Margrete Eknæs defended her PhD thesis on June 10th 2009

Ane Gro Siri Skjelfjord

Free fatty acids and off-flavours in goat milk - effect of energy balance and different feeding regimes

The general aim of this thesis has been to investigate the effect of feeding regimes and feed quality on the frequency of rancid and tart flavour in goat milk. It was of special interest to examine whether milk quality is influenced by the amount and composition of feed concentrate, and to study the effect of pasture intake and plant selection during the mountain grazing period. To evaluate the effects of these factors, four experiments were performed.

The first study was based on the theory that goat energy balance influences the occurrence of flavour defects (Paper I). The energy status of the goats was examined throughout lactation by studying changes in body composition (by computer tomography (CT)) and blood parameters. From day 11 until day 125 of lactation, adipose tissue mass decreased by 3.5 kg on average and there were elevated plasma concentrations of non-esterified fatty acids (NEFA) and acetoacetate. Likewise, the concentration of free fatty acids (FFA) in milk increased, as did the relative proportion of C18 fatty acids (FA). The goats started to produce milk of inferior quality (FFA>2.0 mmol/l) around day 74 of lactation, when approximately 40% of their prepartum adipose tissue mass was mobilized. After three weeks of lowland grazing in the autumn, the goats deposited body fat and the concentration of FFA in milk was lowered.

The effect of roughage availability and contrast levels of concentrate during the grazing season was studied in a 2x2 factorial experiment (Paper II). The effect of roughage availability was tested by comparing continuous grazing with limited periods of roughage restriction and ad libitum hay feeding. Feeding high levels of concentrate reduced the frequency of rancid/tart flavour, but a supply of good quality roughage was of greater importance.

In the third study, the effect of two different concentrate mixtures, containing saturated long chain fatty acids (LCFA) (mainly C16:0/C18:0) and unsaturated LCFA (mainly C18:1/C18:2), was explored during the grazing and indoor feeding period (Paper III). Supplements with saturated LCFA reduced the proportion of 'goat fatty acids' (C6:0 - C10:0) and increased the proportion of C 16 and C 18 FA, thereby reducing the frequency of rancid and tart flavours.

The interaction between pasture intake, grazing behaviour, additional hay feeding and the frequency of off-flavours was studied during a mountain grazing period (Paper IV). As the nutritional value of the grazing plants decreased during the mountain grazing period, the goats tried to optimize the nutritional value of their diet by selecting plant species with the highest nutrient concentration and by increasing their dry matter (DM) intake. The goats walked increasingly longer, the air temperature decreased and the quality of the pasture declined. These factors could have caused an increased imbalance between their energy requirements and provided energy at the end of the mountain grazing period, and the feeding of additional hay reduced the concentration of FFA in the milk.

The main conclusion from the present study is that goats with substantial energy mobilization in early lactation are more likely to produce milk with high levels of FFA and increased frequency of rancid and tart flavours in mid lactation. An increased level of concentrate and feeding good quality hay or silage supplements will reduce the frequency of flavour defects at times when grazing is suboptimal or the pasture is of medium or low quality. The mammary gland supply of fat seems to be of fundamental importance to ensure stable and good milk quality, and it appears that fat supplements, and especially supplements with saturated LCFA, will decrease the proportion of short chain fatty acids (SCFA) and reduce the frequency of rancid and tart flavour in goat milk.

Keywords: Lactating goats; Milk composition; Rancid and tart flavour; Lipolysis; Free fatty acids in milk; Energy balance; Body fat reserves; Blood metabolites; Feed supplementation; Fat supplements; Grazing behaviour; Pasture intake

Updated: 01.07.09
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