. 11.1 mL/ g of dry matter; DM) and stoichiometric CH4 (CH4VFA; 304 vs. 235 moles/mol VFA), with these being also reflected in decreased total gas production per unit of volatile fatty acids (VFA). Microbial structure was assessed on rumen fluid sampled prior to in vitro incubation, by sequencing of the V4 region of 16S rRNA gene. Principal coordinate analysis (PCoA) on operational taxonomic unit (OTU) did not show any differences between groups. Some differences appeared of relative abundance between groups in some specific OTUs mainly related to Prevotella. Genus Methanobrevibacter represented 93.7 ± 3.33% of the archaeal sequences. There were no clear differences between groups in relative abundance of Methanobrevibacter.
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| 8. |
- Cabezas Garcia, Edward Hernando
(författare)
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Methane production in dairy cows : individual cow variability in methane production
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Enteric methane (CH4) emissions vary between individual cows, and this variation is attributed to both animal and dietary factors. In addition, measurement technique of in vivo CH4 emissions from individual animals still represents a major challenge for successful emissions mitigation strategies. This thesis investigated the contribution of different factors to between-animal variation in CH4 production, in order to improve the current knowledge of its biological basis. In a study comparing on-farm systems for measuring CH4 emissions from large numbers of animals and the variation between individual animals, the GreenFeed system was used as the normal set-up (flux method) or modified to mimic gas analysers systems based on CH4 concentrations (sniffer system) to measure CH4 emissions. Measurements taken by the GreenFeed system proved be more reliable than those made by the simulated sniffer method. The GreenFeed data were consistent with literature values determined in respiration chambers, while the sniffer method were poorly correlated to flux method values and were not significantly related to either feed intake or milk yield. Despite GreenFeed being a spot sampling method, it proved to be a promising tool for ranking cows as high and low CH4 emitters. A meta-analysis based on an individual cow dataset investigating the effects of between-cow variation and related animal variables on predicted CH4 emissions from dairy cows. Between-cow variation in fermentation pattern are not likely be the major factor influencing predicted in vivo CH4 emissions. Variation and repeatability for volatile fatty acid concentrations were greater for ruminal concentrations than molar proportions, indicating strong control by the individual cow. Digestion kinetics variables were more repeatable than rumen fermentation or microbial synthesis, as a result of variations in passage rate. In studies in which late-cut silage and rolled barley were gradually replaced with early-cut silage in the diet of dairy cows, production responses and in vivo CH4 emissions were studied in 16 intact lactating cows and possible physiological mechanisms were assessed in four rumen-cannulated cows. Improvements in forage quality by graded addition of early-cut silage was an effective strategy to reduce concentrate supplementation, without compromising performance or increasing CH4 emissions in lactating dairy cows. Differences in intake between treatments were partly compensated by differences in silage digestibility.
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| 9. |
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| 10. |
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| 11. |
- Huhtanen, Pekka, et al.
(författare)
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Comparison of methods to determine methane emissions from dairy cows in farm conditions
- 2015
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Ingår i: Journal of Dairy Science. - : American Dairy Science Association. - 0022-0302 .- 1525-3198. ; 98, s. 3394-3409
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Tidskriftsartikel (refereegranskat)abstract
- Nutritional and animal-selection strategies to mitigate enteric methane (CH4) depend on accurate, cost-effective methods to determine emissions from a large number of animals. The objective of the present study was to compare 2 spot-sampling methods to determine CH4emissions from dairy cows, using gas quantification equipment installed in concentrate feeders or automatic milking stalls. In the first method (sniffer method), CH4and carbon dioxide (CO2) concentrations were measured in close proximity to the muzzle of the animal, and average CH4concentrations or CH4/CO2ratio was calculated. In the second method (flux method), measurement of CH4and CO2concentration was combined with an active airflow inside the feed troughs for capture of emitted gas and measurements of CH4and CO2fluxes. A muzzle sensor was used allowing data to be filtered when the muzzle was not near the sampling inlet. In a laboratory study, a model cow head was built that emitted CO2at a constant rate. It was found that CO2concentrations using the sniffer method decreased up to 39% when the distance of the muzzle from the sampling inlet increased to 30 cm, but no muzzle-position effects were observed for the flux method. The methods were compared in 2 on-farm studies conducted using 32 (experiment 1) or 59 (experiment 2) cows in a switch-back design of 5 (experiment 1) or 4 (experiment 2) periods for replicated comparisons between methods. Between-cow coefficient of variation (CV) in CH4was smaller for the flux than the sniffer method (experiment 1, CV = 11.0 vs. 17.5%, and experiment 2, 17.6 vs. 28.0%). Repeatability of the measurements from both methods were high (0.72–0.88), but the relationship between the sniffer and flux methods was weak (R2= 0.09 in both experiments). With the flux method CH4was found to be correlated to dry matter intake or body weight, but this was not the case with the sniffer method. The CH4/CO2ratio was more highly correlated between the flux and sniffer methods (R2= 0.30), and CV was similar (6.4–8.8%). In experiment 2, cow muzzle position was highly repeatable (0.82) and influenced sniffer and flux method results when not filtered for muzzle position. It was concluded that the flux method provides more reliable estimates of CH4emissions than the sniffer method. The sniffer method appears to be affected by variable air-mixing conditions created by geometry of feed trough, muzzle movement, and muzzle position.
