Recently, feedstuffs for ruminants have been increasing costs and decreasing availability due to global warming effects, raising production of biofuels, and long dry seasons. Plants that use the Crassulacean Acid Metabolism (CAM) pathway have low water requirements and are productive in semiarid regions because they assimilate carbon at night, thereby decreasing the diffusive gradient of water out of leaves and improving water use efficiency (^{Nobel, 1994}). The Agave genus is composed exclusively of obligate CAM plants, which have an increasing interest to many countries for cultivate and industrialize. Agave bagasse have been produced greatly by Mexican alcohol-beverage industries, specifically for production of tequila and mezcal beverages. Agave bagasse is the residual fiber remaining after cooked agave heads are shredded, milled and the sugars water extracted.

It represents about 40% of the total weight of the milled agave on a wet weight basis (^{Cedeno, 1995}; ^{Iniguez-Covarrubias et al., 2001}). This by-product is typically composted and reapplied to Agave fields, returning nutrients but also contributing to acidification of the soil. As well, Agave bagasse has been related to public health problems. Alternative strategies tending to decrease bagasse pollution and to evaluate its nutrient useful arise. Moreover, added information becomes necessary to appropriate uses of this ingredient. For that reasons the main objective of this investigation was to evaluate the nutrient profile of Agave salmiana and Agave weberi cela as alternative feedstuff for ruminants.

Experimental design: samples of Agave salmiana and Agave weberi cela bagasse were collected from mezcal factories at Zacatecas state in Mexico during 2010. Bagasse samples were used for bromatological analysis, mineral composition and, in vitro gas production kinetics, using bean straw as a control group. All lab analysis was effectuated in triplicate.

Analytical procedures: bromatological composition tested included, dry matter, ether extract, crude protein, crude fiber, and ash percentages on Agave bagasse (AOAC, 1990). Total organic carbon (TOC) was obtained by the following equation: TOC= (100 - % ash)/1.8. Neutral detergent fiber was obtained by ^{Van Soest et al. (1991)} technique. Total reducer sugars were obtained using Lane-Eynon technique. Also, pH was measured using preparations of Agave extracts in water on a proportion of 1:5 (weight/volume). Moreover, mineral composition evaluated included: total nitrogen, calcium, cupper, iron, magnesium, manganesium, zinc, sulfur and potassium were accounted by atomic absorption spectroscopy; and phosphor by visible spectrophotometry. Finally, kinetic degradability was evaluated using the in vitro gas production technique described by ^{Menke and Steingass (1988)}, testing by 48 h, reading pressured each 5 min using Ankom^® RFS gas production base kit. Volatile fatty acid (VFA) concentrations were obtained at 24h of incubation by gas chromatography.

Statistical analysis: PROC MIXED of SAS was used to assess statistical differences and Tukey used as a power test (p< 0.05) (SAS, 1991).

Bromatological profile was described at Table 1. ^{Ortiz-Tovar et al. (2007)} reported values of Agave bagasse averaging 5.02, 58.72, 54.14, 19.14, 34.99 and 4.58% of CP, NDF, ADF, lignin, cellulose, and hemicellulose, respectively. Crude protein results (~2.5%) reported in this trial showed that this nutrient is not enough to maximize rumen microbial metabolism, which optimal levels have been estimated between 7 to 8%. However, solid state fermentation adding urea has been proposed to increase crude protein from feedstuffs containing low amounts. Agave salmiana bagasse has been reported to contain 47, 13 and 10% of cellulose, hemicellulose and lignin (^{Garcia-Reyes and Rangel-Mendez, 2009}). Paredes -Ibarra et al. (2007) reported an increment on dry matter intake in lambs fed with a total mixed ratio that included Agave bagasse. Also, ^{Iñiguez-Covarrubias et al. (2001)} concluded that agave bagasse pith could have the same economic value as corn stubble, with the advantage that agave bagasse pith is available all year and corn stubble only after corn harvesting.

Table 1 Bromatological profile of Agave salmiana and Agave webery cela. 

^abRows within unlike superscript differs p< 0.05.

Macro mineral evaluations showed that Agave bagasse can represent a natural source of Calcium. Agave plants are perennial evergreen xerophytes containing calcium oxalate crystals at the mesophyll (^{Blunden et al ., 1973}). ^{Chávez-Guerrero et al. (2010)} concluded that Agave salmiana bagasse is an important source of calcium compounds after its incineration. Calcium requirements of bovines have been established between 0.18 and 1.04%, while sheep requirements are between 0.21 and 0.51% (^{Maynard et al., 1981}). At dry seasons requirements of calcium for ruminants can be cover by Agave bagasse. Iron found at the bagasse analysis can supply ruminant requirements which have been estimated between 40 and 50 ppm for sheep and cattle (^{NRC, 2007}).

Table 2 Mineral composition of Agave salmiana and Agave webery cela byproducts. 

^{Ortíz-Tovar et al. (2007)} obtained (% mol) from Agave bagasse after the in vitro gas production technique during (24 h), 64, 29, 7 and a 2.2 for acetate, propionate, butyrate and acetate:propionate ratio respectively. Unfortunately, those authors did not report what king of Agave was used. Our results are expressed as mmol, which can explain differences between our and reported VFA data. Bean straw is often used as feedstuffs at the dry season, and no statistical differences (p> 0.05) were observed on VFA among bagasse and straw.

Kinetic gas production of Agave bagasse and bean straw is represented in Figure 1. Differences in volume gas (ml) corresponding to complete substrate digestion were observed, between Agave salmiana and bean straw (69 vs 68.5) compared with Agave weberi cela (35.5). Degradation rate (min) was higher on bean straw (606) comparing with Agave salmiana (297) and A. webery cela (442).

Figure 1 Degradability of agave bagasses by in vitro gas production system. 

Table 3 Volatile fatty acid production (mmol) of Agave salmiana and Agave webery cela bagasse, and bean straw.