Morphometric measurements are subjective and used to assess the carcass development and quantitatively measure the muscular distribution in the carcass with estimates of its conformation. In the present study there were not significative differences observed for these parameters or for CCI, inferring that the use of cactus pear silage as well as intermittent water supply combined or alone did not alter animal growth and/or carcass conformation, maintaining the muscle pattern achieved by the control diet (usual) and demonstrating body and carcass uniformity. Since use animals in this study were homogeneous and had similar age and body performance, as indicated by the carcass morphometric measurements and by the difference between the empty carcass and hot carcass weights, which results in the sum of head + limb with an average of 8.2 ± 0.13 kg between treatments, giving an idea that the animals were similar in chronological age, since the allometric growth of the body occurs from the extremities to the interior of the body.
The intermittent water supply, cactus pear silage and interaction between these factors did not result in differences in slaughter weight, BCS, EBW, HCW, CCW, WELC, BCY and LEA. On the other hand, there was a significant increase in HCY and CCY of animals on diets containing cactus pear silage (21 and 42% in DM) as a substitute for Tifton hay. Goats are animals naturally adapted to arid conditions, they have adaptations on their skin, with less dense fur, which allows better air movement by increasing heat dissipation by convection and reducing the surface temperature; reduced density of sweat glands in relation to other ruminant species, which provides less water loss by sweating; physiological adaptation to the control of osmolarity through water intake and urinary excretion, in which, through changes in plasma osmolarity that are signaled to the osmoreceptors in the hypothalamus, mediate the arginine-vasopressin antidiuretic hormone that acts on the kidneys causing more water reabsorption and consequently reduce the excretion of water via urine, so that goats and sheep can be subjected to water restriction for up to 483,6,11−14.
Measurements and evaluations carried out on the carcass, such as the carcass compactness index and LEA, are parameters that quantitatively measure the muscle distribution in the carcass, an edible part of greater financial return, which indicates the conformation of these animals4, while the BCS and the measure C, which are highly correlated, measure the distribution of fat on the carcass, giving an idea of the carcass finish, in which the higher these variables, the greater the proportion of fat that allows less water loss due to carcass cooling15. These variables in the present study were also not influenced by the levels of cactus pear silage and water restrictions, presenting an overall mean of 0.17 kg/cm, 7.68 cm, 2.42 points and 0.7 mm respectively, and consequently did not influence the losses due to cooling, which presented an average 1.48% losses.
There were no effects of diets, water supply or interaction between cactus pear silage and intermittent water supply on weights of the left cold carcass and the commercial cuts of goat meat (shoulder, neck, rib, leg, and loin) in kg or yield percentage. The main cuts of the goat carcass are the neck, leg, shoulder, loin and rib, their economic values differ, and their proportion becomes an important index to evaluate the carcass quality16. The cuts of greatest importance and commercial values are the leg and the loin, called noble cuts because they present greater yield and muscle tenderness, being interesting that they present a good proportion in the carcass, for providing greater edible tissue content, mainly muscle.
Carcasses with similar weight tend to have equivalent proportions of cuts, as they exhibit isogonic growth17. As the CCW and the conformation of the animals were similar, with similar morphometric measurements, they had a direct relationship in the absence of an effect on commercial cuts.
The commercial value of the carcass, whether through carcass yield and/or the proportions of the cuts, is also linked to tissue composition, thus the dissection of the leg represents an estimate of measuring the tissue composition of the carcass, in which is sought a greater proportion of muscle, intermediate proportion of fat and less bone in carcasses17–19. In this way, diets with cactus pear silage and the different levels of intermittent water supply resulted in the constancy in the amount of muscle, fat, and bone in legs of goats. The similarity in muscle proportion is related to the lack of effects on slaughter weight and CCW, as the weight of muscles is highly correlated to carcass weight17. The average muscle yield was above 60% in all treatments, confirming that the animals showed good efficiency to the diets and adapted well to the water supply levels.
As for the weight and proportion of bone tissue, it is believed that because this is a tissue with early development in relation to muscle and fat4, diets in the final stages of growth (average of eight months) would hardly change their participation in the tissue composition, where the relationship of this tissue with the others is usually only increased when there are changes in the proportion of muscle and/or fat.
