Supplementing different types of dietary fat can be a useful strategy to increase the energy supply for lactating animals without resorting to cereals, which can result in costly diets. Therefore, in the present study, the choice was to use a modest amount of dietary fat supplementation (as rumen-protected fats), which was enough to supply adequate amounts of dietary energy without compromising productive traits in lactating goats.
3.2. Nutrient Intake and Digestibility
Metabolic live weight was similar among treatments (Table 3
). Data related to the effects of lipid supplementation on body weight changes in lactating goats are limited, and responses are usually variable between protected and unprotected vegetable oil sources [32
]. In this study, the observed changes were relatively small, showing that animals were, on average, under a positive energy balance, and this was observed with body-weight gains at the end of the experimental trial.
Appropriate nutrient intake and digestibility are key factors influencing productive traits in lactating animals. These factors determine the profitability of animal production systems. Nutrient intakes were similar between treatments, which agreed with Bernard et al. [33
], who fed goats with sunflower and linseed and found similar DM intakes. It is important to note that dietary treatments were formulated to be iso-proteic and iso-energetic and, therefore, no differences were expected with regard to nutrient intake.
Calcium soaps of CO were more digestible compared with SO and PO (Table 3
). Several studies have mentioned that the use of calcium soaps does not have any adverse effect on dry matter digestibility [34
]. Although dietary treatments in this study had similar chemical composition, it may be possible that the fiber contents (NDF and ADF) from SO and PO affected fibrolytic bacteria. However, this contention warrants further investigation. In this study, canola oil had around 65% oleic acid, which is a monounsaturated FA and is therefore less toxic to some of the ruminal microorganisms, which may be the reason for the higher digestibility shown in the CO treatment [36
3.3. Nitrogen Balance
No differences were observed for N intake and excretion (Table 4
). The lack of differences in N utilization attests to the uniformity of energy and crude protein contents of dietary treatments. Dietary energy is one of the most important factors to consider in ruminant diets as it has a direct impact on microorganisms and overall protein metabolism at the rumen level [38
]. Therefore, if dietary treatments were iso-energetic, no changes to N utilization were expected. The animals fed with CO and PO treatments were found in positive nitrogen balance, and only with SO treatment were we able to observe a numerical N loss of 0.03 g/kg LW. In ruminants, it has been reported that dietary FA affects microbial protein synthesis, protozoa counts and methanogen profile [39
], which can contribute to improving the efficiency of energy utilization of the feeding [4
]. However, no differences in N balance were found between treatments in this study. The positive N balance found with CO and PO diets indicated the capacity of the diets to supply the N required for the goat’s maintenance, without the need for energy mobilization from body reserves, and while also supplying adequate amounts of protein to rumen microorganisms [19
3.4. Milk Production and Milk Composition
Milk production (Table 5
) was higher in SO compared with CO and PO. Mir et al. [40
] fed dairy goats with four inclusion levels of canola oil without observing any effect on milk production. Similarly, Chilliard et al. [41
] and Lock et al. [42
] reported that the supplementation of fats in dairy goat diets had no effect on milk production. In contrast, Lu [43
] found a decrease in milk production with the supplementation of animal fat at 5% DM in lactating goats. In order to have more accurate inferences for milk production, milk production efficiency, corrected by fat (fat-corrected milk 3.5%), energy (energy-corrected milk) and feed efficiency (feed efficiency and fat-corrected milk 3.5%), were calculated, and similar results were obtained between treatments. This was a unique feature of the study, as the calculations allowed us to understand milk production from different angles. Thus, it appeared that SO was the best for milk yield, but if we accounted for energy utilization from the diet, all treatments resulted in similar values.
] reported that fat supplementation increases energetic efficiency in lactating cows by increasing total energy intake by generating ATP more efficiently compared with volatile fatty acids, and by directly incorporating long-chain FA into milk fat. Also, one mechanism proposed for the increase in milk yield with fat supplementation is glucose sparing, in which the suppression of de novo FA synthesis in the mammary gland decreases the oxidative use of glucose to generate reducing equivalents for milk fat synthesis [45
]. This extra glucose may be utilized in other milk processes, including lactose synthesis and increased milk yield [46
Contrary to cow research, some articles have mentioned that the use of fat supplements does not change milk production in dairy goats [32
]. With regard to fat and protein content in milk, CO was lower than SF and PO. Although dietary lipid supplementation in dairy goats generally improves milk fat production [32
], in this study, this was not the case for CO treatment, which resulted in the lowest amount of fat. It has been reported that the effect of supplemental fats on milk fat is not always positive since it is influenced by their percentage of inclusion in the diet, type of forages in the diet, type of supplemental fat (rumen-protected or not), physical form (i.e., whole seeds and oils) and the chemical composition of dietary lipids (saturated or unsaturated fatty acids) [41
It is worth mentioning that CO resulted in inferior ruminal fermentation performance, as shown by a reduction in total gas production (Figure 1
) and a reduction in the amount of SCFA, which could have influenced the lower amount of milk fat found for this treatment. This is important since SCFAs, such as acetate and butyric acids, play an important role in the formation of short-chain fatty acids (C4:0 to C14:0) in milk. These fatty acids account for approximately 60 and 45% of the total milk fatty acids on a molar and weight basis, respectively [47
Non-fat solids were lower in CO and PO compared with SO, and total solids were higher for SO compared with PO and CO. The addition of fat leads to a reduction in rumen fermentable organic matter, reduction in precursors of glucose and reduction in the synthesis of microbial protein, and thus a reduction in the amount of amino acids available for the synthesis protein in milk. This explains why a lower amount of protein in milk was observed with regard to the fat content.
