Milk is one of the most produced and most valuable agricultural raw materials in the world. In 2013, with a total production of 770 billion liters valued at 328 billion US dollars, milk ranked third globally in terms of production volume of agricultural raw materials and first in terms of value. Milk contributes 27% to the overall value added of livestock farming and 10% to that of agriculture. The dairy sector relies on natural resources such as land, water, nutrients, and energy. Feeding dairy cows, sheep, goats, and buffaloes requires approximately 1 billion hectares of land, which is 7% of the Earth’s surface. This land primarily consists of grasslands (meadows and pastures). These grasslands provide 77% of forage grass and straw, meaning dairy herds worldwide convert inedible materials for humans into high-quality essential proteins and micronutrients. Therefore, it is necessary to provide them with high-nutritive-value forage. Among the forages meeting these conditions, researchers have selected a grass: Brachiaria sp., which is one of the most important known tropical plants globally. The genus Brachiaria includes approximately 100 recorded species, of which 7 species of African origin have been used for pasture production in South America, Asia, South Pacific, and Australia. It has a high biomass production potential and produces nutritious forage, which increases livestock productivity [2]. Beyond the need to produce food for an ever-growing population, health concerns call for production methods that preserve human health. The lipid content of milk has raised questions about the potential effect of milk fat quality on the frequency of cardiovascular diseases in humans who consume it. Indeed, the composition of milk can be largely influenced by the diet of dairy cows [3]. In ruminants, the plasma concentration of total lipids is approximately between 2.9 and 5.6 g/l [4]. Cholesterol, the fat responsible for cardiovascular diseases, is also found in milk. But its esterified plasma portion remains predominantly associated with HDL (High-Density Lipoproteins) in ruminants, while plasma LDL (Low-Density Lipoprotein) cholesterol has a low concentration [4]. The presence of certain minerals such as sodium (Na) could impact milk production levels [4]. Milk minerals, however, represent a small fraction (8 - 9 g/l) compared to other fractions; this fraction contains calcium, magnesium, sodium, and potassium as the main cations and chloride, inorganic phosphate, and citrate as the main anions. Some forages have a potassium (K) content exceeding the nutritional requirements of ruminants, but the latter have the capacity to tolerate variations in potassium absorption between 3.8 and 20 mmol per kg/day, whereas a value of 40 mmol per kg/day is considered toxic [5]. This study aimed to evaluate the effect of dairy cows consuming Brachiaria on the mineral and cholesterol content of milk. The objective was to evaluate the use of Brachiaria on the composition of milk and serum from dairy cows.

Calcium concentration in milk cows in this trial ranged from 4.49 mmol/L to 4.33 mmol/L, and in serum from 3.7 mmol/L to 4.10 mmol/L. Calcium level in milk of these animals (Holstein crossbreeds on Brachiaria and on natural pasture) approaches the normal level (approximately 120 mg/L) [7]. The others, that is to say the crossbred Montbéliard and Gudali have rates ranging from 160 mg/L to 182,7 mg/L, a rate higher than the normal rate. ANOVA showed a significant difference between treatments (animal groups on different pastures) (P > 0.05); Regarding the media (milk and serum), no significant difference appeared between the media (P < 0.05). The combination between animal groups and media showed that the means presented a significant difference between animal groups and the media from which the element was sampled (P > 0.05). It appears that the calcium level in blood or milk is a trait linked to heritability, that is to say it is a hereditary characteristic that is highlighted by the type of pasture to which the animals are subjected to. In a preliminary study on the genetic parameters of calcium levels, [8] showed that the calcium level in milk presents moderate to high heritability; and much higher than those found by [9]. The difference would also be due to the type of feed received by the cows. Indeed, the ration consisted mainly of grasses (Brachiaria or natural pasture with high floristic diversity) while in the morning during milking or in the evening, cows received a certain quantity of supplement composed either of cottonseed cake, or a mixture of cake, husk, and corn bran.

The phosphorus concentration in milk ranged from 3.31 mmol/L to 4.48 mmol/L, or 102 mg/L to138.8 mg/L for all animal groups. In serum, it ranged from 4.71 to 5.26 mmol/L, or 146 mg/L to 163 mg/L. This level is higher in serum than in milk, although the analyses showed no significant difference. According to [10], the production requirements for dairy cows standardized at 4% fat are 0.90 g for all breeds. The values obtained in this study are similar to those obtained by [8] on Friesian, Normandy and Red Pied (or Holstein) cows. However, the values obtained in this study are lower than those proposed by the NRC. If we consider the NRC values, dairy cows in Vina would require additional phosphorus supplementation. However, according to [9] these values are completely normal. Regarding nitrogen (protein) concentration, the concentrations in milk for Holstein crossbred cows were 3888 mg/L and 5591 mg/L for animals fed on Brachiaria and natural pasture, respectively. Montbéliard crossbred cows, regardless of feed type, had protein concentrations of 7548 mg/L and 7582 mg/L on Brachiaria and natural pasture, respectively. Gudali cows had a concentration of 4297 mg/L. It should be noted that the protein content in milk is higher in animals on natural pasture than those on Brachiaria. If the normal concentration of protein in milk is 32 to 35 g/L, the protein concentration of milk in Vina Division is very low. This means that milking cows should be supplemented with more protein.

