Date of Award

5-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Animal and Veterinary Sciences

Advisor

Andrae, John G

Committee Member

Duckett , Susan K

Committee Member

Pratt , Scott L

Committee Member

Schrick , F Neal

Abstract

Tall fescue [Schedonorus phoenix (Scop.) Holub] is a native grass of Europe and was not widely used in the U.S. until the early to mid 1900's. Toxic tall fescue (E+) is the predominate, cool-season forage used in beef cattle production in the southern U.S., but grazing this forage is known to negatively impact growth and may impair reproductive performance. Two- and 3-yr old (young) beef cattle respond differently to synchronization protocols and have higher energy requirements compared with mature cows under normal management conditions. Interestingly, limited data indicate that young cattle reproductive performance is negatively impacted to a greater extent than mature cattle when grazing E+. Toxic tall fescue effects on pregnancy rate have been documented, but specific timing of exposure and negative events remain unclear. Even though the specific mechanisms associated with decreased reproductive performance are uncertain, alternative grazing systems have been developed to attenuate female reproductive problems and alleviate decreased calf growth performance associated with E+ fescue grazing. The objectives of these studies were to determine if grazing E+ negatively impacts reproduction pre- or post-insemination, and to evaluate calf performance associated with exposure to toxic and non-toxic forage. Preliminary data from a local producer's cattle operation indicated a reduction in timed artificial insemination (TAI) pregnancy rates due to E+ exposure. In addition, 2- and 3-yr old (young) cattle tended (P = 0.07) to have reduced TAI pregnancy rates compared with ≥ 4-yr old (mature) cattle. In a follow-up study, young cattle grazing E+ pre-TAI had lowered (P < 0.05) d 30, 60 and 130 pregnancy rates when compared with grazing non-toxic forages (O). Grazing E+ post-AI lowered d 60 and 130 pregnancy rates compared to O treatment (P < 0.05). Pre-weaning calf growth was negatively affected by E+ treatment post-AI as average daily gain (ADG) and 205-d adjusted weaning weight (ADJ WW) were both decreased (P < 0.05) in calves grazing E+ post-AI compared with calves grazing O post-AI. Grazing 2- and 3-yr old beef cattle on toxic tall fescue prior to the breeding season decreases timed artificial insemination (TAI) and natural service pregnancy rates. Specific aims have been designed to systematically define the point at which E+ negatively impacts beef cattle reproduction and performance.

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