Physical and chemical characteristics of milk from goats supplemented with different levels of total digestible nutrients in the dry period

We evaluated the physical and chemical characteristics of raw milk from goats supplemented with levels of Total Digestible Nutrients (TDN) in the dry period. Samples from 20 goats were divided into 4 groups, 3 groups received 400g concentrate per animal with different levels of TDN (65, 75 and 85%), and the control group (without concentrate). Four samples of milk were used for each animal, in four sampling periods, which were subjected to physical and chemical analysis. There was no significant difference between the treatments for the parameters of acidity and fat. The diet without concentrate presented higher value for the cryoscopic index. Mean values of the four treatments were within the range set by the Brazilian legislation for all parameters evaluated. The use of concentrate, containing 85% TDN in goat supplementation, was favored the physical and chemical characteristics of the milk during the dry period.


Introduction
The Brazilian semi-arid region has about 90% of the country's goat flock (Silva et al., 2016), and the breeding of these animals in the northeast region is related to their adaptability to adverse conditions, especially prolonged drought.In this region, the most diverse production systems can be found (Facó et al., 2011), however the supplementation should always be used to meet the nutritional needs of the animals.
The caprine agribusiness is expanding in the world, especially in relation to the milk and its derivatives (Yamazi, Moreira, Cavicchioli, Burin, & Nero, 2013;Gárcia, Rovira, Boutoial, & López, 2014); among the factors for the growth of the consumption of goat milk and derivatives, are their beneficial effects on human health, which are fully recognized by the scientific community (Gárcia et al., 2014) and is an important part of the economy in many countries (Medeiros et al., 2013).
Goat milk has high biological value and nutritional qualities due to its higher digestibility and its dietary characteristics with smaller diameter fat globules.It presents a chemical composition composed of proteins of high biological value and essential fatty acids, besides its mineral and vitamin content (Haenlein, 2004;Park, Juárez, Ramos, & Haenlein, 2007).
The physical and chemical composition of the milk is directly influenced by the diets given to animals, the forage: concentrate ratio directly interferes with the volume of milk produced as well as the concentrations of the components, especially the fat content (Chilliard et al., 2014).The use of concentrate in diets aims to raise the energy and protein concentration of the diet, increasing the total digestible nutrient content of the diet, promoting a greater input of nutrients for milk production, increasing the volume produced as well as increasing the amount of total solids.
In addition to the great social and economic importance of dairy goat farming for the northeastern semi-arid region, since they are a source of subsistence and sustainability, research must be conducted to attest the quality of the milk produced, and thus contribute effectively to the expression of the product in the consumer market and overcome the barriers that impede the advance of the productive chain (Oliveira et al., 2011).
Studies to set the parameters for goat milk produced, as well as to evaluate the influence of nutrition on the production of goat milk mainly in the semi-arid region of the São Francisco River Valley are very scarce, since producers and consumers do not have information if the use of supplementation allows milk produced and even milk without supplementation to comply with current Brazilian legislation.Thus, this study evaluated the physical and chemical quality of raw milk, from goats supplemented with different levels of total digestible nutrient (TDN) in the diet.

Material and methods
The experiment was conducted in the municipality of Santa Maria da Boa Vista, State of Pernambuco, 8°48'S, 39°49'W, at an altitude of 447 m.
Twenty mongrel dairy goats with 45.05 ± 5.08 kg, newly calved, multiparous, were homogeneously distributed in four groups: Control Group composed of goats that had access only to Tifton 85 (Cynodon spp.) pastures, from 8 to 15 hours and after 4 kg/animal of cactus pear (Opuntia fícus-indica Mill.) fresh, crushed, in addition to water and mineral supplementation ad libitum.The animals of the other three groups were subjected to the same management of the Control Group, and received 400 grams of isoprotein concentrate, containing 20% crude protein and varying TDN (total digestible nutrients) content according to the treatment: Group 65% received concentrate containing 65% TDN, meeting the minimum energy requirement of goats in puerperium, as recommended by National Research Council (NRC 2007); Group 75% received concentrate containing 75% TDN; and Group 85% received concentrate formulated with 85% TDN (Table 1).The concentrate corresponded to approximately 0.89% body weight, the estimated forage: concentrate: ratio was 75:25, with total dry matter intake estimated at 1.6kg corresponding to approximately 3.6% body weight.The average milk production was 0.8 kg milk/animal/day.From the 30 th day postpartum, 5 goats from each experimental group were manually milked with preand post-milking asepsis according to the manual of good milk production practices described by Vallin et al. (2009).Milk samples were taken every 15 days, with 4 samples per goat, totaling 20 samples.The collected samples were stored in sterile tubes of 100 mL and frozen for the subsequent physical-chemical analyses.
The determination of titratable acidity and fat concentration (Gerber method) were performed as recommended by Normative Instruction number 68 of the Brazilian legislation published in 2006.The cryoscopic index was analyzed using the Microprocessed Electronic Cryoscope (MK 540 Flex) previously calibrated, where 2.5 mL sample was added to the reading apparatus and its value recorded.

