The Black Sheep of the Walnut Family: Black Walnuts, Juglone, and Forage Nutritive Value

 

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John Steinbock
Committee: Alisa Hove, Amy Boyd, Mark Brenner

Black walnut (Juglans nigra) trees are known for their allelopathic inhibition of nearby plant growth via juglone toxicity and may adversely affect forage nutritive value in beef cattle production systems. We tested this hypothesis on forage samples of tall fescue (Festuca arrundinacea) from six farms in Kentucky and Tennessee. In total, 111 discrete tall fescue samples were collected at three, five-meter radial increments around 26 black walnut and 11 non-Juglans reference trees. Forage nutritive values, including crude protein (CP), neutral detergent fiber (NDF), acid detergent fiber (ADF), total digestible nutrients (TDN), and relative feed value (RFV), were analyzed for each sample using near infrared reflectance spectroscopy (NIRS) at the University of Kentucky forage lab in Frankfort. Linear regressions were constructed separately for each tree species group to analyze for forage nutritive value differences as a function of black walnut tree age (approximated by tree diameter at breast height) and forage distance from the black walnut tree. We found that mean forage CP and ADF per tree decreased and that mean forage TDN per tree increased with increasing black walnut tree age. We also found that mean forage CP content increased with increasing distance from the black walnut tree. The results are consistent with the hypothesis, but cannot conclusively be attributed to allelopathy since competition or other tree-forage interactions may be responsible for the observed loss of CP and increased digestibility. These results may have significance for producers managing or considering silvopasture systems with black walnut trees.