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 acid and 0á554, 0á716 and 0á892 for total volatile fatty acids 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 acid, lactic acid and alcohol concentrations in grass silage by gas±liquid chromatography using a single injection in an automated procedure that makes the 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 .
A partially balanced change-over design experiment involving 192 beef steers, which were initially 14 months old and 415 kg live weight, was carried out to determine the intakes of 136 silages from commercial farms in Northern Ireland. Each silage was offered ad libitum as the sole food to 10 animals, with eight silages offered in each of 17 periods over 2 years. A standard grass hay was offered to 16 animals in each period to enable period effects on intake to be removed. Detailed chemical and biological compositions of the silages were also determined. The ranges for pH and dry matter (DM), crude protein, ammonia-nitrogen and apparent digestible organic matter fin vivo) concentrations in the silages and silage dry DM intakes were 3-50 to 5-49 (s.d. 0-396); 155 to 413 (s.d. 43-1) g/kg; 79 to 212 (s.d. 24-4) g/kg DM; 45 to 384 (s.d. 63-2) g/kg total nitrogen; 528 to 769 (s.d. 58) g/kg DM and 4-3 to 10-9 (s.d. 1-13) kg/day respectively. Relationships between intake and individual parameters or groups of parameters have been developed using simple and multiple linear regression analysis and partial least-squares analyses. Silage intake was closely related to factors which influence the extent of digestion and rate of passage of the material through the animal, as indicated by the strong relationships (R 2 of regressions = 0-28 to 0-50) with in vivo apparent digestibility and rumen degradability and the concentrations of the fibre and nitrogen factors. Intake was poorly correlated with factors such as pH, total acidity, buffering capacity and the concentrations of lactic, acetic and butyric acids (R 2 of regressions = zero to 0-11). Near infrared reflectance spectrometry (NIRS) provided the best fit relationship with intake (R 2 of relationship = 0-90). The results also indicate that the intake potential of silages can be directly predicted with a high degree of accuracy from the NIRS of both dried and undried samples of silage, provided the appropriate sample preparation and scanning methods are used.
The data set used in the present study was obtained from 20 energy metabolism studies involving 579 lactating dairy cows (511 Holstein-Friesian, 36 Norwegian Red, and 32 Jersey-Holstein crossbreds) varying in genetic merit, lactation number, stage of lactation, and live weight. These cows were offered diets based on grass silage (n=550) or fresh grass (n=29), and their energy intake and outputs, including methane energy (CH(4)-E), were measured in indirect open-circuit respiration calorimeter chambers. The objective was to use these data to evaluate relationships between CH(4)-E output and a range of factors in animal production and energetic efficiency in lactating dairy cows under normal feeding regimens. The CH(4)-E as a proportion of milk energy output (E(l)), E(l) adjusted to zero energy balance (E(l(0))), or intakes of gross energy (GE), digestible energy (DE), or metabolizable energy (ME) was significantly related to a wide range of variables associated with milk production (E(l) and E(l(0))) and energy parameters (energy intake, metabolizability, partitioning, and utilization efficiencies). Three sets of linear relationships were developed with experimental effects removed. The CH(4)-E/GE intake (r(2)=0.50-0.62) and CH(4)-E/E(l) (r(2)=0.41-0.68) were reduced with increasing feeding level, E(l)/metabolic body weight (MBW; kg(0.75)), E(l(0))/MBW, GE intake/MBW, DE intake/MBW, and ME intake/MBW. Increasing dietary ME/DE decreased CH(4)-E/E(l) (r(2)=0.46) and CH(4)-E/GE intake (r(2)=0.72). Dietary ME concentration and ME/GE were also negatively related to CH(4)-E/GE intake (r(2)=0.47). However, increasing heat production/ME intake increased CH(4)-E as a proportion of E(l) (r(2)=0.41), E(l(0)) (r(2)=0.67) and energy intake (GE, DE, and ME; r(2)=0.62 and 0.70). These proportional CH(4)-E variables were reduced with increasing ratios of E(l)/ME intake and E(l(0))/ME intake and efficiency of ME use for lactation (r(2)=0.49-0.70). Fitting CH(4)-E/E(l) or CH(4)-E/E(l(0)) against these energetic efficiencies in quadratic rather than linear relationships significantly increased r(2) values (0.49-0.67 vs. 0.59-0.87). In conclusion, CH(4)-E as a proportion of energy intake (GE, DE, and ME) and milk production (E(l) and E(l(0))) can be reduced by increasing milk yield and energetic efficiency of milk production or by reducing energy expenditure for maintenance. The selection of dairy cows with high energy utilization efficiencies and milk productivity offers an effective approach to reducing enteric CH(4) emission rates.
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