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A Re-evaluation of the Minimum Vitamin A Requirement of Growing-Finishing Lambs^1,2,

Texas Agricultural Experiment Station^,5, San Angelo 76901

Abstract

The minimum vitamin A requirement of growing-finishing lambs was determined by depleting 24 Rambouillet lambs (15.0 ± .59 kg) of their vitamin A stores and then feeding them for 16 wk one of six fixed intakes of vitamin A (2, 4, 8, 16, 32 or 64 µg retinol·kg live wt^–1·d^–1). Plasma retinol concentrations at the end of the depletion phase averaged 6.0 ± .6 µg/dl. The relationship between vitamin A intake (µg retinol·kg live wt^–1·d^–1, X) and plasma retinol concentration (µg/dl, Y) at the end of 16 wk of supplementation was Y = 1.40 + 2.08X – .015X² (r = .98). The relationship between vitamin A intake (µg retinol·kg live wt^–1·d^–1, X) and liver retinol concentration (log₁₀ µg/100 g wet wt, Y) was Y = 1.34 + .058X – .00044X² (r = .90). Cerebrospinal fluid (CSF) pressures were elevated in lambs fed the 2-, 4- and 8-µg levels of vitamin A intake. A broken-line regression model was used to describe the relationship between vitamin A intakes and CSF pressures, and to define minimum vitamin A requirements. The equation obtained was Y = 2.206 – .292 (X – 1.146) , r = .95, where X = vitamin A intake (log₁₀ µg retinol·kg live wt^–1·d^–1) and Y = CSF pressure (log₁₀ mm saline). In this model = 1 when X 1.146 and = 0 when X >1.146. At vitamin A intakes below 14 µg retinol·kg live wt^–1·d^–1, CSF pressures were increased; thus, 14 µg appears to be the minimum vitamin A requirement for maintenance of normal CSF pressures in growing-finishing lambs. Cerebrospinal fluid pressures were also increased at plasma vitamin A concentrations less than 20 µg retinol/dl. Thus, plasma vitamin A concentrations less than approximately 20 µg/dl would be indicative of chronic vitamin A deficiency.

¹ Technical article no. 21510, from the Texas Agric. Exp. Sta., Texas A&M Univ. System, San Angelo, TX 76901. All programs and information of the Texas Agric. Exp. Sta. are available without regard to race, ethnic origin, religion, sex or age.

² The generosity of Clinton Hodges, Sterling City, TX, in providing the lambs used in this study is gratefully acknowledged.

³ Anim. Sci. Dept., Texas A&M Univ., College Station, TX 77843.

⁴ Agric. Dept., Angelo State Univ., San Angelo, TX 76909.

⁵ Texas A&M Univ. System.

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