Several studies have been performed to investigate the effect of Opuntia cladodes as forage on livestock performance and rumen physiology ^[1][2][3][18,60,61].

In a study conducted by Albuquerque et al. ^[4][62] in goats, the dietary inclusion of cactus pear silage up to 42% improved the eating and ruminating efficiency rates (the time spent eating and ruminating decreased from 248 to 160 min/day and from 414 to 303 min/day, respectively) and also improved body water retention, hence reducing drinking water intake without compromising animal performance. The basal diet of forty growing rams was replaced by cactus pear at 0, 20, 40, 60, and 80% on a dry matter basis ^[5][63]. The results of this study demonstrated that cactus pear could optimally substitute pasture hay to a level of 60% and additionally contributed to sustaining the water requirement of the animals. In the study conducted by Costa et al. ^[6][64] the voluntary intake of water decreased with increased levels of cactus pear in the diet of lambs, passing from 4.9 to 2.3 kg/day, demonstrating the importance of O. ficus-indica as a source of water in semiarid regions such as Brazil ^[6][64]. In other studies, it has been observed that when spineless cactus was supplied to sheep exceeded 300 g DM per day, animals consumed essentially no drinking water ^[7][8][15,65]. In Mexico, feeding Opuntia cladodes (untreated or protein enriched, from 500 g to 3 kg/day), was tested in ewes as a substitute for alfalfa hay (1.5 kg), during the last trimester of gestation to evaluate milk yield, birth weight, and lamb growth. Results showed that the enriched one did not show any improving effect, and that Opuntia can largely match the effects from a supplementation with alfalfa hay during the observed period showing similar birth weight (3.76 as average of CON and Opuntia groups); lambs fed both Opuntia experimental diets (untreated or protein-enriched) were on average heavier at weaning than the control group (alfalfa hay) (about 10 kg vs. 8 kg) representing a cost-saving option for industries based in arid and semiarid regions where forage supply is limiting ^[9][66]. The study conducted by Souza et al. ^[10][67] in Brazil evaluated the effect of three different addition ratios (0, 21, and 42%) of cactus silage on the carcass traits and meat quality of lambs. The experimental period was 84 days, with an adaptation period of 10 days, with a starting body weight equal to 19.8 kg. A higher weight at slaughter was observed for animals who received 42% cactus silage level (33.41 kg vs. 28.39 CON), probably connected with the higher diet intake found in this group. Lambs fed 42% cactus silage showed a larger rib eye area and a reduction in saturated fatty acids (C18:0 and C21:0), which is a positive characteristic from a health point of view. As mentioned above, the indiscriminate supplementation of cactus as fodder in ruminant feed could cause several critical issues, including diarrhea, decreased milk fat content, reduced DM consumption, and weight loss ^[2][11][12][60,68,69]. To address these problems, the combination of O. ficus-indica with other forages in the diets of ruminants has been evaluated in several studies with successful results. In dairy cattle farming, lactating Holstein cows fed a diet with different replacement levels (0, 12, 24, and 36%) of forage cactus (O. ficus-indica Mill.) for sorghum silage showed a quadratic effect of levels of forage cactus on the milk fat concentration, with a maximum milk fat of 4.08% with 24% of forage cactus ^[13][70]. In lambs, replacement of 33% maniçoba hay by spineless cactus can be recommended as optimal level, because it improved the fattening of the carcass, without causing negative effects on performance or meat quality ^[14][71]. Cardoso et al. ^[15][72] in a trial on lambs fed with increasing levels (0, 150, 300, and 450 g/kg of DM basis) of fresh spineless cactus found that the inclusion of the spineless cactus in the diet up to 450 g/kg of DM improved the microbial efficiency, nutrient utilization, and the growth performance of the lambs.

A significant reduction in water consumption (−16%) was found in lambs for whom saltbush and spineless cactus cladodes (1.7 to 1) replaced 60% of barley straw and 16% of the concentrate mixture (on a DM basis) compared to the control group ^[16][73]. In experimental diets performed on goats, the association of 74.9, 57.6, 37.0, and 7.2% of fresh spineless cactus with 8.4, 18.8, 31.2, and 48.3% of saltbush hay (more concentrate) was, respectively, tested. It was observed that the combination of 37% of fresh spineless cactus, 31.2% of saltbush hay, and 31.8% concentrate provided the highest final weight, daily gain, and total weight gain ^[17][74].

In arid and semiarid regions of the northeast of Brazil, where goat husbandry represents one of the important economic activities, it has been observed that the addition of 8.4% of saltbush (Atriplex nummularia L.) hay and 74.9% of O. ficus-indica can represent strategic alternative sources of fodder able to supply the nutritional deficiencies of these small ruminants ^[11][68]. The combination of these two sources of forage to the diet promoted a linear and quadratic effect (p = 0.02) on dry matter intake (DMI) and other nutrients. The diet composed of these percentages of saltbush and O. ficus-indica presented the lowest water balance (1.54 kg/day), water intake per kg of DM consumed (1.60 kg/day), and DM digestibility ^[17][74].

