Submitted to: Sixth Edition of Forages, Volume II The Science of Grassland Agriculture
Publication Type: Book / Chapter
Publication Acceptance Date: 3/8/2005
Publication Date: 1/2/2007
Citation: Coleman, S.W., Sollenberger, L. 2007. Plant - herbivore interactions. In R.F. Barns, C.J. Nelson, K. J. Moore, M. Collins, editors. Forages, Volume II The Science of Grassland Agriculture. Sixth Edition. Ames Iowa: Blackwell Publishing. p.123-136. Interpretive Summary: The interaction of plants in a pasture or landscape and the grazing animals is a complex situation. Animals interact to the available herbage to satisfy their nutritional needs, but the interaction is dependent on the characteristics of the landscape and the needs of the animal. For instance, animal drives such as the need for water, food, temperature modification, social, rest, and rumination all require time. Some landscapes offer such a sparse amount of grazeable herbage that the grazer cannot meet its requirements and satisfy the other important drives. The presentation of nutrients in the landscape influence the strategy that the animal employs to gain the needed nutrients. For instance, in a pasture of one or two plant species such as grass or grass and clover with forage above 1000 lbs per acre most domestic ruminant animals (cows, sheep or goats) can adequately meet their requirements. If the available forage appreciably falls below that level, then the amount of forage taken with each bite is reduced and either the animal must graze faster or longer to make up the difference. Compensation has been observed to about 15%, and when forage becomes so short that bite size is critically low, then animal performance is affected. Other factors that influence the interactions of the animal with the forage landscape is the need and desire to selectively graze (looking for more nutritious plants or plant parts), fouling by dung or urine, and trampling. Another part of the interaction is that over long periods of time under grazing, plants might adapt mechanisms to either avoid (prostrate growth form, chemicals that deter grazing) the grazer or mitigate the effects of grazing (storage of nutrients below ground level). While some of the mechanisms are being discovered, the prediction of an animal's reaction to the landscape is still quite difficult.
Technical Abstract: The interaction of plants and animals at the grazing interface is a complex and dynamic situation. Although considerable achievements have been made in understanding interactions that take place at the plant-herbivore interface, integration of the dynamic processes that instantaneously drive the various components remains unsolved. Accuracy of intake predictions will likely remain limited until a better understanding exists of how animals reconcile the costs and benefits of grazing activity and the integration of grazing with inherent herbage constraints to digestion and processing of residues. That being said, much more progress has been made in ascertaining the short-term impacts of the sward on rate of intake and selectivity, than has been made on determining the immediate response of plants to defoliation and other disturbance by grazing. Mechanistic models on plant growth and animal intake should be integrated to evaluate our current understanding of the dynamics of the plant/animal interface in quantifiable form.