Results
The acidity was influenced by the days of collection, however it was not affected by the treatments (Table 2).Regarding the time of collection, there was a difference between the initial collection and the others.Similar results were found in Queiroga et al. (2007), Sahoo and Walli (2008), Araújo et al. (2009) and Silva et al. (2013) who used different percentages of energy in the diet.Differing from Costa et al. (2008), whose study found no variation for the acidity parameter.The results are in accordance with the recommended by the Brazilian law in the normative instruction number 37 of 2000 that determines that the titratable acidity ranges from 0.11 to 0.18% expressed in lactic acid, which correspond to 11 to 18 ºD.
For the fat parameter, there was no difference between the energy levels tested and also in relation to lactation time (Table 3), corroborating with other studies that evaluated milk fat (Zambom et al., 2005;Costa et al., 2008;Sahoo & Walli, 2008;Araújo et al., 2009).Morgan et al.(2003) obtained from goats reared in France, but according to this same work, breed and breeding conditions can cause a variation in the milk fat content from 5.1 to 3.2%.It is observed that the means of the groups comply with the Brazilian legislation in the normative instruction number 37 of 2000 that determines the minimum content of 2.9% fat in the goat milk.There were differences in the means between treatments with different energy levels for the cryoscopic index, in which milk samples from the control group had the highest mean (Table 4).The means of the treatments are in agreement with the standard established by the Brazilian legislation in the normative instruction 37 of 2000, which recommends a range between -0.550°Hovert to -0.585°Hovert, corroborating with Andrade, Souza, Penna and Ferreira (2008) and differing from the works of Mayer and Fiechter (2012) and Silva et al. (2013).Figure 1 illustrates the correlation between cryoscopy and acidity, in which it was possible to verify an inversely proportional relationship, i.e., as the acidity increases the cryoscopic index decreases.In this study, by means of principal components analysis (Figure 2), it was observed that the milk from goats that received concentrate with 85% TDN had better values for acidity and fat.

