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Accueil du site → Doctorat → États-Unis → 1972 → RELATIONSHIP OF THE BOTANICAL COMPOSITION OF STEER DIET TO DIGESTIBILITY AND FORAGE INTAKE ON A DESERT GRASSLAND

University of Arizona (1972)

RELATIONSHIP OF THE BOTANICAL COMPOSITION OF STEER DIET TO DIGESTIBILITY AND FORAGE INTAKE ON A DESERT GRASSLAND

Galt, Henry Deloss

Titre : RELATIONSHIP OF THE BOTANICAL COMPOSITION OF STEER DIET TO DIGESTIBILITY AND FORAGE INTAKE ON A DESERT GRASSLAND

Auteur : Galt, Henry Deloss

Université de soutenance : University of Arizona.

Grade : Doctor of Philosophy (PhD) 1972

Résumé
The diets of steers grazing a desert grassland were evaluated for botanical and nutrient composition on mesquite and mesquite-free ranges from December 1966 to September 1967. Four rumen fistulated steers were paired and placed on two adjacent pastures of the Santa Rita Experimental Range near Tucson, Arizona. The pastures were similar in vegetation except for a 177o crown canopy of velvet mesquite [Prosopsis juliflora var, velutina (Woot.) Sarg,] on one pasture. Forage samples were collected by the rumen evacuation method in conjunction with forage intake and digestibility trials. Forage collections and digestion trials were made for two periods in each of four seasons using a cross-over experimental design. A total of 32 forage samples were analyzed for species and plant part composition by a modification of the microscope point technique. Digestibility was determined for dry matter, acid-detergent fiber, crude protein, and gross energy using the lignin ratio method. Forage intake was measured by the chromic oxide-cellulose indicator method. Masticated forage mixtures of known composition by weight were prepared from whole plant species collected from the experimental pastures in October 1966 and 196 7. The species were fed and recovered from rumen fistulated steers. The known mixtures were analyzed by the microscope point technique to determine point-weight, point-volume, and density relationships of species. Species in known mixtures were estimated to within 5% points of the mean and plant parts to within 4.37o points of the mean with 400 microscope points per sample. Application of point-weight regressions developed from known mixtures underestimated percent weight of species in forage samples collected from grazing animals. The low estimates were found to be primarily related to density differences of species when the botanical composition of species mixtures were unlike the composition of the known masticated mixtures. Percent volume was determined to be near a 1:1 ratio with point estimates of species in mixtures of different botanical composition. Percent weight of species in rumen forage samples was determined by applying a measured density constant (weight/volume) to the point estimates of species in forage samples. Species composition of the steer diets was usually very different from the composition of the available species on the experimental pastures. Grasses were the predominant plant group in the diets. Arizona cottontop [Trichachne californica (Benth.) Chase] was the most consistently selected and predominant species in the diets. Large amounts of shrub species were selected primarily during spring and early summer. Selectivity was also shown for certain plant parts. Leaves were the predominant plant part in the diets and increased from winter to summer. Stem parts in diets decreased for the same seasons. Apparent digestibility of dry matter, acid-detergent fiber, crude protein, and gross energy increased significantly during the summer. Forage intake did not change appreciably from winter to early summer but was significantly higher during summer. Total crude protein content of diets was adequate for maintenance during all seasons except winter and was more than adequate for maintenance and weight gains obtained in summer. The higher protein content of the steer diets compared to available forage was related to selective grazing by the animals for species and plant parts containing higher amounts of protein. Botanical composition was a primary factor related to nutrient composition and forage intake. The animals consistently selected species and plant parts of high nutrient quality. The high leaf content in the diets contributed to less acid-detergent fiber and more crude protein content compared to other plant parts in the forage samples.

Mots clés : Cattle — Feeding and feeds. ; Cattle — Arizona. ; Santa Rita Experimental Range (Ariz.)

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Page publiée le 24 novembre 2014, mise à jour le 23 mars 2018