The early lactation dairy cow is highly susceptible to experiencing a negative energy balance, which has an adverse impact on milk production [1]. Feeding rumen protected fat (RPF) provides more energy without affecting rumen functions, which may improve production and alleviate the negative effects of feeding low forage diets [2,3]. However, some studies have shown that adding RPF does not affect milk yield or composition [4]. Commercial products containing calcium salts produced from palm oil, which is rich in saturated fatty acids (SFA), are the most commonly recognized RPF supplements in ruminant.

Milk and dairy products are the main sources of SFA, which have been associated with an increased risk of cardiovascular disease (CVD) and cardiometabolic syndrome [5]. Currently, there is a growing trend among consumers toward an increased interest in healthy food products. Omega-3 (ω3) and omega-6 (ω6) polyunsaturated fatty acids (PUFA) have shown beneficial effects in CVD, chronic inflammation, and autoimmune disorders [6]. Linoleic acid (LA) and α-linolenic acid (ALA) have been considered essential fatty acids for humans [7]. However, the transfer efficiency of PUFA from the diet into milk in lactating cows is relatively low due to microorganisms from the rumen that are toxic to unsaturated fatty acids (UFA) because they damage cell integrity [8]. Therefore, following hydrolysis, rumen microorganisms convert UFA to SFA via a process known as biohydrogenation. The manipulation by using rumen-protected linseed or tuna oil has the potential to prevent rumen biohydrogenation and enhance the performance of milk fat with PUFA from ruminant [9,10].

The rubber tree (Hevea brasiliensis), primarily cultivated in Southeast Asia, produces rubber seed, a by-product consisting of shell and kernel. Due to its composition, which includes 40.0% LA and 21.2% ALA, the rubber seed could be used as a PUFA source [11]. In dairy cows, a previous study found that adding 4% rubber seed oil to the diet could manipulate the rumen fermentation pattern by increasing the proportion of propionate while reducing acetate and total volatile fatty acids [12]. Moreover, adding rubber seed oil increased milk yield and PUFA in milk, especially LA and ALA, while lowering milk fat and total solids in mid-lactating dairy cows [13]. Our hypothesis is that rumen-protected rubber seed oil (RPRSO) supplementation increases feed utilization, milk production, milk fat concentration, and the milk fatty acid profile. Therefore, the objective of this research was to evaluate the effect of supplementation with RPRSO on digestibility, milk yield, and milk composition in tropical dairy cows.

The contents of OA, LA, and ALA in RPRSO were 27.68%, 40.98%, and 11.24%, respectively. Our previous studies found that heat treatment of rubber seed kernels had an amount of OA, LA, and ALA of 22.50%, 40.00%, and 21.20%, respectively [11]. The ALA content in the RPRSO was lower than in the previous report, which may be due to the different physical forms of rubber seed, the location of harvest, harvest period, and so on [21]. Another possibility was that rubber seed oil was the main ingredient to create RPF, along with calcium hydroxide, sulfuric acid, and butylated hydroxytoluene as minor ingredients, resulting in lower levels of ALA in RPRSO. In addition, Pi et al [12] reported that rubber seed oil contained UFA and PUFA and had contents of 83.00% and 59.00%, respectively. Similarly, our results found that RPRSO contained UFA and PUFA of 81.16 and 52.77, respectively. This suggests that ruminant diets could potentially use RPRSO as a feed additive.

Early lactation is characterized by a cow's lower feed intake and increased body condition score (BCS) mobilization in order to satisfy their milk requirement. Dairy cows use the BCS as an indicator of their energy balance status, and there is a correlation between the BCS, BW, and milk production [22]. In the present study, adding RPF did not influence dry matter intake (DMI), while increasing milk yield and milk fat in dairy cows. Similarly, Ranaweera et al [1] observed that tropical dairy cattle showed increased milk production, but supplementation with RPF had no effect on BW and DMI. Kowalski et al [23] reported that the addition of RPF had no effect on BW, BCS, or DMI in early lactation dairy cows. This may indicate that cows supplemented with RPF may have utilized fewer body reserves throughout the early lactation period, resulting in a maintained BW and BCS while providing energy for milk production and milk fat synthesis. However, the present experiment did not evaluate the BW and BCS of the cows, leaving the situation unclear.

Ruminants extensively use internal markers, especially AIA, to assess the nutrient digestibility from fecal samples. Fecal collection provides an accurate estimate of the diet's digestibility at any time during the day. Previous studies suggested spot sampling at least six times was accurate of fecal output in dairy cows [24]. Nevertheless, using fecal sampling twice a day for five days in our investigation may result in an estimation bias in the digestibility of nutrients. The digestibility of OM increased with the inclusion of RPRSO. The results agree with Ghoniem and Atia [25], who found that adding RPF to the diet of crossbred ewes increased the OM digestibility. However, previous studies showed that the addition of rubber seed oil did not affect OM digestibility in dairy cows [12]. RPF supplementation increased the digestibility of nutrients such as CP, EE, and crude fiber, which in turn increased the digestibility of OM in ruminants [21]. In the current study, this might increase the digestibility of CP, NDF, and ADF of RPRSO, despite their non-significant differences, and also lead to an increase in OM digestibility in dairy cows.