Water restriction, as long as it is moderate and acute, mainly affects the loss of body water and not tissues, which does not cause deleterious effects on animal productivity and growth.
The muscle: fat ratio indicates the state of leg fattening, while the muscle: bone ratio estimates the carcass muscularity, both being attributes of quality6. The similarity previously reported in the weight of fat, bone and muscle corroborates that these relationships also do not have differences. The same occurs for the LMI, due to the weight of the five muscles used to determine the index and the length of the femur to have been similar between the animals.
Nevertheless, when considering fat as a percentage of participation in leg weight, it is possible to observe that the intermittency in water supply in both intervals (24 and 48 hours) reduced the proportion of fat in the leg. Although water supply levels do not sometimes affect the daily intake of dry matter, during days of water deprivation fat mobilization for energy availability may occur, possibly offsetting water stress and influencing not only feed intake, these days of deprivation, but also affecting energy metabolism, which results in the mobilization of energy reserves3.
When the physicochemical composition of the meat was evaluated, it was observed that the diets and water supply levels probably did not affect the reserves of muscle glycogen during the pre-slaughter management as can be seen through pHinitial and pHfinal. The pHinitial right after slaughter should be close to neutrality, as well as in the live animal, indicating that the animal did not suffer from stress during the pre-slaughter period. The pHfinal, on the other hand, is expected to show a considerable variation, between 5.55 and 6.2 for goat meat; due be inversely proportional to the concentration of muscle glycogen at the time of slaughter, that is, a more intense expenditure of glycogen stores results in less lactic acid production and higher pHfinal20,21. In this research, the pHfinal had average 5.74, pH higher than the isoelectric point of muscle proteins (5.2–5.3), this result being favorable, since it is above the neutral charge and presenting an excessive negative charge that provides the repulsion of filaments, that allows water molecules to bind and improve the organoleptic characteristics of the meat through succulence and texture of meat22 evaluated by cooking loss, moisture, and shear force, principally. The cooking loss, moisture and shear were within the recommended (20 to 35% CL, moisture above 70% and SF up to 44.13 N for goat meat) to classify the meat as soft and tender23. Statistically, interactions were found between the supply of silage and intermittent water supply, in which goats on a diet without cactus pear silage and without intermittent water supply showed higher values of cooking losses and shear force.
Higher concentrations of collagen content and/or greater activities of calpastatin (which inhibit the action of calpains) can lead to reductions in meat tenderness20,24. However, it is believed that the differences for CL and SF were mainly caused by a slight shortening of muscle fibers, occurring during the cooling period of carcasses. Because goat carcasses are generally small and with a thin layer of subcutaneous fat, there is rapid heat dissipation at the beginning of the post-mortem period, leading to cold shortening and subsequent muscle hardening15,25.
pHfinal of the meat has a high correlation with color parameters (L*, a*; b* and Chroma), as the pHfinal can affect the reaction of myoglobin to oxymyoglobin, but all the parameters related to color corroborate the literature. The b* index or yellow pigmentation index in meat, on the other hand, may be related to the concentration of fat and/or the presence of carotenoids in the diet which can be affected by forage preservation processes, such as silage- and hay, which significantly reduces by up to 80% carotenoids levels26. It is believed that the carotenoid concentrations in the diet of this study were similar between treatments and consequently in values of b* of meat. Values of a* and Chroma directly depend on the content and state of the heme pigments in the muscle, due to the chemical state of iron (Fe), playing an important role in meat color20. These parameters showed no significant difference between treatments (P > 0.05), however, higher values of a* and Chroma in meat are desired, as a result of the increase in oxymyoglobin and decrease in metmyoglobin that provides the meat's "bloom". According to Dawson et al.27 and Velasco et al.28, the minimum critical value for meat luminosity (L*) is 34, lower values of L are related to elevating pHfinal, which results in the high concentration of metmyoglobin making the meat darker, which causes rejection by consumers for associating dark meat as old meat.