In serum of animals from the 5 groups, the concentration of protein was higher in Holstein cows on natural pasture than in any other group. For the remaining 4 groups, the concentrations were almost identical, ranging from 1057 to 1327 mg/L. These levels are higher than those obtained by [10] on exotic and local cows (Boran, Nguni, Tuli, Afrikaner...). However, these levels are lower than those observed in animals in tropical environments like in this study by [11]. Indeed, the latter found concentrations of 64,500 to 89,200 mg/L in the serum of tropical animals (cows). However, he notes that variations in serum protein levels are due either to season or breed...But during the dry season, the levels decrease before starting to rise again at the beginning of the rainy season. We collected our samples during the dry season (specifically in January). The values obtained for both milk and serum could also be explained by the scarcity of forage in the pastures. The protein levels, represented here by nitrogen levels, also vary depending on the breed and type of pasture. However, it should be noted that for exotic breeds, protein levels are generally 30,000 - 35,000 mg/L. Protein concentrations in cows of Vina, regardless of breed and forage type, are therefore very low. This may be due to the fact that the amount of energy or protein provided to the animals through their feed is low and thus does not allow the microbial flora to increase significantly. Moreover, during the period when the samples were taken, the animals were primarily fed straw, which has low nutritional value. Hence the supplementation that is often provided to animals in general and to dairy cows in particular. But most often, for economic reasons, this supplementation is insufficient.

In the serum, the lowest concentrations were those of Montbéliard crossbred fed on natural pasture (1057 mg/L) and Holstein fed on Brachiaria (1091 mg/L). The protein content of Brachiaria in the Vina department is around 8.5%. Nevertheless, the Student’s T-test revealed that there is no significant difference between the nitrogen (protein) levels in animals from the different groups. The fact that animals on Brachiaria have lower protein levels than those on natural pasture can be explained by the floristic and Ecophysiological diversity of forages in natural pastures. Indeed, in natural pastures, one finds both legumes and grasses, and the animal feeding on all these species also benefits from their richness in nutrients. This indicates the need for farms seeking improvement through the selection of high-yield forages for animals to enrich rations with protein ingredients such as oilseed cakes or tropical alfalfa (Stylosantheshamata). However, it should be noted that the Protein Content of milk is largely determined by the cow’s genetics [12]. Indeed, the supply of crude protein in the ration, across a range of different feeding systems, appears to have an inconsistent effect on the protein content (PC) of milk [12]. However, the PC of milk can be reduced when rations are supplemented with UIP (Undegraded intake Protein) that is not well balanced in amino acids (AA) relative to the cow’s requirements [12]. The availability of energy in the rumen also influences the milk PC. Indeed, experiments (11) have shown that the inclusion of corn silage significantly increases milk total protein TP. Also, during experiments [12] during which diets with high starch content were fed to cows, milk production and milk TP increased. The authors of these studies believe that a greater microbial production (Pmi) would be the cause of these results. Also, there are factors such as fat supplementation that can reduce milk TP [12]. Low fat concentrations in cow’s milk are often reported in the case of range-grazing cows; this is attributable to a high intake of linoleic acid which leads to the formation of trans isomers which are associated with the decrease in milk fat [11]. This phenomenon could be amplified by the fact that less mature forages contain a lower NDF rate than mature forages, leading to less pronounced chewing [12].

The sodium content in milk and serum is very high compared to the levels obtained by [13], who reported values of 400 mg/L. In the latter’s study, milk used was either a small batch milk or a large batch milk. The authors were unable to draw a conclusion about the limits within which normal sodium concentration should be judged. Indeed, the sodium content in milk varies from 2.3 to 2.6 g/kg while that in serum varies from 6.8 to 7 g/kg, regardless of the type of pasture on which the animals were raised. These results are higher than those obtained by [9] for the Normande, Friesian, and Red Pie breeds regarding milk. On the other hand, [8] gives milk sodium content equal to 500 mg/L. According to a study by [14], the lower the flow rate of milk, the richer the milk is in ion. This means Gudali the local breed has lower milk ion content than Holstein and Montbeliard crossbreds. The lower the flow rate, the richer the milk is in ions. This means that if the cow produces less milk, its milk is richer in ions. In our study, Gudali milk, which is supposed to produce less milk than exotic breeds (Holstein and Montbéliard), contains less sodium than these breeds, both in serum and in milk. The Student’s T- test showed that there is a significant difference (P < 0.005) between sodium concentrations in milk and serum. These differences could be due the fact that exotic cows receive slightly more salt than Gudali cows. This means, salt provided to cows in the Vina department should be reduced.

