Submitted to: Grass and Forage Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 1, 1996
Publication Date: N/A
Interpretive Summary: Several shrub species that are prevalent in the Chihuahuan Desert are avoided by livestock. We are currently studying the role of plant chemistry in regulating consumption of these shrubs by livestock, with the long-term goal of increasing shrub consumption. Tarbush is being used as a shrub model for studying diet selection because sheep and cattle do occassionally yeat small amounts of tarbush and because our previous work has shown certain plant fractions are related to diet selection. Because tarbush is being used as a model shrub, we must know its nutritional value and its content of antiquality factors. Leaves were removed from 40 tarbush plants harvested each of three years during four growth stages 1) early, 2) midpoint, 3) late and 4) curtailed growth. The crude protein content of tarbush was relatively high. Fibrous components, lignin and condensed tannins were generally low, but total phenolic content was quite high. Browsing by livestock early in the growing season would be most advantageous based on these results, because crude protein, in vitro digestibility and phosphorous content were highest at that time, and fibrous components and other antiquality factors increased later in the season. Tarbush chemistry fluctuated considerably from year to year. Tarbush represents a potential high-protein forage, but its nutritional value may be limited by its high phenolic content.
Technical Abstract: Tarbush (Flourensia cernua DC) is an abundant Chihuahuan Desert shrub but is used sparingly by livestock. Leaves were removed from 40 tarbush plants harvested each of three years during four growth stages 1) early, 2) midpoint, 3) late and 4) curtailed growth (10 plants per growth stage each year). Plants were air-dried and all leaves were removed. In vitro DM disappearance was greater for the early growth stage than other stages (P 0.05). The fibrous fraction increased with maturity, with NDF, ADF and ADL contents all less for the early growth stage (P < 0.05) than other stages. The latter three growth stages did not differ for NDF or ADF, but ADL content was lower for the midpoint growth stage than for the latter two stages (P < 0.05). Calcium content increased with advancing season (P < 0.05), while phosphorous concentration exhibited an inverse pattern (P < 0.05). Nitrogen content of tarbush was greatest in the early growth stage (P < 0.05) and declined substantially thereafter. Soluble N content was greater and ADIN content was lower for early growth (P < 0.05) than other stages. Insoluble N concentration followed a pattern similar to N, declining with advancing season (P < 0.05). However, when expressed as a fraction of total N, insoluble N decreased and soluble N increased with maturity (P < 0.05). Condensed tannin concentration tended (P < 0.10) to increase with advancing season. Total phenolic content was lowest for early growth (P < 0.05), and did not differ among the other stages. Chemical analysis revealed tarbush to be relatively high in N concentration. Early use by livestock would be most advantageous in terms of greatest nutrient availability and lowest total phenolic concentration. Tarbush chemistry fluctuations due to year to year variability appear to be substantial.