The oxidative stability of milk and the derived dairy products plays a major role in the dairy industry. Milk lipid oxidation is highly influenced by long chain unsaturated fatty acids, which are particularly susceptible to this biological process and can give rise to the development of off-flavor that leads to milk quality deterioration [1]. The composition of the milk is affected by several factors, such as breed, management, stage of lactation, health status and nutrition. Diet components and certain feeding regimes provided to the dairy animals can therefore increase the content of polyunsaturated lipids of milk and its susceptibility to oxidation [2] or offer additional protection against oxidation procedures [3,4].

      Among the factors that affect milk composition is the forage type [5,6]. As already shown, milk from cows fed fresh green forage had a much higher unsaturated: saturated Fatty Acids (FA) proportion, with increased levels of polyunsaturated FA and Conjugated Linoleic Acid (CLA) (in particular C18:2 cis-9, trans-11), compared to the milk produced by silage-fed cows [7]. Moreover, according to a previous study [8], molar proportions of acetate, butyrate and valerate decreased linearly, whereas that of propionate and 2-methyl butyrate increased linearly with the increase of wheat straw levels as a replacer of alfalfa hay in the diet of cows.

      Part-time grazing with indoor supplementation is the predominant, most frequently used, sheep flock management system during the main period of lactation in Greece [9]. The two main sources of supplemental forage used in the diets of small ruminants are alfalfa hay and wheat straw. However, recent literature is mainly focused on the effects of concentrate/ forage ratios and little information is available on the effect of the nature of forage on milk composition in dairy animals others than dairy cows. The aim of the present study was therefore to examine the impact of partial replacement of alfalfa hay by wheat straw without changing concentrate/forage ratio on milk yield, composition and oxidative stability in sheep.

      Eighteen 3-year-old Karagouniko dairy ewes at the mid lactation (100–105th day after parturition) and at their second parity were used in the present experiment. Animals were fed with alfalfa hay and concentrates, were housed at the premises of the Agricultural University of Athens and handled according to the Ethical Committee guidelines of the Faculty of Animal Science and Aquaculture. The ewes were allocated into two homogeneous sub-groups (n = 9) based on their mean body weight (46.3 ± 2.4 kg) and milk yield (1.11 ± 0.15 L). Each ewe of control and straw groups was individually fed with forage (100% alfalfa hayor alfalfa hay: 35% wheat straw, respectively) and concentrates (Table 1) for a period of 28 days. The quantities of feed provided to the animals were adjusted according to their individual requirements, based on their body weight and milk yield. All animals had ad libitum access to fresh water. Diet selectivity did not occur, and no refusals of forage and/or concentrates were observed. Individual samples from control and straw groups were collected at the beginning of the experiment. These samples were analyzed for organic matter (OM; Official Method 7.009), dry matter (DM; Official Method 7.007) crude protein (CP; Official Method 7.016) and ether extract (EE; Official Method 7.060), according to the Association of Official Analytical Chemists International [10]. Composition and analysis of the concentrates (Viozokat S. A., Katerini, Greece) are presented in Table 1.

Table 1

Table 1: Composition and analysis of concentrates, alfalfa hay and wheat straw.

      Animals were milked twice a day at 6 am and 6 pm by a milking machine. Individual milk samples were collected at 0, 7, 14, 21 and 28 days of the experiment for chemical composition and oxidative stability analyses. Milk sample was obtained after mixing the volume of milk collected during the morning and evening milking and was analyzed for fat, protein and lactose by IR spectrometry (Milkoscan 133; Foss Electric, Hllerod, Demark), after calibration according to Gerber [11] and Kjeldahl [12]. Oxidative stability of ewe milk was evaluated by measuring the levels of Malondialdehyde (MDA), a secondary lipid oxidation product formed by hydrolysis of lipid hydroperoxides. MDA concentration was determined by applying a selective thirdorder derivative spectrophotometric method, previously developed by Botsoglou et al [13].

      Data were subjected to ANOVA with the nutritional treatment as the fixed effect. Ewe was considered as the experimental unit. The level of significance was set at 0.05. Results are presented as least square means ± SEM. All analyses were performed by SAS/STAT Version 9.1.3 (2011).

      As illustrated in Table 2, partial replacement of alfalfa hay by wheat straw (35%) did not have a significant effect on milk yield and composition (fat, protein, lactose) during the experimental period. No effect on milk yield, fat, protein and lactose content was also found in cows after the partial replacement (30%) of alfalfa hay by wheat straw [8]. On the other hand, MDA content of milk was decreased after 14 days of wheat straw supplementation indicating an improvement in milk oxidative stability (Table 2). Milk MDA values remained lower in S compared to C group till the end of the experiment. However, milk malondialdehyde levels of S group were similar on days 14, 21 and 28. A possible explanation for the improved milk oxidative stability is that ewes fed with the wheat straw produced milk with lower levels of polyunsaturated fatty acids. This hypothesis is supported by the fact that alfalfa hay contains higher levels of polyunsaturated and lower levels of saturated fatty acids compared to wheat straw (Tsiplakou, personal communication).

Table 2

Table 2: Effect of forage (100% alfalfa hay - C or 65% alfalfa hay: 35% wheat straw – S) on milk yield (L), composition (fat, protein, lactose - %) and MDA content (ng mL^-1).

      The significant effect of forage type on milk oxidative stability has already been shown in previous studies. Increased levels of lipid hydroperoxides in milk were observed from cows fed grass-clover silage compared to that fed meadow hay, possibly due to the significantly higher concentration of linolenic acid [14,15]. Singlet oxygen attacks the double bonds of the unsaturated fatty acids that are responsible for the amount of lipid hydroperoxides [16]. Moreover, diets based on red clover and alfalfa silages are susceptible to a rapid loss of α-tocopherol and increased production of oxidation products, such as Thiobarbituric Acid Reactive Substances (TBARS) during the storage of cow milk in comparison with the milk produced by cows fed diets based on grass silage [17]. Finally, water-soluble Total Antioxidant Capacity (TAC) of milk appears to increase when the ewes consume fresh grass mainly through grazing instead of alfalfa hay [18].

      Partial replacement of alfalfa hay by wheat straw improves milk oxidative stability without negative effects on milk yield and composition in sheep. This effect was evident after two weeks of wheat straw dietary inclusion and is possibly associated with the lower concentration of polyunsaturated fatty acids in wheat straw than alfalfa hay.

      The authors would like to thank the Department of Nutritional Physiology and Feeding in Agricultural University of Athens and especially Mrs M. Georgiadou for forage analyses and the Department of Dairy Research in Agricultural University of Athens for milk composition analyses.