Potassium (K) concentrations in milk from animals of all groups and pasture types combined range from 0.08mmol/L to 0.11mmol/L, or 14 to 19.8 mg/L, except for Holstein crossbred on natural pasture where it is 9.74 mmol/L or 1700 mg/L. However, according to [15], the average potassium concentration in cows would be 1 g/l while [8] found a value in milk of 1400mg/L. The value is high in Holsteins crossbred on natural pasture and very low for all animal groups on all other types of pasture. The value for Holstein crossbred on natural pasture is close to that of Red Pie (1.73 g/l or 1730 mg/L) and French Friesian (1.62 g/l or 1620 mg/L) found by [9]. [13] found concentrations ranging from 1433 mg/L to 1760 mg/L, which is the same result as in Holstein crossbred on natural pasture. The hypokalemia observed in the 4 other groups may be due to several causes. Indeed, according to literature, the forage that constitutes the basis of these animals’ diet should provide them with sufficient potassium. But its low level could be explained either by the intake of medications that act as laxatives, or by a low magnesium level in the blood of these animals. Potassium levels as low as these should cause muscle cramps leading to complete cardiac arrest. However, we did not observe anything like this in any of these animals. Another cause of hypokalemia could be stress. Regardless of breed or pasture type concentrations vary from 0.20 to 0.24 g/l, or 200 mg/L to 240 mg/L. The normal potassium level in blood is 1.26 g/l. However, the results obtained in our trial show very low concentrations compared to this normal level. These low concentrations may be due to high concentrations of Ca [9]. It is known that potassium plays an important role in muscle contraction, particularly those of the heart. A deficiency or excess of this element in the blood could therefore cause cardiac disorders in cows, while a deficiency in milk could cause a deficiency in humans if they are not supplemented with potassium. Hence, the supplementation of dairy cows in Vina should take potassium supplementation into account. In serum, it ranges from 0.059 g/l to 0.07 g/l; with the lowest concentration found in crossbred Holstein on Brachiaria. However, the normal value of HDL cholesterol (or good cholesterol) in dairy cow blood ranges between 1.063 and 1.21 g/l [15] and in milk would be 14.5mg/L. Low HDL concentrations contribute to the development of atherosclerosis (with the occurrence of complications such as hypertension, myocardial infarction, and stroke) whose causes can be genetic or secondary (dietary) [16]. Yet it has been demonstrated that the fat content of Bos indicus (Gudali) milk is higher than that of Bos Taurus (Montbéliard and Holstein) [17]. In this trial, there is no significant difference between HDL cholesterol concentrations in all these animals, regardless of pasture type. The results obtained here are much lower than those reported by [4]. Indeed, in the article by [4], the concentration of HDL in the plasma of lactating dairy cows is 3270 mg/L. Indeed, plasma is much more concentrated than serum, which is the liquid portion of the former. The majority of plasma lipids in ruminants is transported in the HDL fraction (d = 1.063 - 1.21 g/ml). Although ruminants are not an exception in the animal world, they differ from humans in whom the majority of lipids are transported in light lipoproteins [17]. The low concentrations observed here could indicate that these animals’ diet is low in fat. Hence the need to review the fat content in the supplement given to dairy cows. The difference could also be due to the medium from which HDL is measured.

The concentration of glucose in milk ranges from 200 mg/L for all breeds to 300 mg/L for Montbéliard crossbred cows on natural pasture. In serum, it ranges from 400 mg/L to 500 mg/L regardless of breed or forage type. According to a study by [18], the normal concentration in cow’s milk 4 weeks after calving would oscillate around 3 mmol/L and less than 3.5 mmol/L, or approximately 540 mg/L. This shows that glucose level is very low in the analyzed milk and serum samples. Part of the explanation would come from the fact that the amount of forage provided or gleaned by the animals is insufficient while production is high. Therefore, forage quantities should be increased or the amount of supplements should be increased, especially since the effect of prolonged hypoglycemia could have consequences such as the cessation of milk production [18].

In conclusion, we can say that there is no major difference in the composition of milk or serum from animals that consumed Brachiaria or natural pasture. Therefore, Brachiaria can safely substitute natural pasture with the advantage that it produces more biomass. But in both cases, the diet should be supplemented with certain minerals like K. Na should be reduced. The low HDL concentration may indicate that the animals are accumulating more LDL. And the glucose level in the sera and milk clearly shows that the amount of energy ingested and produced by these animals is lower than their needs. Hence the need to increase energy sources in the diet of dairy cattle in Vina Division Cameroon.