Soybean meal (SBM) is one of the most common protein supplements used in livestock diets. It contains high level of lysine, but low in methionine, valine, and isoleucine, which are the first, second, and third limiting amino acids (AA), respectively in dairy cows (Schingoethe, 1996). Also, the use of SBM in animal diets increases the cost of livestock production because it is considered as an imported product in most countries and is expensive. Sesame meal (SM), a by-product of sesame oil extraction, can replace SBM at a lower cost with a positive effect on milk production and nutrient digestibility (Abo Omar, 2002; Obeidat et al., 2019). The amounts of dry matter (DM), crude protein (CP), ash, ether extract (EE), nitrogen free extract and crude fiber in SM are about 83-96%, 23-46%, 7.5-17%, 1.4-27%, 25-32% and 5-12%, respectively (FAO, 1990). As a fat source, SM is valued for its high levels of unsaturated fatty acids (UFA), especially linolenic acid and linoleic acid, which can modify some bioactive components in milk, such as conjugated linoleic acid and omega-3 fatty acids (Medeiros et al., 2014).

Decreasing the degradability of protein in the rumen has the potential to improve milk production by providing more AAs for absorption in the small intestine. Many measures are taken to protect feed against degradation in the rumen (Haryanto, 2014). Among these, treatment with formaldehyde is the most common, efficient and least expensive method (Walli, 2005). Formaldehyde toxicity has been studied in rats, rabbits, and dogs after oral feeding, and the LD₅₀ was 800, 270, and 550 mg/kg of body weight (BW), respectively (NCBI, 2023), indicating the low toxicity of formaldehyde. Wales et al. (2010) concluded that formaldehyde, when applied as an antimicrobial feed additive, has not been generally shown as a cause of adverse responses in animals. Formaldehyde significantly reduces the solubility of protein and makes it very resistant to microbial attack in the rumen without affecting its digestibility in the small intestine (Sanjukta & Rai, 2016). There are few reports on the effects of formaldehyde-treated SM on dairy. However, a positive effect of feeding formaldehyde-treated SBM or canola meal on nutrient intake, milk production and composition was observed in cattle, goats and sheep (Tajaddini et al., 2021). Thus, the objective of current study was to evaluate the effect of substitution of SBM by treated or untreated SM in the diet of Murciano-Granadina dairy goats on feed intake, nutrient digestibility, milk yield, milk fatty acid (FA) profile, blood metabolites, and rumen fermentation characteristics.

Today’s ongoing global challenge is to optimize the use of protein and fat supplements in various production systems, which is mostly focused on the quality of produced milk/meat in developed countries, rather than raising the quantity of production in developing countries (Gulati et al., 2005). The results of the current study demonstrated that goats fed diets containing formaldehyde-treated SM, especially those with higher levels of formaldehyde, had grater DM, OM, CP, and ME intakes than those fed diets containing untreated SM or SBM. This may be due to the processing with formaldehyde since this processing decreases protein solubility in the rumen and enhances the number of peptides and essential AAs available in the intestine and increases animal feed intake (Sanjukta & Rai, 2016). In other words, preventing the degradation of dietary CP by rumen microorganisms, increases the flow of by-pass protein and AAs to the small intestine, which helps balance of absorbed AAs and thus stimulates feed intake (Baker et al., 1996). Similar to our results, Tajaddini et al. (2021) reported that DMI increases when goats are fed with formaldehyde-treated canola meal, which can be mainly attributed to higher dietary rumen undegradable protein (RUP) content of diets by using formaldehyde-treated canola meal, leading to an improved balance of amino acids post-ruminally (Forbes, 1995). Furthermore, increased milk production in goats fed a diet containing treated SM (Table 3) leads to a rise in nutrient requirements, especially energy, requiring higher nutrient consumption (Baker et al., 1996). It is evident that the goats with higher milk production will consume more DM and ME. Furthermore, substitution of SBM with formaldehyde-treated SM increased RUP content of diet and RUP intake of the goats compared to control. Our results agree with previous researchers’ findings who fed formaldehyde-treated canola meal or SBM to dairy goats and cows (Baker et al., 1996; Tajaddini et al., 2021). However, Sirohi et al. (2013) reported no effect on feeding formaldehyde-treated mustard cake to lactating crossbred cows.

Partial substitution of SBM with either untreated or treated SM with different levels of formaldehyde had no effect on nutrient digestibility. These findings might be attributed to the similarities in chemical composition of four dietary groups as mentioned by Mahmoud & Bendary (2014). This result support those obtained by Obeidat et al.’s study (2009) in which addition of different levels of SM (0, 8, and 16%) to the diet of fattening lambs have no significant effect on nutrient digestibility. Furthermore, our results are consistent with findings of Mahmoud & Ghoneem (2014). Throat et al. (2016) also reported that the formaldehyde-treated rapeseed meal do not affect feed digestibility in high-yielding dairy cows. Furthermore, Obeidat et al. (2019) reported that there is no significant difference in nutrient digestibility when SBM was replaced by SM in the diet of Awassi sheep.