The study conducted by Ragab ^[18][75] reported that the inclusion of prickly pear peel (PPP) in the diets of male chicks (15 and 30%) improved their performance, since inclusion of PPP caused a significant (p < 0.05) decrease in live body weight gain (1061.1 g vs. 996.5 g) and a significant increase in feed intake (2736.2 g vs. 2885.2 g) during the total period of supplementation (from 14 to 70 days of age). Moreover, its integration, by minimizing the amount of expensive yellow corn grain needed in poultry diets, reduced the cost of feeding.

The supplementation of 1% of O. ficus-indica as a source of dietary fiber in gilts during late gestation and lactation determined a variation in the biochemical parameters (increase in insulin and high density lipoproteins and a decrease in triglyceride concentration at gestation, farrowing, and lactation) and voluntary feed intake, which was higher compared to the control group ^[19][76]. Table 1 summarizes the application of Opuntia in animal nutrition and the main effects.

In the study conducted by Ortiz-Rodriguez et al. ^[20][77], the dietary supplementation of 14.9% of fresh Opuntia-ficus indica in Holstein cows improved both the quantity and quality of raw milk and fresh cheese. In fact, milk yield at 225 day of lactation was 10.864 L/day in the group supplemented with cactus pear, while in the control group this value was equal to 7.200 L/day. Additionally, the storage life of the fresh cheese also was improved, since the bacterial colony forming units (CFU) were significantly lower (p < 0.05) in the fresh cheese obtained from the group of cattle receiving the diet with Opuntia (4.0 CFU/mL vs. 5.1 CFU/mL; ^[20][77]). However, it cannot be ignored that both contents found were above that (2.0 CFU/g⁻¹ [log10]) allowed by the Official Norm of the country in which the cheese was produced, which is indicative of poor hygiene and, therefore, not suitable for human consumption. The use of prickly pear byproducts (juice, peel, pulp, and seeds) as dietary supplement could be a way to enhance a more sustainable animal production system by reducing waste production. However, it was also reported that the poor shelf life of these products was due to their high levels of moisture and fermentable carbohydrates ^[31][88]. In fact, prickly pear byproduct outdoor storage is only possible for a few days due to bacterial fermentation ^[32][22]. It is advisable to ensile prickly pear byproducts with dry fodder or mature crop residues, such as wheat straw, to partially absorb the water, due to their high level of humidity. It has been observed that the silage obtained with 5% of straw is the best preserved, since it helps in achieving a significantly lower pH and ammonia nitrogen concentration. Moreover, the inclusion of only small amounts of straw is economically advantageous, as the cost on forages is greatly reduced ^[31][88].

The incorporation of Opuntia in ruminant diets also had effects on the quality of meat. The study conducted in Morocco by Otmani et al. ^[33][89] investigated the effect of the incorporation of cactus cladodes (CC; 30% concentrate diet) in the diet of Beni Arouss goat kids, during three months of a feeding trial starting from an average weight of 10.5 kg, showing higher proteins and lower fat and ash content in the CC group than the control (conventional supplementation). With reference to fatty acid, a decrease in C16:1 and an increase in C20:3n3 was observed in the L. dorsi of the CC group.

Pascoal et al. ^[34][90] conducted a study in which different levels (0%, 10%, 20%, and 30%) of forage cactus meal were added to the diet of rabbits in the growing phase to evaluate the effects of its inclusion on the productive performance, carcass characteristics, and economic evaluation. There was no effect of the dietary inclusion on average daily feed consumption, average daily weight gain, feed conversion, and the final weight of rabbits, suggesting that the nutritional quality of the diets were maintained as the level of inclusion of forage cactus meal increased. Equally, no effect was found on carcass weight, carcass yield, viscera weight, and liver weight.

The effect of substitution of yellow corn grains by prickly cactus pear (O. ficus-indica) fruit peel at the rates of 0, 5, 10, and 15% in broiler diets on the feed conversion ratio, meat quality, and its biological value was investigated by Badr et al. ^[35][91]. The results showed that the dietary inclusion of prickly cactus pear fruit peel enhanced the live body weight, feed intake, and feed conversion ratio (p < 0.05). The observed increments in live body weight and body weight gain (1765 g in the control group vs. 1865 g in 15% Opuntia integration) may be attributed to the increasing digestion of all nutrients, suggesting that the inclusion of this product in chick diets did not have a negative effect on diet palatability. Concerning the slaughter parameters, the dietary inclusion did not show any detrimental effect and achieved high carcass weight and dressing percentage, with a higher crude protein content (75.08% in the control group vs. 79.9% in the 15% Opuntia integration). Moreover, broilers fed diets containing 15% prickly pear products reached higher scores of taste, color, odor (aroma), texture, and overall acceptability.