**4. Discussion**

#### *4.1. Growth Performance and Carcass Characteristics*

The slaughter weights of lambs fed different experimental diets were within the range of 22–30 kg BW, as established in the European Regulation for "Cordero Manchego" protected geographical indication [27]. The average feed intake, average daily gain and feed conversion ratio values from the present study were more favorable than those obtained previously by Avilés et al. [2] using lambs from the same genetic background, with similar ages and slaughter weights, raised in on-farm conditions.

The overall performance of the lambs was not affected by the addition of algae meal to the diet in the present study, which is in agreemen<sup>t</sup> with previous research, wherein the diets of fattening lambs were supplemented with *Aurantiochytrium limacinum*, *Arthrospira platensis*, and *Isochiris* sp. [8,11,28,29]. However, the results in the literature regarding algae meal supplementation to the diet of lambs are somehow contradictory. Other researchers found that lambs fed diets containing *Aurantiochytrium limacinum* algae had slower growth rates at 1.7% of inclusion [7], or lower average feed intakes and daily gains at 2 to 6% of inclusion [10,13,17], in comparison with the control lambs. In contrast, increased average feed intake, daily gain and final body weight in lambs fed *Arthrospira platensis* algae meal at 1 g/10 kg BW/d were also reported [30]. These discrepancies could be mainly attributed to the type and amount of algae used, its palatability, the trial duration, and/or the weights and ages of the lambs following algae treatments.

As in the present study, previous research did not find differences in HCW and dressing percentage between control and algae-added diets [7,8,11,13,29]. Nonetheless, the means of administration of the algae meal showed a marked influence on HCW and dressing, being significantly higher when algae meal was fed in the concentrate (ALGCON) and reduced when the same amount of supplement was fed in the milk replacer (ALGMILK). Regarding the carcass composition, the present research is in agreemen<sup>t</sup> with previous results [2], and the reported values are within the normal ranges for this type of meat. On average, the algae meal treatments showed ~12% more carcass fat than NOALG, which might be related to a higher percentage of fat in the former's daily gain due to the extra energy consumed via milk replacer and algae meal [31].

#### *4.2. Meat Quality Characteristics and Oxidative Stability*

Ultimate pH (24 h after slaughter) and its fall rate are widely used to evaluate raw meat quality, due to its strong relationship with meat quality characteristics such as color, water-holding capacity and tenderness [32,33]. In the present study, neither treatments nor ageing time affected meat pH (Table 2). The ultimate pH ranged from 5.70 to 5.77 (Table 2), values typically observed in non-stressed sheep at the time of slaughter [34,35]. The observed pH values (Table 2) were within the expected range for this type of lamb meat, and were in agreemen<sup>t</sup> with those reported by other authors [2,8,17,29]. The lack of changes in meat pH due to ageing is in line with previous studies [19,36,37].

DL was not influenced by the addition of algae meal to the diet, and remained stable throughout ageing (Table 2). The DL values measured at day 1 were in agreemen<sup>t</sup> with those reported by other authors [8,13]. Regarding aged samples, the DL values were slightly higher than those shown by Avilés et al. [2] and Vergara et al. [35,37] in Manchega lambs.

Treatments had no effect on CL, or loss of water with cooking. The CL values (Table 2) differed from those obtained by other authors [2,38,39]. The slaughter weight, fatness, pH, cooking procedure and cooling time, among others, are factors to which these differences could be attributed [40]. The numerical trend of lower CL in day 7 samples matched with the observed trend of carcass fat percentage, suggesting that as fat increased, the aged samples were protected from losing water [41,42]. Conversely, Hopkins et al. [29] found that the CL values were significantly higher in algae-fed lambs, but those animals had no increased subcutaneous fat depth.

Meat tenderness, measured as WBSF, was not affected by the inclusion of algae meal in the diet, neither in fresh nor in aged meat samples, which is in agreemen<sup>t</sup> with Valença et al. [13] and Hopkins et al. [29]. The average WBSF value at day 1 was greater than the threshold (5 kg/cm2) reported by Shorthose et al. [43] to classify lamb meat into tender or tough, while, after 6 days of ageing, it was similar to that value. The maximum shear force recorded in the present study was similar to that obtained by Avilés et al. [2], Blanco et al. [38] and Linares et al. [44]. As expected, ageing time caused a significant decrease in WBSF, as previously described [45].

The meat color coordinates L\*, a\*, C\* and h◦ were not affected by the algae meal treatments (Table 2), in agreemen<sup>t</sup> with previous research [8,12,13,29]. Although b\* was higher on average in the algae meal treatments, it did not affect h◦ and C\* values, and thus did not alter the perception of color (from red to yellow) or its vividness, respectively [46]. Desirable changes in color indices were observed during ageing in the present study (i.e., higher L\* and a\* values and lower b\* and h◦ values), as observed by other authors [39].

Algae meal in the diet tended to increase meat oxidation measured as TBARS in day1 samples (Table 2), whereas ageing significantly raised TBARS regardless of the treatment. All TBARS values were lower than the acceptability limit of 2 mg MDA kg−<sup>1</sup> muscle to detect rancidity or oxidized flavors in cooked lamb meat by consumers [18,47,48]. Previous research has shown that algae meal supplementation increases meat lipid oxidation levels [8,12,13,17], which can be attributed to the higher long chain omega-3 polyunsaturated FA content in the IMF of algae meal-supplemented lambs [14].

Meat antioxidant activity was evaluated through the DPPH radical scavenging capacity of both lipophilic (DPPH fat) and hydrophilic (DPPH water) extracts (Table 2). Day1 ALGMILK samples, which contained 5 g of omega-3 FA/100 g of total fat [14], showed less DPPH fat than those from ALGCON treatment, which contained 20% less omega-3 FA. This difference would be mainly related to the need to eliminate the free radicals generated by the extra omega-3 FA in ALGMILK samples [18]. Ageing time decreased both the DPPH fat and the DPPH water of NOALG and ALGCON samples, but a similar antioxidant capacity was observed in all aged meats.

The meat's total polyphenols contents were not affected by dietary treatments (Table 2), which is in agreemen<sup>t</sup> with Muiño et al. [49], who did not observe differences after the supplementation of sheep diets with red wine polyphenols (900 mg of red wine extract/kg of feed). On the contrary, Luciano et al. [50] reported higher concentrations of polyphenols in the meat of lambs fed a diet enriched with tannins for 60 days (8.96% quebracho supplement rich in proanthocyanidins), compared to control lambs. Although algae are recognized as a good source of polyphenols, it may have been the case that the amounts of marine algae included in the current experimental diets or the length of supplementation were not sufficient to trigger any significant response [51].
