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Goossen, Caleb

Plant and Soil Science

2018

Ph.D.

Demand for ruminant-derived products high in beneficial fatty acids (FA) has led to a desire to maximize the alpha-linolenic acid (ALA) and total FA ([Sigma]FA) content of forage crops. Several management factors affect the ALA and [Sigma]FA content of forages, including phenology, species, season, and nitrogen fertility. Yet, the influence of harvest management decisions that affect wilting time of conserved forages is under-studied. Similarly, the majority of published research regarding ALA and [Sigma]FA content is of cool season (C3 photosynthetic) temperate perennial forage species, and not warm season (C4 photosynthetic) annual species. Sample preservation methodologies used in published research are often too expensive and time consuming for desired practicality, or unreliable. This dissertation aids in addressing these deficiencies. In the first study, forced hot air drying of forage samples was shown to be unreliable for accurate FA analysis, and an alternative methodology was established utilizing brief microwave pretreatment of small fresh weight samples prior to forced hot air drying, yielding results similar in accuracy to lyophilized samples. Oxidative losses of ground dried forage samples were also examined, again suggesting that microwave pretreatment prior to forced hot air drying is a fast, inexpensive and otherwise desirable choice for forage sample preservation in anticipation of later FA analysis. A second study investigated two warm season annual forage species (sorghum-sudangrass and pearl millet), showing that maturity-associated declines in whole plant ALA and [Sigma]FA content are largely a product of an increasing ratio of ALA- and [Sigma]FA-scarce pseudostem fractions, and only secondarily resultant of maturity associated declines within individual plant fractions. Lamina mass ratio was identified as a correlate with ALA and [Sigma]FA content, at least as useful as two common correlates - crude protein and neutral detergent fiber content. A third study also showed the critical influence of crop maturity upon ALA and [Sigma]FA content in two warm season annual forages (pearl millet and sudangrass), in addition to differences between species and those resultant from differing nitrogen fertility. Conserved forage harvest decisions were evaluated in the fourth study. No difference was found between wide and narrow swath treatments (70% and 40% of mower width, respectively) of AM and PM mown reed canarygrass, but there was evidence to suggest that AM mowing may allow for a higher content of ALA and [Sigma]FA content relative to PM mowing. Ensiling was also found to decrease ALA content or proportion. In conclusion, management choices promoting grazing and/or harvesting of a higher laminae proportion, optimizing nitrogen fertility, and suitable choice of species for meeting these goals may be the best way to maximize the ALA and [Sigma]FA content of forages grown for livestock. AM mowing may reduce ALA and [Sigma]FA content losses otherwise caused by overnight wilting of forages mown for conservation, and microwave pretreatment prior to forced hot air drying is an advisable sample preservation methodology for researchers furthering the study of forage ALA and [Sigma]FA content, when lyophilization is impractical or too expensive.