Feeding trial

Following 69 d of treatment, bucks on the P. laevigata treatment had a decreased daily weight gain compared with bucks offered all other leguminous trees or alfalfa. This response was due, in part, to the reduced feed intake of goat bucks offered P. laevigata, a response previously observed with increasing levels of this forage in goat´s diets (Mahgoub et al., 2005). In fact, at the end of the feeding trial, bucks consuming P. laevigata showed inappetence, drowsiness, dehydration, and shaggy hair. The lower feed intakes of the P. laevigata ration in the present study may be due to poor ration palatability due to the excessive ingestion of toxic chemicals present in the foliage of this leguminous tree (Shachi, 2012; da Silva et al., 2018). The ingestion of these secondary compounds also explains the adverse health effects that eventually caused the death, at the end of the feeding trial, of four young bucks consuming P. laevigata. Even though Prosopis spp. is used mainly for feeding ruminants and horses around the world (William & Jafri, 2015), serious toxicity problems in goats have been reported in different countries when this plant constitute >50% of the diet (Tabosa et al., 2000; Washburn et al., 2002; Misri et al., 2003).

However, diets containing 75% P. cineraria did not reduce goat weight gain (Bhatta et al., 2007). Likewise, contrary to the present study, other authors have reported that daily weight gain of lambs (Obeidat et al., 2008) and kids (Sirohi et al., 2017) was not affected when P. juliflora pods were included at rates of 20% in diets. Under range conditions, despite the abundance of P. glandulosa, goats typically restrict the consumption of this tree to <20% of their diet (Mellado et al., 2003, 2004), a strategy for reducing toxins ingestion through limiting the consumption of the foliage of this tree.

The daily weight gain of bucks did not differ between animals receiving alfalfa, A. farnesiana, and L. leucocephala, which showed that the foliage of these leguminous trees provided enough nutrients to maintain a body growth rate comparable to alfalfa and that these leguminous trees at the rate fed in the present study did not hamper the health of goat bucks. An extensive review by Patra (2009) showed that foliage of trees can be added to goat diets at levels up to 500 g/kg for improving feed utilization and animal performance. In coincidence with the present study, Srivastava & Sharma (2002) did observe pathological changes in vital organs in goats receiving pelleted diets containing 97% of L. leucocephala. However, diets containing 40-60% L. leucocephala (dry matter basis) resulted in body weight loss, salivation, dullness, and alopecia in Myanmar goat bucks (Zun Wut Hmohn et al., 2017). Also, Phaikaew et al. (2012) documented subclinical toxicity of goats fed L. leucocephala as the primary dietary component. The considerable discrepancies in animal responses to supplementation of L. leucocephala are the high variability in mimosine content of this leguminous tree (Soltana et al., 2017).

Feed intake varied amply with diets; the highest feed intake was observed in bucks fed A. farnesiana, while goat bucks with the lowest feed intake were those offered P. laevigata. Again, secondary compounds of this leguminous tree such as alkaloids, flavonoids, prosogerin D, procyanidin, ellagic acid, tannins, and polystyrenes, among others (Zapata-Campos et al., 2020) reduce palatability (Misri et al., 2003) which explains the decreased feed intake. It is unknown specifically which of these secondary compounds negatively affect palatability of P. laevigata, but this effect derives from a post-ingestive feedback (Baptista & Launchbaugh, 2001).

It is worth mentioning that condensed tannins of A. farnesiana, L. leucocephala, and P. laevigata were far below the forage level in the forage to acts as a feeding deterrent (Provenza, 1995). Compared to the diet based on alfalfa, the higher feed intake of the diet, including A. farnesiana, can be due to the high nutritional quality of A. farnesiana for goats because of its high crude protein content (17.3%; Zapata-Campos et al., 2020) and medium tannin contents (2.8%; Zhou et al., 2011). This tannin concentration benefits ruminants in improving protein utilization without negatively affecting feed intake and nutrient digestion (Waghorn, 2008). Ramírez & Ledezma-Torres (1997) reported that replacing alfalfa with A. farnesiana did not affect forage intake and N utilization of goats. Also, the higher intake of diets containing A. farnesiana could be due to a lower nutritional quality of this diet, which forced bucks to increase feed intake to fulfill their nutrient requirements. Indeed, kg of feed/kg of weight gain was higher in bucks offered A. farnesiana than alfalfa (apparently bucks needed more feed to obtain a weight gain similar to that obtained with the diet containing alfalfa).