Goats fed diets containing formaldehyde-treated SM had higher milk production than other groups. This may be due to the fact that when dietary protein is treated with formaldehyde, it leads to increased absorption of essential AAs and metabolizable proteins from the duodenum (Yörük et al., 2006), and hence providing the limiting AAs for milk synthesis to the mammary gland. Increased milk production in goats fed with formaldehyde-treated SM, which were fed diets with higher contents of protected protein, may be a direct result of more postruminal supply of those amino acids involved in limiting milk production (Tajaddini et al., 2021). In addition, increased milk production in response to formaldehyde-treated SM might be attributed to an increase in protein available for digestion in the intestines increasing milk precursors (Mishra et al., 2006). As previously stated, an increase in DM, CP and ME intake in goats fed the formaldehyde-treated SM diets could explain their increased milk production because higher ME intake is responsible for higher milk production (Tajaddini et al., 2021). In accordance with our findings, Petit (2003) demonstrated that adding formaldehyde-treated flaxseed and sunflower seeds to the diet of dairy cows increases milk production up to 2.7 kg/day when compared to untreated flaxseed and sunflower seeds. Tajaddini et al. (2021) also reported that giving 1.2% formaldehyde-treated canola meal to lactating goats increased their milk, FCM, ECM and TSCM production. Similar findings were also obtained in cows, buffalos, and goats by feeding rapeseed meal treated with formaldehyde (Sherasia et al., 2010), formaldehyde-treated mustard and groundnut cakes (Shelke et al., 2012b), formaldehyde-treated SBM (Dosky et al., 2012), and formaldehyde-treated sesame cake (Bugalia & Chaudhary, 2010). Throat et al. (2016) reported that lactating cows supplemented with formaldehyde-treated rapeseed meal on iso-nitrogenous rations have higher daily milk yield and FCM, and improved production efficiency. However, our findings are inconsistent with those of Ashes et al. (1992), who found no change in milk production in response to formaldehyde-treated canola seeds. Substitution of 15% SM in the diets of lactating dairy cows decreases milk yield and feed efficiency than control diet (Shirzadegan & Jafari, 2014). Variations in the type and quantity of protein, as well as the animals’ stage of lactation, may cause different outcomes.

Goats fed diets containing SM (either treated or untreated) had higher milk fat content than those fed control diet. The results of present study agree with Hejazi & Abo Omar (2009), who found that Anglo-Nubian goats fed diets containing 10 and 15% sesame oil cake have significantly higher milk fat than goats fed basal ration. The increasing in milk protein content in goats fed diets containing formaldehyde-treated SM may be due to increased RUP levels through treatment with formaldehyde, which is then optimally utilized in milk protein synthesis. In line with the results of the present study, Ababakri et al. (2021b) indicated that higher levels of dietary RUP (40%) has beneficial effects on ewes’ milk protein contents, but did not affect milk production.

Addition of untreated and formaldehyde-treated SM to the diet of lactating goats decreased SFA, SCFA and MCFA in milk, while UFA, MUFA, LCFA, and PUFA contents as well as UFA/SFA ratio significantly increased. Since the mammary gland’s epithelial cells primarily produce SCFA from de novo, their synthesis can be prevented by raising the concentration of specific LCFA (Grummer, 1991). Indeed, the high amount of C18:1 (oleic acid) and C18:2 (linoleic acid) in SM causes an inhibition in the expression of genes involved in the de novo synthesis of milk SCFA and MCFA (acetyl-Co A carboxylase alpha and fatty acid synthase) (Thakur et al., 2018) and reduces milk SCFA and MCFA contents. On the other hand, it is possible that due to the high concentration of PUFA in SM, these FAs escape rumen degradation and enter the blood and increase the milk UFA content. It is also possible that the decrease in ruminal degradation in goats fed formaldehyde-treated SM diet, led to the escape of fatty acids from rumen biohydrogenation and absorbed in the small intestine and entered the milk, and as a result, LCFA is higher in the milk of these goats. Similar to our results, Kim et al. (2013) found that the ratio of UFA/SFA is higher in steers fed the diet containing SM. Also, Liu et al. (2008) also showed a reduction in SCFA and MCFA concentrations in milk when cows are fed diets containing SM. Given that MCFA constitutes the hypercholesterolemic component of milk fat; therefore, reducing MCFA may be recommended as a potential to improve milk’s FAs profile from the perspective of human health (Shingfield et al., 2013).