The diets had a significant effect on the content of triglycerides and total cholesterol in the cows’ blood serum. Animals with high triglycerides often have a high total cholesterol level [26]. Our results demonstrate the relationship between triglyceride and cholesterol levels. The concentrations of cholesterol and triglycerides were increased in cows fed RPPO, as found in the present study. Others have also observed that the concentrations of cholesterol and triglycerides in the blood will increase in cows fed RPPO in buffalo [27]. These results may be attributed to triglyceride-rich lipoproteins being secreted into the lymphatic system and eventually released into the main blood circulation via the thoracic duct [13]. However, supplementation with RPRSO can lower levels of triglycerides and cholesterol compared to RPPO. Clearly, the level of cholesterol in the milk is closely dependent on the quantity of fat matter.

Feeding of RPF had the maximum effect on milk yield during the first quarter of lactation, when feed intake is usually low, and the effect was less prominent as lactation advanced, probably due to the DM intake starting to increase after 6–8 weeks of calving [28]. The increased milk yield observed in the RPF group may be attributed to the increased energy density of the ration, resulting in reducing the deleterious effect of negative energy balance [3]. Previous studies reported that adding RPF could increase milk yield in tropical crossbred dairy cows [1]. In the current study, adding RPF increased milk yield in lactating cows when compared with the control group. However, it was not affected by time or the interaction of treatment×time effect. There are two plausible reasons. First, the difference in milk yield was not caused by the treatments but rather by the different groups of cows that were selected and assigned to different treatments. Hence, the difference was maintained throughout the experiment, resulting in effects that were not affected by time and interaction effects but by treatment effects. Secondly, the treatments immediately altered the milk production once the treatment diets were fed from the first week of the experiment. Then, it could have a significant treatment effect without any effect of time or interaction. These findings are consistent with those of Price et al [29], who found that Holstein dairy cows increased milk production when changing the treatment diets in the immediate period (days 3 and 4) and short-term adaptation period (days 7 and 8). Nevertheless, it is very difficult to determine milk production adjustments during the initial week, as animals require a minimum of two weeks to substitute gut digesta with treatment diets.

Milk fatty acid is synthesized either from fatty acids taken up from the blood or by de novo synthesis in the mammary gland. In addition, among all components of milk fat content, it is most sensitive to dietary changes. The addition of RPRSO to a diet generally increases the milk fat percentage and milk fat yield due to an increase in fatty acid uptake. There are two plausible explanations for this result. First, adding RPRSO, which contains major long-chain fatty acids, will increase the milk fat content. Second, the use of RPRSO may be non-toxic to microbes, especially cellulolytic bacteria, and have no negative effect on fiber intake and digestibility when compared to the control, which ultimately results in increased milk fat.

The rubber seed oil by-products from tropical rubber plantations contain a high level of OA, LA, and ALA, which makes them viable sources of UFA for animals [30]. However, the biohydrogenation may change LA and ALA to C18:0 in the rumen when ruminants are fed C18 UFA in their diets. The inhibited biohydrogenation of C18 UFA in rubber seed has been found in our previous studies. Gunun et al [11] found that the inclusion of rubber seed kernel heated in a hot air oven reduced biohydrogenation in the rumen and increased levels of OA, LA, and ALA in vitro. So, a study using rumen-protected UFA might enhance milk fat with PUFA and also be used for functional foods. In the current study, the addition of RPPO and RPRSO resulted in an increase in OA compared to the control group. The contents of OA in RPPO and RPRSO were 37.16% and 27.68%, respectively. When dietary RPF supplementation in cows leads to increased OA in milk. Furthermore, adding RPRSO increased the LA and ALA in milk. Similarly, Pi et al [13] found that adding 4% rubber seed oil increased OA, LA, and ALA in early lactation dairy cows. The levels of LA and ALA in milk increased in cows fed RPRSO (2.8% and 0.38%, respectively) when compared to the control (1.5% and 0.14%, respectively), which correlated with higher concentrations of LA and ALA in these diets. This could be due to the fact that using RPF has the potential to protect UFA against ruminal dissociation. As a result, there is an increased supply of UFA in the small intestine, allowing it to be available to the mammary gland for incorporation into milk fat with a higher LA and ALA content [31].

Milk fat synthesis is nutritionally modulated to increase the concentration of PUFA [32]. According to Pi et al [13], adding rubber seed oil increases PUFA in milk, whereas reducing milk fat in dairy cows may affect biohydrogenation in the rumen. The strategies to use rumen-protected oil lead to an enhanced transfer of PUFA to the small intestines while having no effect on rumen fermentation or milk fat concentration [33]. In the present study, adding RPRSO increased milk PUFA and also raised milk fat in dairy cows. These findings are in agreement with Contreras-Solís et al [9], who found that supplementation with rumen-protected linseed oil in sheep increases milk PUFA. This indicated that RPRSO enabled intestinal absorption in the gut and tissue distribution, which includes the udder [32], and also enhanced milk PUFA in early lactation dairy cows. However, this study used a small sample size for dairy cattle (n = 4 per treatment), probably showing the weak point. The impact of this investigation on the digestibility and performance of cows may be greater if the use of RPF was consistent with a large sample size.

The RPRSO supplementation increases OM digestibility. Cows fed RPPO had the highest concentrations of blood cholesterol and triglycerides. Moreover, adding RPRSO increases milk fat concentration and PUFA, particularly LA and ALA, in milk. Therefore, supplementing RPRSO during early lactation enhances feed digestibility and milk fat with PUFA in tropical dairy cows.