Abstract
Concerning health statistics, coupled with increasing consumer interest in product origin and quality, has led researchers to investigate natural methods to improve lipid composition in red meat. Countries such as New Zealand, Australia and Ireland, rely on fresh forage and extensive finishing systems that naturally increase polyunsaturated fatty acids (PUFAs) in meat and milk. In New Zealand, inconsistent summer rainfall has seen farmers adopt novel summer-safe forages that require intensive grazing management but enable high lamb performance at crucial times. While herbs and legumes have been studied extensively for their agronomic characteristics, few studies contrast their fatty acid performance and the effects of feeding fresh forage on lamb fatty acid composition, especially for novel species such as Cichorium intybus (chicory).
This thesis sets out to investigate the variation of fatty acid composition in commonly used lamb finishing species and understand their effects on lamb fatty acid composition. Throughout the thesis, it was important to contribute further scientific understanding of fatty acid composition in plants and animals, but also acknowledge the practical implications of these findings.
Investigations into the effects of plant species and sampling time on fatty acid composition, found that natural declines in fatty acid concentrations occurred when pastures were sampled monthly throughout summer and were most apparent for Medicago sativa (alfalfa) and grasses. When the effects of moisture restriction were assessed under glasshouse conditions, both nutritional composition and fatty acid content were marginally affected for grasses, herbs, or legumes. Assessment of chicory composition under controlled conditions and field conditions in various years, highlighted the ability of chicory to maintain high nutritional quality, comprising elevated levels of metabolisable energy (ME) and digestibility that occurred regardless of low moisture under simulated and field environments. Consequently, fatty acid composition of chicory was more stable than grasses, especially compared with unirrigated Lolium perenne (perennial ryegrass) x Trifolium repens (white clover) pastures, which diminished in fatty acids and nutritional quality throughout the lamb finishing period. Under these conditions, lambs that grazed chicory for 8 weeks had higher PUFA concentrations than those that grazed perennial ryegrass x white clover or chicory for 2, 4, or 6 weeks, and resembled the effects of 6 weeks grazing on chicory in a separate study under irrigation. The reduced fatty acid content for lambs that grazed unirrigated chicory for less than 8 weeks was thought to occur because of inconsistent feed allocations that were at times restricted due to unseasonably low rainfall.
In the final experiment, lambs that grazed chicory as a single sward or a chicory x Trifolium pratense (red clover) mix for 6 to 8 weeks had faster growth rates and consequently achieved heavier carcass yields compared with lambs that grazed ryegrass, Plantago lanceolata (plantain) x white clover, or red clover, under identical feed allocations. Lambs also had consistently higher muscle concentrations of PUFA compared with lambs that grazed a traditional perennial ryegrass pasture or other herbs and legumes, that remained after adjustment to a constant live weight, carcass weight, and growth rate. Pasture composition differed for chicory and chicory x red clover, and unlike the previous study, lambs with the highest fatty acid concentrations did not always correlate to pastures with the highest PUFA concentrations or the highest nutritional value.
When fatty acid composition was contrasted from lambs that grazed unirrigated perennial ryegrass x white clover or chicory in the initial field trial, it appeared that the high fatty acid concentration and nutritive value of chicory provided lambs with greater quantities of C18:3n-3, which enhanced the concentrations in the muscle. However, in the final experiment forage diets with the highest fatty acid concentrations and nutritive value did not correspond with the highest muscle concentrations under identical feed allocations, suggesting a plant specific interaction between diet and the rumen environment that requires further investigation.
Irrespective of substantial changes in muscle fatty acid composition in each study, the ability to advertise lamb as a source of ALA (Alpha-linolenic acid; C18:3n-3) or EPA + DHA (Eicosapentaenoic acid; C20:5n-3, Docosahexaenoic acid; C22:6n-3), was not altered by diet. Pasture-based finishing was estimated to achieve labelling requirements for “source” of EPA + DHA under international regulations, however further investigation is required to understand the restrictions imposed for lambs, due to meat from lambs containing high proportions of SFAs. Estimated ALA concentrations per serving of lamb was not attained for any animals, regardless of finishing regime. World Health Organisation recommendations for the ratio of omega-6:omega-3 were achieved by all pasture diets, however the PUFA:SFA ratio remained approximately half the recommended level. Possibly improvements in marketable fatty acids, particularly ALA, would be associated with improved PUFA:SFA, however further experimentation is required to assess the possibility of this.