Even though feed intake was higher in bucks offered A. farnesiana or L. leucocephala as compared to alfalfa, the highest feed efficiency was recorded in bucks fed either alfalfa or A. farnesiana, signifying the overall adverse effect of secondary compounds on the growth performance in bucks offered L. leucocephala and P. laevigata forage. The lack of differences between diets containing alfalfa and A. farnesiana in terms of feed efficiency indicates that A. farnesiana forage would likely have similar digestibility. The high in vitro dry matter disappearance of A. farnesiana compared to other fodder leguminous trees has been previously documented (Garcia-Montes de Oca et al., 2011). Other studies offering ≥40% L. leucocephala (Kanani et al., 2006) or A. farneisana (Velázquez et al., 2011) presented similar feed efficiency than diets containing alfalfa.

Reproductive traits

P. laevigata showed a detrimental effect on the health of bucks, which means that bucks were incapable of regulating absorption of ingested toxin or could not biotransform the secondary metabolites of this leguminous tree. The deteriorated health of the two survival goat bucks receiving P. leavigata impeded semen collection, and therefore this treatment was excluded for analysis of reproductive variables.

Both scrotal circumference and volume of ejaculate of bucks offered alfalfa was greater than that of bucks receiving leguminous trees. The mechanism of leguminous trees to substantially reduce these traits is uncertain; it could be that plant secondary metabolites of the trees fed to bucks in the present study somehow slowed down testicular functions. It is worth mentioning that, despite the highly toxic effect of P. laevigata on bucks in the present trial, consumption of this leguminous tree did not reduce testicular development when compared to bucks fed A. farnesiana and L. leucocephala. Partially in agreement with the present study, Mellado et al. (2006) observed that bucks on rangeland with higher levels of Acacia greggii in their diets yielded up to 50% less semen than bucks with a low proportion of this shrub in their diets. Contrary to the present results, other researchers have found an increased semen volume with supplementation of Acacia spp. or L. leucocephala in breeding rams (Dana et al., 2000b; Gebreslassie et al., 2021). The reduction in ejaculate volume with the addition of leguminous trees to diets could not be considered adverse since semen volume values were within the normal range for various breeds of goat bucks (Mellado, 2020).

Except for pH, which was lower in semen of bucks fed alfalfa, all other semen characteristics were not influenced by the forage source. In ruminants, pH of semen is slightly acidic, even though semen has a very high buffering capacity, higher than that of most other body fluids (Zhou et al., 2015). It is important to note that the high semen pH values found in the present study do not seem to alter sperm physiology, and therefore, can be interpreted as standard, because this value is within the pH range in ram semen (Oláh et al., 2013; Ozer Kaya et al., 2020).

Results of the present study are in line with Akingbade et al. (2002), who did not find evidence that feeding L. leucocephala was detrimental to semen quality of Nguni goat bucks. Likewise, Dana et al. (2000b) documented the improvement of semen quality supplemented with L. leucocephala leaf hay compared with bucks receiving only chickpea haulm (Cicer arietinum). On the contrary, Zun Wut Hmohn (2017) observed a marked reduction in sperm concentration in the ejaculate of bucks fed 60% L. leucocephala compared to bucks not receiving foliage of this tree.

All sexual conducts of bucks did not differ among groups, which implies that bucks' libido, mating ability, and copulatory capacity were not impaired with 33% of A. farnesiana and L. leucocephala in the buck diets. These results also suggest that the plant secondary compounds of L. leucocephala and A. farnesiana did not perturb the synthesis of androgens and the release of LH and FSH from the pituitary gland.

In summary, these data indicate that alfalfa can be totally replaced by A. farnesiana and L. leucocephala in diets for young goat bucks where these legumes constitute 33% (dry matter basis) of the diet, without altering daily weight gain, feed efficiency, scrotal circumference, most semen characteristics, and copulating ability. Thus, these leguminous trees constitute important sources of nutrients for goats in confinement. However, 33% of P. laevigata in the diet for young bucks severely impaired the health of these animals, although it took nearly three months to become a recognizable health problem. Clearly, high levels of foliage of this leguminous tree should not be included in diets for growing goat bucks in confinement.