3.1. Diet Composition (Individual Feedstuffs and Treatment Diets)
Only numerical differences in the chemical composition of individual feedstuffs and dietary treatments are reported. In terms of individual feedstuff’s chemical composition, DM contents for grass hay and grass silage were expected (
Table 1). Interestingly, contents for NDF from linseed and sunflower seeds were almost in the same magnitude as those from hay and silage. Linolenic acid was the most predominant fatty acid in whole linseed. Sunflower seed was mostly composed by oleic and linoleic acids whereas Megalac-R
® was composed by palmitic and oleic acids. Regarding each treatment’s chemical composition, as initially formulated, protein and energy contents were similar between treatments. Compared to control diet, ether extract from SF and LS was slightly lower than control. One could expect ether extract to be higher in those diets with oilseed-by products, however, one possible explanation is that canola meal inclusion was higher for control in order to formulate iso-energentic and iso-proteic diets.
Compared to control, SF was characterized by higher contents of oleic and linoleic acids and LS by its higher contents of linoleic and linolenic acids (
Table 2). Analyzing individual feedstuffs and dietary treatments is important as it provides a wider picture of what animals eat and how the animals use nutrients. In this study, the inclusion of oilseed is relevant as they provided dietary protein, ether extract and neural detergent fiber.
3.2. Animal Performance (Intake and Digestibility)
Goat’s BW did not differ between the groups along the trial and no oil supplement refusals were found. Total DM intake tended (
p = 0.06) to increase (
Table 3) with SF and LS. OM intake was lower (
p < 0.05) for control and LS compared with SF; NDF and ADF intakes were lower for control compared with LS and SF. Similarly, when lactating goats [
20], lactating ewes [
21], and dairy cows [
22], were fed with diets supplemented with sunflower oil or linseed, no differences in DM intake were found.
Grass silage intakes were not affected by treatments (p > 0.05), however, the grass hay intake was higher (p < 0.05) for LS diet compared with control and SF diets. On the contrary concentrate intake was higher for SF diet (p < 0.05) followed by control and LS diets. Although fat intake (g/d) was higher for control (p < 0.05), followed by SF and LS, it is possible that the amount of supplemental FA sources was enough to promote shifts in the rumen biohydrogenation pathways and later this was reflected in reduced milk concentrations of C16:0 and C18:2 in LS compared to that of control.
Total digestibility (g/kg) for DM (641), OM (668), NDF (629), ADF (567) and ADL (375) were similar between treatments. The lack of differences on nutrients digestibility from dietary treatments somehow indicated that the treatment formulations were appropriate. This is similar to Haro et al., [
23] who did not find differences in digestibility in lambs fed with sunflower seed or sunflower meal. However, others have reported changes in the NDF digestibility when goats are fed with oilseed by-products. For example, Silva et al. [
24] observed no differences in NDF digestibility at different inclusion levels of soybean oil in dairy goat diets, whereas Karalazos et al., [
25] observed that NDF digestibility increased in diets containing 17%, 35% and 53% of cottonseed inclusion when compared to the control treatment. The discrepancies between similar studies and our results may be related to the differences in the composition of the concentrate used in the different trials.
Nitrogen intake was higher in control, compared to SF and LS diets. N urine excretion tended (
p = 0.06) to be lower for SF and LS (
Table 4), while N excretion in feces was similar (
p > 0.05) between treatments. Nitrogen balance was similar (4.4 ± 1.8 g N/d) between treatments (
p > 0.05). There was no negative N balance from dietary treatments, which suggests that the protein intake met the protein requirements of the animals. Although no differences were found in N excretion, it can be observed that N in feces was higher than in urine, which suggests that there was a greater use of ammonia in the rumen, causing a transfer from urine to feces [
26].
3.3. Milk Composition
No differences in milk yield, FCM 3.5%, protein and NFS were found between treatments (
Table 5). Fat content (g/100g) tended to be higher (
p = 0.09) for control compared to LS. Milk urea N was reduced by SF and LS and this was in line with the observed trend for a reduction in N excretion in urine.
Milk production, milk fat and milk protein responses to SF and LS were consistent with previous studies [
4,
20,
27], supporting the fact that normally oilseeds have no effect on milk yield, enhance milk fat secretion, and induce variable effects on milk protein concentrations in goats.
In the present study, fat and protein concentrations in milk were higher than those reported by Bernard et al., [
20] and Luna et al., [
27], when dairy goats were supplemented with SF and LS oils, but similar to Economides, [
21] when goats were supplemented with SF meal. Our results point at the fact that the proportion of fiber in the diet was adequate to promote the formation of acetate and butyrate that are the main precursors of the FA synthesized in the mammary gland [
5]. This is also supported by the NDF contents found in both SF and LS. Another explanation for the lack of difference in milk fat contents could be a dilution effect, as in this study milk yield was low (around 0.75 kg/d) compared to the cited studies [
20,
27], which resulted in greater concentrations of fat and protein in milk.
