The nutritive values of tumbleweed (Gundelia tournefortii L.) hays harvested at three stages were evaluated by chemical composition, in vitro (gas production) and in situ dry matter degradation methods. Gas production or dry matter (DM) degradation were calculated at 0, 3, 6, 12, 24, 48, 72 and 96 h and their kinetics were described using the equation p = a + b (1 - e(ct)). Maturity had a significant effect on both the chemical composition and degradability of tumbleweed hay. Neutral detergent fibre (NDF), acid detergent fibre (ADF) and ether extract (EE) increased with increasing maturity whereas the crude protein (CP) and ash content decreased. The gas produced after 96 h incubation ranged between 66.33 and 76.67 ml per 0.200 g of dry matter. The gas production (ml) at all incubation times and estimated parameters decreased with increasing maturity of tumbleweed hay. The gas production at all incubation times and estimated parameters (c, a, organic dry matter digestibility (OMD), metabolizable energy (ME)) were negatively correlated with NDF and ADF. The DM disappearance after 96 h incubation ranged between 56.19 and 72.77%. The in situ DM disappearance at all incubation times and estimated parameters decreased with increasing maturity of tumbleweed hay. The in situ dry matter disappearance at all incubation times and estimated parameters (c, a, b and effective dry matter degradability (P)) were negatively correlated with NDF, ADF and EE but positively correlated with CP and ash content. There were significant correlations between in vitro gas production and in situ DM disappearance at all incubation times. The nutritive value of tumbleweeds continually changed as it matures. Tumbleweeds, harvested at the proper stage of maturity offers considerable potential as a high quality forage for ruminant during winter feeding period. (c) 2004 Elsevier B.V. All rights reserved.