The meat's presentation and more precisely its color is an important factor that can influence a consumer’s purchase decision, as it gives us the idea of freshness and meat' quality, the L* and a* color parameters are the most representative for these characteristics29. Although in our research it did not have a significant effect for the color parameters, we can indicate that the meat obtained in this research would be well accepted by consumers, because30 suggests that consumers will consider meat color acceptable when the L* value is equal to or exceeds 34, and a* value below 19 or equal to or exceeds 9.5 according to Khliji et al.29. In the present study, all values for L* remained above this aforementioned threshold and the values of a* remained within these values which suggests that meats for all diets and water supply levels had an acceptable color for consumers.
When evaluating the chemical composition of meat, no significant difference was observed between treatments, except the ash content, that remained above the average values found in the literature, which is 0.99 to 1.10%31. It is believed that because cactus pear is a rich source of Ca, Mg, K and with increasing level of cactus pear silage in the diet32, these minerals were consumed in larger amounts, which can have resulted in a higher proportion of minerals in the meat of animals that received 42% cactus pear silage.
The fatty acid lipid profile in meat has a major impact on sensory properties and nutritional quality, influencing acceptance and health for consumers33,34. Intermittent water supply, cactus pear silage, and interaction between water supply and cactus pear silage did not influence most fatty acids present in the Longissimus lumborum muscle of the animals under study, except only a few saturated fatty acids so docosanoic acid (C22:0), tricosanoic acid (C23:0), branched-chain fatty acid total (BCFA), anteiso-tridecanoic acid (C13:0 anteiso) and anteiso-pentadecanoic acid (C15:0 anteiso).
Biohydrogenation of ruminal bacteria results in a circumstantial variety of fatty acids, which will be absorbed in the intestine and later incorporated into the meat of goats. In addition to the diet and the biohydrogenation, the meat lipid profile can vary due to de novo synthesis, desaturation, duration of the feeding period and differences in pathways of various FAs by the animal organism35.
A high concentration of saturated fatty acids present in meat is not desirable, as there is evidence that saturated fatty acids, mainly C16:0, as well as myristic (C14:0) and lauric (C12:0) increase the blood cholesterol and LDL concentration, due to interferences with hepatic LDL receptors36, however, in studied treatments, there was no significant difference for these fatty acids. On the other hand, C18:0 has no impact on cholesterol levels, due to being poorly digested and easily desaturated to C18:1 by Δ9-desaturase37, present in the cell endoplasmic reticulum. This fatty acid is not harmful to health and is considered the only desirable SFA. As the levels of C18:0 in diets tend to be minimal, their main origin is the biohydrogenation of PUFA and de novo syntheses in diets with a high energy pattern38.
In addition to carrying out the biohydrogenation process, ruminal bacteria synthesize a series of FA, mainly those of odd and branched chain, that comprise mainly the lipids of the bacterial membrane39–41, to maintain membrane fluidity42. Linear odd-chain fatty acids are formed when propionyl-CoA, instead of acetyl-CoA, is used as a de novo synthesis initiator39. On the other hand, iso and anteiso FA are synthesized by the precursors branched-chain amino acids (valine, leucine, and isoleucine) and their corresponding branched- short-chain carboxylic acids (isobutyric, isovaleric and 2-methyl butyric acids)43.
There is an increasing interest to study odd-and branched-chain fatty acids (OBCFAs) from animal products, mainly in milk due to its higher concentration compared to meat. Researchers reported that several OBCFAs have potential health benefits in humans44,45 as improve the gut health46 and presents anti-cancer activity47,48, as well as improve the sensory characteristics of the meat, providing a greater sensation of tenderness and juiciness, because BCFA content are associated with a less consistent fat in meat from lambs due to its lower melting point and its chain structure42,49.
The FAs profile in the ruminal bacteria is largely composed by OBCFAs (C15:0; anteiso C15:0; iso C15:0; C17:0; iso C17:0; C17:1 and anteiso C17:0) in the bacteria membrane lipids39. Thus, the higher concentration of OBCFAs might be the result of the difference in the rumen bacterial populations induced by variation in the dietary carbohydrate, that is, a higher concentration of cellulolytic bacteria in relation to amylolytic bacteria, due to the high NDF content in the diet with 0% cactus forage silage. It is also known amylolytic bacteria produce more linear odd chain and anteiso FAs than iso FAs, whereas cellulolytic bacteria produce more iso FAs43,49. As the Tifton hay-based diet had the highest NDFap and starch content (highest % of ground corn), the meat of those animals had higher concentrations of anteiso C15:0 and anteiso C13:0 compared to animals fed diets with the inclusion of cactus pear silage, also influencing the total sum of branched chain fatty acids.