Replacement of formaldehyde-treated SM in the dairy goats’ diet reduced ruminal pH, NH₃-N concentration, acetate production as well as acetate/propionate ratio while increasing total VFA and propionate production as compared with SBM and untreated SM. The decrease in rumen pH in goats fed formaldehyde-treated SM is influenced by the increase in total VFAs and especially the increase in propionate production in the rumen. Previous studies showed that formaldehyde-treated SBM (Yörük et al., 2006) and formaldehyde-treated canola meal (Tajaddini et al., 2021) have no effect on sheep’s ruminal pH, but significantly decrease rumen NH₃-N concentration. The reduction of ruminal NH₃-N in goats fed formaldehyde-treated SM diets was predictable because treating with formaldehyde protects the protein from microbial degradation. Previous results on feeding formaldehyde-treated oilseed protein meals were reported by Shelke et al. (2012a) who indicated that rumen protected protein supplements save more energy, which could be wasted through urea synthesis in the liver. Similar to our findings, Wright et al. (2005) found a decrease in ruminal NH₃-N production when lactating cows were provided heat and lignosulfonate-treated canola meal.

Increased production of VFA in goats fed formaldehyde-treated SM diets could be attributed to higher DM intake (McDonald et al., 2011). Our findings are consistent with those of Bhatt & Sahoo (2019), reporting that feeding formaldehyde-treated SBM to lambs increases the production of rumen VFAs and propionate. On the other hand, reduced acetate production in rumen of goats fed formaldehyde-treated SM is mainly due to lower ruminal pH which in turn diminishes rumen cellulolytic activity and lowers ruminal acetate concentration (Purdie et al., 2008). Moreover, an increase in ruminal acetate production of goats fed untreated-SM and SBM diets may be due to the fact that feeding more rumen degradable protein (RDP) may enhance the deamination of AA in the rumen and the supply of branched-chain VFAs, which may promote fiber digestion and lead to an increase in ruminal acetate production in these goats (Misra & Thakur, 2001).

In the present study, experimental diets had no significant effect on hemato-chemical parameters of lactating goats. The concentration of blood triglyceride and cholesterol is an indicator of an animal’s energy state and equivalent amounts of these two parameters among different dietary groups suggest animals with comparable energy status. Our results agree with those reported by previous studies (Shirzadegan & Jafari, 2014; Ashjae et al., 2021) in which the substituting SM with SBM in the diet of dairy cows have no significant effect on blood glucose, cholesterol, or calcium levels. However, our findings contradicted those of Ababakri et al. (2021a), who found that sheep given extruded flaxseed containing high levels of RUP have higher blood glucose and cholesterol concentrations.

Similar to our results, Mahima et al. (2017) found no significant changes in serum creatinine, total protein, albumin, and globulin levels among heifers fed different levels of formaldehyde-treated mustard oil cake. Estimation of blood AST and ALT activity is an important factor for evaluating liver health. When the liver is damaged, these enzymes are released in high proportions into the blood, resulting in hepatopathy (Venukumar & Latha, 2004). The ineffectiveness of the experimental treatments on liver enzymes (ALT and AST) suggested that either formaldehyde-treated or untreated SM consumption had no adverse effects on the liver.

The BUN content is commonly used to assess nitrogen balance in ruminants. High BUN levels are associated with increased dietary RDP, which is degraded into NH₃-N and absorbed from the rumen to the blood (Wanapat & Pimpa, 1999). In the present study, goats fed formaldehyde-treated SM showed lower ruminal NH₃-N concentration, but their BUN concentration was similar to other groups. Perhaps the reason for this difference is related to other factors such as the energy in the rumen that affected the BUN concentration. Our findings contradicted the study of Shelke et al. (2012b), in which BUN content is reduced in buffalos fed diets containing formaldehyde-protected mustard and peanut meals.

Experimental treatments did not influence antioxidant activity of milk and blood in lactating dairy goats. Sesamin is one of the most important lignan furfuran compounds, which has various vital roles and is found in sesame seeds, oil, and meal. Some properties of sesamin include its antioxidant capacity (Jin et al., 2005) improving immune system function and anticancer activity (Hristov & Giallongo, 2014) as well as reducing blood pressure and serum lipid content (Ordway et al., 2009). Tsiplakou et al. (2021) reported that feeding goats a diet rich in SM, selenium and vitamin E improves their health and their milk oxidative status. Additionally, Mitsiopoulou et al. (2021) found that supplementing the diet with a high level of sesame seed (10% vs. 5%) increased blood antioxidant activity and milk oxidative stability in goats.

In conclusion, partial substitution of costly conventional protein sources like SBM with less expensive ones like formaldehyde-treated SM in the diet of lactating Murciano-Granadina goats improved their nutrients intake, milk yield and composition, and rumen fermentation characteristics. Furthermore, the addition of formaldehyde-treated SM (1.2 g formaldehyde/ 100 g CP) to the diet of dairy goats led to an increase in PUFA and a decrease in milk SFA, which could be beneficial to human health.