Discussion
The inclusion of energy sources, especially starch, can affect ruminal fermentation by increasing release of propionic acid in the rumen, produced by the degradation of starch.This is the precursor of lactose in the mammary gland, which through osmolality affects the amount of milk produced, because the concentration of lactose in milk is very little variable.Thus, the higher level of lactose in the mammary gland increases water drainage for milk production, resulting in increased milk volume (Cannas, Pulina, & Francesconi, 2008).Goetsch et al. (2001) explains that the concentration of lactose in milk can be affected by the phases of lactation, as well as the content of fat and proteins, especially to meet the needs of the kids in the different post-partum phases.
According to the aforementioned, there is a possibility of variation of the data found for the milk acidity, resulting from the possible reduction in lactose concentration in the milk due to the advancement of the lactation curve and that added to the information that this sugar is fermented by microorganisms that transform a lactose molecule into 4 molecules of lactic acid (Kondyli, Svarnas, Samelis, & Katsiari, 2012;Fagnani, Battaglini, Beloti, & Araújo, 2016), which may be one of the possible explanations for the acidity variation along the collections.This is a parameter that is directly linked to the milk storage, so the Brazilian legislation in the normative instruction number 37 as of 2000, accepts that raw frozen milk has a higher acidity variation than raw milk without freezing, thus all samples of the treatments presented acidity (Table 2) within the acceptable range by the aforementioned legislation.
The evaluation of milk acidity is a parameter that associated to others can help in the indirect verification of the microbiological quality and associated to other analyses can be indicative of possible adulterations by water or ammonia (Abrantes, Silva Campêlo, & Silva, 2015).
In this study, the concentration of energy in the concentrate of the treated groups (65, 75 and 85% TDN) (Table 1) had no influence on fat percentage, but Lu, Kawas and Mahgoub (2005) and Morand-Fehr, Fedele, Decandia and Frileux (2007) state that the fat content of goat milk is not related only to the dietary fiber content, but also the dietary fiber effectiveness, besides the energy intake by the goats and the production of volatile fatty acids in the rumen, as mentioned above.It was verified that the supply of concentrate with different TDN contents did not cause changes in the fat content of milk from supplemented goats and neither did the lactation phase or the combination of these factors (Table 3).
The lack of effect of supplementation of goats with concentrate containing different TDN contents can perhaps be explained by the forage: concentrate ratio in the diet, which was 75:25, not enough to affect the milk fat concentration.Morand-Fehr et al. (2007) supplied diets with different proportions of concentrate in the total diet: 40-60% and 60-80% and observed that the proportion representing 40 to 60% of the total diet causes a slight drop in the milk fat concentration, while the contents 60-80% cause an abrupt drop in the milk fat concentration.
The source of energy is another factor that combined with the forage: concentrate ratio may affect milk composition, both in fat content and in milk fatty acid profile (Costa, Queiroga, & Pereira, 2009).
Cryoscopy is a measure relative to the freezing temperature of milk and is influenced by the elements soluble in milk, especially lactose (Abrantes et al., 2015), so it can be inferred that among the main causes for reduction in the cryoscopic index of milk is the availability of energy in the diet of females, in the form of non-fiber carbohydrates, which are the precursors of lactose in milk.
There was an increased production of lactose by the mammary gland and consequent higher milk production in animals supplemented with concentrates, which are generally rich in non-fiber carbohydrates.Due to this fact, there may have been small variations in lactose concentration in the milk % TDN in the diet for dairy goats of the supplemented animals, thus explaining the slight but significant difference (p<0.05) in the milk from goats supplemented with concentrate compared with the milk from goats of the control group, without concentrate (Table 4) (Gonzáles et al., 2004;Abrantes et al., 2015).Vargas et al. (2015) reported that the season may enhance the stress condition of the animals, either by reducing dry matter intake so that the animals cannot reach their daily needs, mainly in energy, or because of the hormonal mechanisms triggered by the stress causing weak immune response to the microorganisms, making them susceptible to diseases, such as mastitis that can also affect the quality and physical and chemical parameters of milk.
The groups that had access to the diet with the highest energy level presented lower values for the cryoscopic index, which may be related to higher total milk solids production caused by the higher energy supply in the form of non-fiber carbohydrates, possibly raising lactose more specifically, which has a direct influence on the depression of the freezing point of milk, as evidenced by the cryoscopic index (Abrantes et al., 2015).
The inverse correlation detected between the variables acidity x cryoscopic index (Figure 1) can be explained stoichiometrically, as previously reported.The microorganisms, when carrying out the fermentation, transform a molecule of lactose into 4 molecules of lactic acid, thus increasing the quantity of dissolved particles and causing a decrease in the cryoscopic index (Fagnani et al., 2016).
According to the principal component analysis, the supplementation of goats with the concentrate containing 85% TDN was responsible for promoting a milk with more suitable physical and chemical characteristics, thus standing out from the other treatments (Figure 2).
The literature states that animals in dry periods have less availability of forage with adequate quality for milk production, and pastures are characterized by plants containing higher content of neutral and acid detergent fibers as well as lignin (Grant, Kreyling, Dienstbach, Beierkuhnlein, & Jentsch, 2014;Zhang, Whish, Bell, & Nan, 2017), resulting in lower intake of crude protein and non-fiber carbohydrates in the diets.Raynal-Ljutovac, Gaborit and Lauret (2005) emphasize that goat supplementation produces better quality milk, which will result in better quality products.As previously explained, the energy from the non-fiber carbohydrates that are degraded in the rumen.The non-fiber carbohydrates become energy source for the growth and microbial proliferation, resulting in the production of propionic acid, which is substrate for the production of lactose and thus can increase the volume of milk, as well as, increase the availability of protein since it will favor the greater contribution of bacteria in the abomasum, and thus increasing the total solids production of the milk.

Conclusion
The use of concentrate containing 85% TDN in goat supplementation favored the physical and chemical characteristics of the milk during the dry period.

Figure 1 .
Figure 1.Correlation between acidity (°D) and cryoscopy (°H) of milk from goats supplemented with concentrate of NDT contents.

Figure 2 .
Figure 2. Principal Component Analysis (PCA) based on the percentage of TDN in the diet for dairy goats.

Table 1 .
Proportion of ingredients in concentrate and chemical composition of ingredients.
*Estimated according to literature data.

Table 2 .
Acidity of milk (ºD) from goats supplemented with concentrate of TDN levels.
a,b different letters in the same row are significantly different (p < 0.05) by Scott-Knott test.NS : non-significant.

Table 3 .
Quantification of milk fat from goats supplemented with concentrate of TDN contents.

Table 4 .
Cryoscopy values of goat milk supplemented with concentrate of TDN contents.
NS -0.566 NS -0.565 NS -0.567 NS -0.566 0.001 a,b different letters in the same row are significantly different (p < 0.05) by the Scott-Knott test.NS : non-significant.