Although levels of intermittent water supply have generated punctual changes in tricosanoic acid (C23:0) SFA, the same was not observed for MUFA and PUFA, due to changes in the rumen environment, promoted by water restrictions, which were not sufficient to circumstantially modify biohydrogenation, resulting in similarities in concentrations of unsaturated fatty acids in goat meat.
The animals subjected to 24h IWS presented the highest concentration of C23:0 in relation to other treatments, which is interesting because involved in the synthesis of ceramide and reduces the risk of diabetes in human50,51.
The cactus pear has high NFC content (mainly pectin), that provides changes in rumen bacterial populations because have 59.5% high and medium rumen degradation carbohydrates52. Thus, there is probably a higher rate of biohydrogenation, reducing the percentages of cis-13 C18:1 in goat meat in animals’ cactus pear silages-based diets. Furthermore, diets with high proportions of CPS, such as 42% CPS diet, can decrease ruminal pH and affects the final stages of biohydrogenation, resulting in escape of intermediate fatty acids isomers, that are absorbed in the small intestine53, which can explain the similarity of C20:1 in 42% CPS diet from the Tifton hay-based diet, with differences between goat meat from 21% CPS diet and Tifton hay-based diet.
Oleic acid (c9-C18:1) was the MUFA with the highest participation in the lipid profile of goat meat, which is interesting because it has a hypocholesterolemic effect, being a DFA for not reducing the serum HDL levels and thus prevent cardiovascular disease by reducing LDL levels54. The high concentrations of c9-C18:1 in ruminant meat come from the food intake, the effect of biohydrogenation, and mainly of the high activity of Δ9-desaturase, necessary for animal biosynthesis through desaturation of C18:0 to c9-C18:141. This fatty acid in the lipid profile of red meat varies between 30 and 43%55, confirming that the meats in the present study had a good concentration of this fatty acid.
Much of unsaturated fatty acids, which have 18 carbons or 16 carbons, are largely converted to C18:0 and C16:0 through biohydrogenation, and when this process is not 100% completed, in addition to the AGPI that pass through this process intact, some products intermediates are formed, reach the duodenum and are absorbed by the animal, in which significant amounts of cis and trans-monounsaturated, such as vaccenic fatty acid (t11-C18:1), reach the duodenum and are absorbed, later composing the muscle tissue34.
The literature indicates that the precursor of conjugated linoleic acid (CLA) in the meat of animals is trans vaccenic acid (t11-C18:1), so the enzyme ∆9-desaturase, besides acting in the conversion of stearic into oleic fatty acid, also converts the trans-vaccenic acid to its corresponding CLA isomer, c9t11-C18:256. This pathway is more expressive in the mammary gland, and as the concentration of vaccenic acid (t11-C18:1) was not different, the concentration of CLA was not affected by the supply of silage and intermittent water supply, in the same way, that there are also no differences in the activity of ∆9-desaturase. Nevertheless, it is worth noting that in the human adipose tissue there is also the presence of ∆9-desaturase, and therefore, increased intake of vaccenic fatty acid could have the same beneficial effects associated with the intake of CLA, where the dietary vaccenic fatty acid shows 19 to 30% conversion rate57.
Tifton hay is a natural source of n-3 fatty acids, mainly C18:3 n-3 with up to 20% participation in the lipid profile4, allowing a certain part of these PUFAs to be absorbed and increased in the tissue muscle, with 10 to 30% PUFAs in the diet generally escaping from biohydrogenation.
