Extracts of Saccharomyces cerevisiae (Sc) yeast were generated using a biotechnological solid-state fermentation method. The organic material used for fermentation consisted of a variety of fibrous substrates of agricultural origin, and their effect on the chemical composition and productivity of cellular biomass generated was evaluated. The substrates studied included the following: wheat straw, WS; barley straw, BS; chili stubble, CS; oat hull, OH and starch powder, SP (control). For incubation with Sc, 2 g of (dry) vegetable substrate were added to 12 mL of cultivation medium. The fermentation time was seven days under aerobic conditions (pH 5, 26ºC). After extraction of the yeast biomass, its chemical composition and productivity were analyzed. The results showed an effect (P≤0.01) of the type of vegetable substrate fermented on the crude protein (CP) and true protein (TP) contents of the yeast biomass. The highest concentration of CP was obtained by fermentation of the control TSP (treatment with the starch powder substrate ) (506.3 g kg -1 DM), followed by the treatment TOH (474.5 g kg -1 DM). The productivity of the yeast biomass was affected by the type of substrate fermented (P≤0.001). The highest value was obtained with TSP (327.10 mg g -1 ), followed by TOH (treatment with the oat hull substrate) (207.08 mg g -1 ). The lowest efficiency was measured in TCS (156.30 mg g -1 ). On the basis of these results, it may be inferred that an extract of high CP can be obtained from yeast (Sc) by the fermentation of fibrous substrates.
The high content of crude protein (CP) of pastures in southern Chile creates a nutritional imbalance that results in large amount of nitrogen (N) being excreted into the environment. Two regression equations and a dynamic model, available from the literature, were utilized to simulate the N use efficiency (NUE) in pasture-finished steers. A medium frame animal (Angus breed) with an initial and final body weight (BW) of 350 and 500 kg, respectively, was used to estimate nutrient requirements and excretions. Average DMI was fixed at 2.2% of animal BW with 150 days on feed (October to February). Energy and protein requirements, as well as the retained protein, were estimated by using Beef NRC software (tabular system level 1), while the nutritional characteristics of the pasture were obtained from studies based in Chile. The quality of the pastures was good enough to allow daily weight gain (GDP) of 1.0 kg/d. However, metabolizable protein balance was positive across the whole finishing period (average of 149 g/d). Total production of microbial protein increases with time because of the greater DMI, even when the PC content in forage decreases with time. This also explains the lower contribution of UIP during the last months of the finishing period. The estimated N intake was 123 g/d, whereas N excretion was 106 g/d. In summary, the combination of high levels of CP of Chilean southern pastures and the low NUE of the finishing cattle (13.4-16.3%), represent a strong challenge in terms of contamination and production.
SUMMARYA mechanistic model (COWPOLL) was used to estimate enteric methane (CH 4 ) emissions from beef production systems in Chile. The results expressed as a proportion of gross energy intake (GEI) were compared with enteric fermentation data reported in the last Chilean greenhouse gases inventory, which utilized an earlier the Intergovernmental Panel on Climate Change Tier 2 approach. The simulation analysis was based on information from feedstuffs, dry matter intake (DMI), body weight (BW) and average daily gain (ADG) of steers raised and finished at two research facilities located in Central and Southern Chile, as well as three simulated scenarios for grass-based finishing systems in Southern Chile. Data for feedlot production systems in the central region were assessed by considering steers fed a forage : concentrate ratio of 23 : 77 using maize silage and wheat straw as roughage sources during the stages of backgrounding and fattening. Average DMI were 7·3 ± 0·62 and 9·2 ± 0·55 kg/day per steer for backgrounding and fattening, respectively, whereas ADG were 1·1 ± 0·22 and 1·3 ± 0·37 kg/day for backgrounding and fattening. For the Southern Chilean fattening production systems, the forage : concentrate ratio was 56 : 44 with ryegrass pasture as the sole forage source. In this case, average DMI was 9·97 ± 0·51 and ADG was 1·1 ± 0·24 kg/day per steer. Two of the grass-based scenarios used the same initial BW information as that used for the Central and Southern Chilean systems, but feedlot diets were replaced by ryegrass pasture. The third grass-based scenario used an initial BW of 390 kg. In all the grass-based scenarios an ADG of 0·90 kg/day, with maximum DMI estimated as a proportion of BW (0·01 of NDF, kg/kg BW), was assumed. The results of the simulation analysis showed that emission factors (Y m ; fraction of GEI) ranged from 0·062 to 0·079 of GEI. Smaller values were associated with finishing systems that included a lower proportion of forage in the diet due to higher propionate production, which serves as a sink for hydrogen in the rumen. Cattle finished in feedlot systems had an average of 0·062 of GEI lost as CH 4 , whereas grass-based cattle had losses of 0·079 of GEI. Enteric CH 4 emissions for the systems using grass-based and concentrate diets were 261 and 159 g/kg weight gain, respectively. The Chilean CH 4 inventory employs a fixed Y m of 0·060 to estimate enteric fermentation for all cattle. This value is lower than the average Y m obtained in the current simulation analysis (0·071 of GEI), which results in underestimation of enteric CH 4 emissions from beef cattle. However, these results need to be
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