Some studies have reported a reduction of protein in milk in which long-chain FAs were abomasally infused in cows, and showed that the magnitude of changes in milk components depended on the amount and type of FA supplied [48
]. Despite the fact that dietary treatments supplied similar protein and energy, the type of FA in each calcium soap elicited differential effects on protein contents in milk, and in this case, CO resulted in a detrimental effect on this milk component.
For lactose content in milk, no differences were observed among treatments (p
> 0.05); however, Luna et al. [49
] reported differences in goat milk lactose when the diets were supplemented with whole linseed (1.84% DM) and sunflower oil (0.81% DM). Ayeb et al. [50
] found no differences in the lactose content of goat milk when animals were fed dry olive leaves ad libitum. These authors reported an increase of the total solids when dry olive leaves were included in the diet.
3.5. In Vitro Gas Production
The parameters of the in vitro gas production of the ingredients used in the diets are presented in Table S1
. Differences in gas production for each ingredient (p
< 0.001) were observed, being higher (p
< 0.001) in corn silage and corn grain, and lower in PO. The fractional rate of degradation (c) and lag time was similar among ingredients (p
> 0.1), DMD 96 h was higher (p
< 0.001) for soybean and corn grain, followed by barley hay and corn silage, with DMD being lower for safflower oil, canola oil and palm oil. PG 96 h was higher (p
< 0.05) for corn grain than for the other ingredients. Gas production is an indirect measure of the degradation of substrates, particularly from carbohydrates. Furthermore, it is an estimator for the production of short-chain fatty acids [30
]. GY 24 h, SCFA and MCP synthesis was higher (p
< 0.001) for soybean, corn grain and barley hay compared to SO, CO and PO (Figure S1
). The results obtained mirrored the chemical composition of each feedstuff, which could increase or decrease microbial fermentation in addition to the fact that SO, PO and CO are protected fats, and protected fats in the form of calcium soaps usually depress gas production [31
The in vitro gas production parameters were different between treatments (p
< 0.002) (Table 6
). Increased gas production (mL gas/g DM) and the “c” fraction were higher for SO (p
< 0.001) followed by PO, where the inclusion of CO affected the gas production and fermentation rate. Lag time was higher (p
< 0.001) for CO than SO. Fat, oils and grease have negative effects on rumen fermentation, which are associated with the inhibition of microbial activity, particularly microorganism methanogenic activity [52
]. It has been mentioned for many years that oil supplementation, particularly with unsaturated oils, affects and modifies the microbial activity of the rumen, which is involved in cellulose degradation. Also, it has been explained that these factors are due to the fact that fats coat the ruminal bacteria, disrupting cell membrane functions, and that long-chain fatty acids are toxic to cellulolytic bacteria [53
In the present study, less gas production was observed in the treatment of CO, followed by SO, which are unsaturated fatty acids such as linoleic acid. As mentioned above, this type of fatty acid is toxic, causing fewer protozoa and bacteria in the rumen, which explains our findings. The DMD 96 h, PF 96 h and MCP were not affected among treatments (p
> 0.1). GY 24 h and SCFA were higher (p
< 0.001) for SO and PO than CO. Under the conditions of this study, results from in vitro gas production pointed at the fact that there were differential effects on rumen fermentation based on the degree of FA saturation as palm oil was a saturated FA source, CO was mainly a monounsaturated FA and SO was primarily formed by polyunsaturated FA. In this study, treatments exerted different magnitudes of change that, to some degree, led to the inhibition of ruminal fermentation processes and affected carbohydrate digestion [54
]. Overall, the discrepancies between the higher digestibility detected with CO and its lower gas production remain unknown, and perhaps analyzing the rumen microbiome will clarify these findings.
Taken together, our study confirms pioneer studies (i.e., [17
]) reporting that calcium soaps are an effective source of fat for dairy rations because ruminal fermentation is normal or at least not negatively affected, digestibility of fatty acids is high and soaps are mixed easily with other feed ingredients (as observed when diets and treatments are mixed and supplied).