Linoleic fatty acid (c9c12 C18:2) and α-linolenic acid (C18:3 n-3) are essential fatty acids for humans, that serve as precursors of the n-3 and n-6 pathways, distinct families, but synthesized by some of the same enzymes (∆4-desaturase, ∆5-desaturase, and ∆6-desaturase)37. Arachidonic fatty acid (C20:4 n-6) comes from elongation and desaturation of linoleic acid, where its concentrations even close to that of its precursor may indicate that there was a high activity of ∆6-desaturase (desaturation to γ-linolenic), elongase (elongation of γ-linolenic to dihomo-gamma-linolenic) and ∆5-desaturase. This fatty acid was influenced by the diets, presenting lower concentrations in the meat of animals fed the 42% cactus pear silage when compared to the Tifton hay diet (0% cactus pear silage).
A higher concentration of long-chain PUFA n-3, docosahexaenoic (C22:6 n-3), was observed in the muscle of animals fed on Tifton hay, this was probably due to the high concentration of C18:3 n-3, precursor of C22:6 n-3, that the hay presents in relation to the palm silage.
Diets with cactus pear silage, intermittent water supply, as well as the interaction between water supply and the diets with cactus pear silage showed not differences for ∑SFA, ∑MUFA, ∑PUFA as well as for the ratios PUFA/ SFA; PUFA/ MUFA; MUFA/ AGS; n-6/ n-3; ∑n-3; ∑n-3.
The ratios and proportions of fatty acids are used to determine nutritional and nutraceutical values of the product or diet, and mainly, to indicate the cholesterolemic potential4. It is interesting that the n-6/n-3 ratio is low due to the pro-inflammatory properties of n-6; it is recommended to decrease its intake to assist in disease prevention58, while n-3 fatty acids are anti-inflammatory, antithrombotic, antiarrhythmic and reduce blood lipids, with vasodilating properties, being interesting that they present a higher proportion36. n-6 fatty acids tend to have a higher percentage in meat, and this directly influences the formation of n-3 isomers, since linoleic acid, when in excess, can reduce the synthesis of linolenic acid metabolites59. The percentage of FA in one family can interfere with the metabolism of the other, reducing its incorporation into tissue lipids and altering its general biological effects60. Therefore, it is not recommended that the n-6/n-3 ratio be kept above 5 or 661, demonstrating that the averages of the current research remained acceptable.
In relation to AI and TI, Ulbricht & Southgate62 proposed that sheep meat should have values of up to 1.0 and 1.58, respectively, and the lower the values for these indices in the lipid fraction, the greater the prevention of early stages from cardiovascular diseases63. In the present study, the general averages observed were 0.29 for the AI, and 0.81 for the TI, although there was no significant difference, all treatments are within the recommended range, despite having been used the comparative standard to sheep, due to the absence of the proposed standard for goat meat.
The h:H ratio did not differ for diets and water supply levels, but had an average of 1.90, below the reference value for meat products, which is 2.0. Values above 2.0 are recommended and favorable64, as it indicates a higher proportion of hypocholesterolemic fatty acids, that are beneficial to human health.
The ∆9-desaturase enzyme that acts on both the mammary gland and adipose tissue, responsible for the transformation of SFA into UFA, as well as in the endogenous conversion of CLA56 did not differ between treatments, on the other hand, the elongase showed less activity. Probably there was a greater "de novo" synthesis which resulted in a greater accumulation of palmitic fatty acid, and a reduction in the activity of the elongase enzyme.
The crossbred goats demonstrated to present efficient mechanisms for adapting to water restrictions, especially when receiving feed with higher water content, such as cactus pear silage, being able to replace Tifton hay with 42% cactus pear silage in the diet for goats in confinement without negatively affecting the carcass traits and meat quality. Because although these animals have shown some differences in the indices of tenderness and juiciness of your meats, however, all presented values of juiciness and tenderness compatible with meat extremely appreciated by the consumer market. And even goat meat showing some fatty acids with different concentrations induced by the supply of silage and water intermittence, the final lipid profile was appropriate to the health of consumers, observed by the absence of differences in the total concentrations of PUFA and in the main nutraceutical parameters (DFA, n-6/n-3; h: H; AI and TI).
These results are relevant, indicating that goat feedlots in regions with low water availability may adopt strategies of lesser demand for drinking water and diets with considerable concentrations of cactus pear silage in the diet, reducing production costs without considerably affecting the product to be marketed, and therefore, provide higher profitability of the system.