In this study, milk protein was unaffected by dietary treatments, and one explanation could be that fat supplementation did not affect energy intake, which is one of the most important nutritional factors affecting milk protein [
4,
28]. For the same reason, NFS were not affected by dietary treatments. Studies that have used oilseed by-products supplementation in goat diets have different milk production traits outcomes, as there is a wide variety of combinations of basal diets (forage and concentrates), and amounts of oilseed by-products in the diets. Our results agree with those studies using a modest amount [
29,
30] of supplementation enough to supply energy and provoke changes in milk fatty acids contents.
3.4. Milk Fatty Acid Profile
In this study, lipid supplementation did not (
p > 0.05) influence the concentrations of C4:0 to C14:0 milk FA (g/ 100 g FA) (
Table 6). Control increased C16:0 (
p < 0.001) while LS and SF increased (
p = 0.01) oleic acid (C18:1).
Palmitic acid is the main FA in Megalac-R
® (control) and is often related to increases in milk fat contents. In goats, feeding C16:0 increases milk C16:0 considerably at the expense of C10:0 to C14:0 and C18:1 [
4]. In fact, in this study, content of milk C18:1 was lower (
p < 0.05) in control. Nudda et al. [
30] and Luna et al., [
27] did not detect significant changes in the concentration of FA from C6:0 to C12:0 in goat´s milk after supplementation with different amounts of extruded linseed cake or SF respectively.
In this study, the lack of changes in the contents of short-chain FA (C4:0 to C8:0) was unexpected as they are partially synthesized by metabolic pathways that are dependent of acetyl-CoA carboxylase [
31]. Similarly, the decrease of C18:2 observed for LS diet was probably due to the extensive hydrogenation of this acid, abundant in LS and SF, as observed numerically in C18:0 contents in milk. Chilliard and Ferlay, [
32] compared the effects of including sunflower oil and oilseeds in goat diets and revealed that oilseed C18:2 was more hydrogenated to C18:0 than oils C18:2. This suggests that the desaturation ratio of stearic acid in the mammary gland is decreased by oil supplemented diets which increase the availability of either PUFA or trans FA, as these FA are putative inhibitors of the delta 9-desaturase [
33].
Contrary to the present study, Bernard et al., [
20] used plant oils in goat diets resulting in increases (
p < 0.05) in milk C4:0 and decreases in (
p < 0.05) C8:0 concentrations. In this study, changes in milk FA composition attributed to SF and LS were characterized by a numerical increase in C18:0 concentration. Those responses are comparable to those reported by Bernard et al., [
20] and Luna et al. [
27]. Contrary to the present study, in an in vitro study, Zened et al. [
34] found an increase in C18:0 when including SF oil compared to diets with starch ranging from 22 to 35% of dietary starch. Our results on C18:0 could be explained by the fact that adding unsaturated FA to the diet cause adaptations of rumen microorganism increasing their ability to hydrogenate unsaturated FA [
35].
While C18:2 was reduced in response to LS, C18:3 was not affected by the inclusion of oilseeds (
p = 0.17). Similarly, decreases in milk C18:2 were reported when goat diets were supplemented with sunflower oil [
27] or extruded LS cake [
30]. On the contrary, when goats and sheep are fed with linseed [
24,
36,
37], increases in rumenic acid (conjugated linoleic acid; CLA) have been observed, for which the main structure is based on C18:2. In this study, the gas chromatograph program was not able to detect CLA isomers and this is something that future research should take into account as CLA has very high biological value (i.e., cardioprotective) for human health [
1].
Although, previous studies [
38] suggested that when rations are supplemented with linoleic acid rich seeds such as SF, the linoleic acid proportion in milk fat rarely exceeds that observed with un supplemented diets by more than 1.5 %. In this context, Mir et al. [
39] added canola oil up to 6 %DM in goat diets and reported no changes in the secretion of linoleic acid in goat’s milk.
Concentrations of total SFA tended to be lower (
p = 0.08) in SF and LS (
Table 6). This was explained by the observed decrease of C16:0 in milk from SF and LS compared to that of control. Total PUFA content was higher (
p = 0.03) in milk from control, and was consistently lower in milk from animals on LS treatment (
Table 5). Nowadays there is a current debate on the effects of dietary SFA on increasing the risk of development of cardiovascular diseases or coronary heart diseases [
40]. However, results from the present study suggest that feeding dairy goats with SF or LS is a feasible feeding strategy that tends to produce milk fat with less negative impacts on human health. One dilemma that this study faced is the fact that the inclusion of SF and LS was meant to provoke changes in milk fatty acids without disruptions to the overall animal’s performance. Therefore, if stronger effects on milk fatty acids were sought, it would have been possible to observe negative effects on animal´s productive traits.