Nutritive evaluation of wastes of brassica vegetables and other agroindustrial by-products for sheep feeding

Resumen

The fruit and vegetable sector in Spain constitutes an important activity, as our country is the leading European producer and the sixth worldwide. According to Food and Agriculture Organization of the United Nations estimations, about half of the initial production of vegetables and fruits is lost or wasted at different stages of the food chain supply (cultivation, harvest, distribution, marketing and consumption). Besides that, the new presentation formats of vegetables (ready to eat, bagged, canned, frozen, lyophilized, …) that are progressively present in the market contribute to increase the amount of wastes due to the discarded fractions (i.e. leaves, stems) generated in these marketing processes. Both wastes and discarded vegetables may be used in animal feeding, but the knowledge on their nutritional value is still limited in some cases. In addition, consumers increasingly demand safe and healthy animal products produced with minimal environmental impact, and the use of vegetable wastes and by-products in livestock feeding can contribute to this demand. The general objective of this Doctoral Thesis is to obtain information on the nutritional value for sheep of wastes from different Brassica vegetables and the effects of including some agroindustrial by-products (dehydrated citrus pulp (DCP), corn distillers dried grains with solubles (DDGS) and exhausted olive cake (EOC)) in the diet of fattening light lambs. Three in vitro experiments (Experiments 1, 2 and 3) were conducted to assess the nutritive value of wastes from Brassica vegetables, and the effects of the selected agroindustrial by-products on growth performance and meat quality of fattening light lambs were analyzed in Experiment 4. The first objective of this Doctoral Thesis was to analyze the variability in chemical composition, in vitro ruminal fermentation and in vitro intestinal digestibility of discarded pieces and different fractions of Brassica vegetables produced in Spain. A total of 12 different samples (3 replicates per each) were analyzed: four leave cabbages (Brussels sprouts (BS), white cabbage, savoy cabbage, and red cabbage), three fractions (leaves, stems and florets) of both cauliflower (CAU) and romanesco (ROM), and two fractions (stems and florets) of broccoli (BRO). In vitro incubations were carried out by using batch cultures of ruminal microorganisms from sheep to measure the gas production kinetics (144 h incubations) and the main ruminal fermentation parameters (24 h incubations). One sample of each barley grains, sugar beet pulp and wheat DDGS was included to serve as reference. All samples had low dry matter (DM) content, which ranged from 5.64 for white cabbage to 16.3 % for BS. The DM was rich in crude protein (CP; 15.0 – 30.0 %) and sugars (16.3 – 41.4 %), had medium values of neutral detergent fiber (NDF; 17.5 - 32.3 %), and low lignin content (< 4.1 %). Within the group of leave cabbages, BS had the greatest DM content (16.3 %), as well as the lowest NDF (17.4 % of DM) and the greatest sugars levels (41.4 % DM). For CAU and ROM, the leaves contained more (P < 0.05) NDF and less (P < 0.05) sugars than stems and florets. Compared to florets, the stems had lower CP (P < 0.05; 22.2 vs. 29.6 % of DM; values averaged for CAU, ROM and BRO) and greater sugars content (P < 0.05; 20.6 vs. 30.1 % of DM). All samples were rapidly and extensively fermented in vitro by ruminal microorganisms. Within the leave vegetables, BS was the most extensively fermented. Stems and florets of CAU and ROM were more quickly fermented than leaves, but there were only minor differences in fermentation profiles of both vegetables. In general, stems and florets had similar fermentation patterns. With the exception of CAU and ROM leaves, all tested samples had DM effective degradability values similar or even greater to that of barley grains. The 12-h in situ degradability of CP was high for all samples, and ranged from 80.9 % (ROM florets) to 95.1 % for (BRO florets) indicating a low by-pass CP fraction. The in vitro intestinal digestibility of CP ranged from 61.4 % for red cabbage to 93.2 % for ROM florets. The estimated metabolizable energy (ME) content varied from 9.4 (CAU leaves) to 11.8 (ROM florets) MJ / kg DM. The ME content in BS, white cabbage, and CAU and ROM leaves was similar to that for sugar beet pulp and wheat DDGS, whereas all florets had ME content close to that of barley grains. Altogether, these results indicate that Brassica vegetables can be a good source of energy for ruminants, but their high moisture content could limit their use in practical feeding. The objective of Experiment 2 was to assess the in vitro fermentation of diets containing increasing amounts of dried BS, CAU or BRO. A mixed diet composed of 40 % alfalfa hay and 60 % of a high-cereal concentrate was used as control, and all experimental diets were formulated to have similar CP and NDF content. Each vegetable was included at 0, 8, 16 and 24 % of the concentrate (4.8, 9.6 and 14.4 % of total diet) replacing cereals and protein feeds. In addition, the in situ ruminal degradation kinetics of DM and N was determined for the diets containing BS and BRO. A similar response was observed for the three vegetables, and increasing their inclusion level in the diet augmented both potential gas production and total volatile fatty acid (VFA) production, indicating that the diets including BS, CAU or BRO were more extensively