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| 12. |
- Huhtanen, Pekka, et al.
(författare)
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Effects of ruminal digesta retention time on methane emissions: a modelling approach
- 2016
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Ingår i: Animal Production Science. - 1836-0939 .- 1836-5787. ; 56, s. 501-506
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Tidskriftsartikel (refereegranskat)abstract
- The reasons for among-animal variations in methane (CH4) emissions are not fully understood. There is experimental evidence that ruminal digesta mean retention time (MRT) can affect CH4 emissions. The objective of the present study was to evaluate the contribution of among-animal variations in MRT on CH4 emissions and nutrient supply for dairy cow (default MRT = 34 h) and sheep (default MRT = 41 h), using the mechanistic Nordic dairy cow model Karoline. The simulations (n = 100) were made for a cow (bodyweight 600 kg) and for a sheep (bodyweight 60 kg) eating 20 kg and 1.0 kg DM/day, respectively. The diet for the dairy cow consisted of grass silage, barley and rapeseed meal (60:30:10 on a DM basis; crude protein 156 g/kg DM, neutral detergent fibre 450 g/kg DM) and the sheep diet was grass alone. Normal distribution of MRT values was assumed. Variability (coefficient of variation (CV) = 0.086) on default MRT was introduced by random-number generator of Excel. Intake, diet composition and digestion kinetic parameters were constant in all simulations, only ruminal MRT variables were changed in each simulation. Predicted CH4 emission increased with increased MRT for dairy cow (range from 407 to 488 g/day) and sheep (from 25.0 to 29.2 g/day). Increases in predicted CH4 emissions were partly associated with enhanced organic matter (OM) digestibility in dairy cow (from 0.715 to 0.758) and sheep (from 0.731 to 0.773). Greater CH4 emissions per kilogram digested OM with increased MRT were mainly related to reduced efficiency of microbial cell synthesis in the rumen both for dairy cows (22.8 +/- 0.91 g N/kg OM truly digested; CV = 0.040) and for sheep (20.7 +/- 0.92 g N/kg OM truly digested; CV = 0.044). Predicted CH4 yield was 20% and 17% greater in dairy cow and sheep, respectively, with the short (n = 10) compared with the long (n = 10) ruminal digesta MRT. Linear regression indicated that CH4 emissions increased by 0.37 (dairy cow) and 0.33 (sheep) g/kg DM intake per 1 h increase in ruminal digesta MRT. It is concluded that among-animal variation in MRT can markedly contribute to among-animal variation in CH4 emissions from ruminants.
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| 13. |
- Huhtanen, Pekka, et al.
(författare)
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Evaluation of between-cow variation in milk urea and rumen ammonia nitrogen concentrations and the association with nitrogen utilization and diet digestibility in lactating cows
- 2015
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Ingår i: Journal of Dairy Science. - : American Dairy Science Association. - 0022-0302 .- 1525-3198. ; 98, s. 3182-3196
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Tidskriftsartikel (refereegranskat)abstract
- Concentrations of milk urea N (MUN) are influenced by dietary crude protein concentration and intake and could therefore be used as a biomarker of the efficiency of N utilization for milk production (milk N/N intake; MNE) in lactating cows. In the present investigation, data from milk-production trials (production data set; n = 1,804 cow/period observations from 21 change-over studies) and metabolic studies involving measurements of nutrient flow at the omasum in lactating cows (flow data set; n = 450 cow/period observations from 29 studies) were used to evaluate the influence of between-cow variation on the relationship of MUN with MNE, urinary N (UN) output, and diet digestibility. All measurements were made on cows fed diets based on grass silage supplemented with a range of protein supplements. Data were analyzed by mixed-model regression analysis with diet within experiment and period within experiment as random effects, allowing the effect of diet and period to be excluded. Between-cow coefficient of variation in MUN concentration and MNE was 0.13 and 0.07 in the production data set and 0.11 and 0.08 in the flow data set, respectively. Based on residual variance, the best model for predicting MNE developed from the production data set was MNE (g/kg) = 238 + 7.0 x milk yield (MY; kg/d) - 0.064 x MY2 - 2.7 x MUN (mg/dL) - 0.10 body weight (kg). For the flow data set, including both MUN and rumen ammonia N concentration with MY in the model accounted for more variation in MNE than when either term was used with MY alone. The best model for predicting UN excretion developed from the production data set (n = 443) was UN (g/d) = -29 + 4.3 x dry matter intake (kg/d) + 4.3 x MUN + 0.14 x body weight. Between-cow variation had a smaller influence on the association of MUN with MNE and UN output than published estimates of these relationships based on treatment means, in which differences in MUN generally arise from variation in dietary crude protein concentration. For the flow data set, between-cow variation in MUN and rumen ammonia N concentrations was positively associated with total-tract organic matter digestibility. In conclusion, evaluation of phenotypic variation in MUN indicated that between-cow variation in MUN had a smaller effect on MNE compared with published responses of MUN to dietary crude protein concentration, suggesting that a closer control over diet composition relative to requirements has greater potential to improve MNE and lower UN on farm than genetic selection.
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| 15. |
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