Fatty acid composition of adipose depots of suckling lambs raised under different production systems
Introduction
It has been amply proven that, with the exception of stearic acid (Bonanome & Grundy, 1988), saturated fatty acids (SFA) tend to increase cholesterol low-density lipoprotein (C-LDL) and total cholesterol levels in plasma, while polyunsaturated fatty acids (PUFA) decrease those same levels (Grundy, 1987, Gurr et al., 1989, Mensink and Katan, 1992, Hegsted et al., 1993, Yu et al., 1995). For this reason, the PUFA/SFA index is used to calculate the risk factor of foods with regard to blood cholesterol elevation, and the n-6/n-3 PUFA index indicates the role of fatty acids in atherosclerosis (Carleton et al., 1991, Sanders, 1988). The principal effect of n-3 PUFA is to reduce triglyceride plasma levels (25%); this effect is mediated by the reduction of cholesterol very-low-density lipoprotein (C-VLDL) levels (Kris-Etherton, 1996).
Although it is generally accepted that the fatty acid composition of the adipose depots of suckling animals depends on the composition of the milk they consume (Velasco, 1999, Zygoyiannis et al., 1992), this composition can be modified by supplementary foodstuffs, as these reduce the intake of milk (Doney, Smith, Sim, & Zygoyiannis 1984), or by management (drylot or pasture), which conditions fatness (Ely, Glenn, Mahyuddin, Kemp, Thrift, & Deweese 1979).
The kind of food consumed by the lambs can modify the proportion of fatty acids deposited in their tissues. When the concentrate/forage ratio increases, enriching the diet, the proportion of monounsaturated fatty acids in corporal lipids augments as a result of a greater proportion of propionic acid in the rumen (Aurousseau, 1981).
The n-6/n-3 PUFA ratio is also modified by the kind of food consumed; lambs eating grass display higher n-3 PUFA levels, while the proportion of n-6 PUFA increases in those fed concentrate (Mitchell, Reed, & Rogers, 1991). This phenomenon has also been observed in cattle (Enser, Hallet, Hewett, Fursey, & Wood, 1998). These differences reflect the fat composition of the diet, as grass contains a high concentration of linolenic acid (C18:3), precursor of the n-3 series, while concentrates contain high levels of linoleic acid (C18:2), precursor of the n-6 series.
This study was undertaken to analyse the influence of the production system (pasture or drylot, addition or not of concentrate to the diet) on the fatty acid composition of various adipose depots of suckling lambs raised with their dams, whose meat is very highly regarded by Spanish consumers.
Section snippets
Material and methods
One week after birth, 49 male Talaverana-breed lambs were selected from 120 lambs born after oestrus synchronisation of the dams and raised as singles for the present study.
The effects of two different lamb management systems (pasture and drylot) and diets (maternal milk alone and maternal milk supplemented with starter concentrate) were studied. Groups consisted of 12 lambs each, except one of 13 drylot lambs not given supplementary feed.
Each of the two pasturing groups (12 lambs each) were
Fatness
Table 1 displays refrigerated carcass weight (RCW) in addition to the parameters relating to fatness. Pasture-raised lambs presented a carcass weight of 7.24 kg while drylot animals weighed 7.20 kg. Drylot lambs obtained a higher carcass fatness score (P⩽0.001) and displayed greater mean dorsal fat thickness (mm; P⩽0.05) than those raised at pasture. The percentage of total leg fat obtained through dissection and that of the various fat depots was also higher in the drylot lambs, while the
Fatness
The greater visually scored carcass fatness of drylot lambs (Table 1) corresponds with their greater objective fatness (mean dorsal fat thickness); dorsal fat thickness is related to the quantity of carcass fat (Starke & Joubert, 1961). Similarly, Rhodes (1971) observed greater overall fatness, as well as greater dorsal fat thickness (measured at the sixth rib), in drylot lambs than in lambs raised at pasture.
Furthermore, lambs raised at pasture presented a lower proportion of overall leg fat,
Conclusions
Suckling lambs raised at pasture displayed lower overall carcass fatness and qualitatively better fat than their drylot counterparts due to their higher PUFA/SFA ratio in intramuscular fat and their lower n-6/n-3 PUFA index in intramuscular and subcutaneous fats. Addition of concentrate to the diet of these very young pasture and drylot lambs is advisable as it does not influence fatness and because lambs fed concentrate present a higher PUFA/SFA ratio. With regard to carcass components, lamb
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