fermented than the control diet. There were differences among vegetables in their effects on VFA profile, as the inclusion of BS in the diet resulted in greater (P < 0.05) proportions of propionate and less (P < 0.05) of butyrate compared with the control diet, whereas CAU and BRO increased (P < 0.05) molar proportions of acetate and decreased (P < 0.05) those of propionate and butyrate. Concentrations of NH3-N after 24 h of in vitro fermentation were only increased for BRO diets, which is in accordance with the greater rumen degradability of BRO-CP observed in Experiment 1. No differences (P > 0.05) among diets were detected in CH4 production, indicating a lack of antimethanogenic compounds in the studied vegetables. The measurement of the in situ degradability of BS and BRO diets confirmed the in vitro results. The inclusion of increasing amounts of BS or BRO in the diets resulted in significantly augmented values of the soluble fraction (a) of both DM and CP, and decreased the non-soluble but potentially degradable fraction (b), with the total potentially degradable fraction (a + b) remaining unchanged. The results from Experiments 1 and 2 are included in the Scientific papers 1, 2 and 3 of this Doctoral Thesis. The objective of Experiment 3 (Scientific paper 6) was to assess the effects of replacing 14.8 % of conventional feed ingredients in a mixed diet for dairy sheep (control diet) with dried discarded pieces of either BS (BS15 diet) or BRO (BRO15 diet) on ruminal fermentation and microbial protein synthesis (MPS) in Rusitec fermenters. All experimental diets were composed of alfalfa hay and concentrate in 1:1 ratio, and were formulated to have similar CP and NDF content. Six Rusitec fermenters received the diets (30 g DM / d) in two 14-d incubation periods. The MPS was determined using 15N as a microbial marker. Fermenters fed the BRO15 diet had greater pH values (P < 0.05; 6.19) than those receiving the control diet (6.10), but similar to BS15-fermenters (6.15). There were no differences among diets in total VFA production, but VFA profile was markedly affected. Compared with the fermenters fed the control diet, those fed the BS15 and BRO15 diets had greater (P < 0.05) molar proportions of acetate and valerate and lower (P < 0.05) of propionate, resulting in greater (P < 0.05) acetate:propionate ratios for BS15 and BRO15 diets. The BRO15 diet promoted the greatest (P < 0.05) NH3–N concentrations, which was in accordance with the greatest (P < 0.05) CP degradability observed for this diet (0.616, 0.639 and 0.673 g / g for control, BS15 and BRO15, respectively). Digestibility of DM was unaffected by the diet (P = 0.109), but BS15 showed greater (P < 0.05) NDF digestibility than the control diet. Neither daily CH4 production nor the CH4 / total VFA ratio differed among diets (P ≥ 0.134), confirming the lack of antimethanogenic compounds in the tested vegetables observed in Experiment 2. There were no differences (P = 0.312) among diets in MPS in the solid phase of fermenters, but BRO15 promoted greater (P < 0.05) MPS in the liquid phase compared with control and BS15 diets. Efficiency of microbial growth ranged from 29.4 to 32.2 g microbial N / kg organic matter fermented, and did not differ (P = 0.253) among diets. Altogether, the results indicate that replacing 14.8 % of highly degradable conventional feed ingredients in mixed diets with dried BS or BRO can increase the degradability of some dietary fractions without reducing microbial growth in the rumen. These results encourage further investigations to assess the influence of both vegetables on ruminant performance, animal health and products quality. Finally, the objective of Experiment 4 (Scientific papers 4 and 5) was to evaluate the effect of replacing cereals and protein concentrates in a high-cereal concentrate (control) for fattening light lambs with DCP (18 %), corn DDGS (18 %), and EOC (8 %) in a BYP concentrate on growth performance, digestibility, ruminal fermentation, plasma metabolites, and meat characteristics and fatty acid (FA) profile. Two homogeneous groups of Lacaune lambs (13.8  0.25 kg) were fed one of each concentrates and barley straw ad libitum until reaching about 26 kg body weight. There were no differences between groups on feed intake, average daily gain, or feed conversion ratio, but control diet had greater (P < 0.001) DM digestibility. The type of concentrate had no effect on post-mortem ruminal pH and total VFA concentrations and profile, but NH3-N concentrations were lower (P = 0.003) for the BYP-fed lambs compared with the control ones. However, plasma concentrations of amino acids, total proteins, urea and hepatic enzymes were not affected by the concentrate. In addition, there were no differences (P ≥ 0.130) between groups in the pH, chemical composition, color, and texture parameters in the Longissimus dorsi. Feeding the BYP concentrate reduced the concentration of thiobarbituric acid reactive substances (TBARS) in the meat after 6 days of refrigerated storage (unmodified atmosphere), probably due to the greater polyphenol content in this concentrate. Compared with control lambs, the meat and the subcutaneous fat from BYP-fed lambs had lower saturated and greater polyunsaturated FA content as well as greater n-6/n-3 FA. In conclusion, replacing 44 % of feed ingredients in the concentrate for fattening light lambs with a blend of DCP, corn DDGS and EOC can improve the antioxidant status and FA profile of meat without affecting negatively growing performance, animal health or meat composition.