In this study the effects of supplemental crystalline niacin on sow reproduction and sow and litter performance were evaluated using 240 litters produced by 67 sows. A basal 12.80% CP corn-soybean meal-oat diet was supplemented with 0 or 33 mg of crystalline niacin/kg during gestation and lactation. The sows were grouped to equalize initial BW and parity among the two treatment groups. Each sow was fed 2.0 kg/d during gestation and 1.8 kg plus .45 kg per nursing pig during lactation. Sows not culled due to reproductive or structural problems remained on the experiment for five parities (average 3.6). Supplemental niacin did not improve (P |is greater than~. 20) any of the sow reproductive or sow and litter performance variables evaluated, including the subjective scores for sow lameness, toe cracks, and hair and skin condition. In conclusion, these results suggest that a 12.80% CP corn-soybean meal-oat diet provides adequate niacin during gestation and lactation without supplementation with crystalline niacin. Introduction Niacin, or nicotinic acid, is required for the synthesis of nicotinamide-adenine dinucleotide (NAD) and NAD phosphate, two coenzymes essential for the metabolism of proteins, carbohydrates, and lipids (White, 1982; Henderson, 1983). Young pigs require a dietary source of niacin when fed diets deficient in tryptophan (Luecke et al., 1947, 1948; Powick et al. 1948; Firth and Johnson, 1956). However, a niacin deficiency is unlikely in growing-finishing and adult pigs (Braude et al., 1946; Luecke et al., 1948; Yen et al., 1978) because of the metabolic conversion of excess tryptophan to niacin (Firth and Johnson, 1956) and(or) the synthesis of B-complex vitamins by bacteria in the intestinal tract (March, 1979). Crystalline niacin is usually added to gestation and lactation diets for sows because the niacin requirement is unknown (NRC, 1988). The addition of niacin to sow diets produced inconclusive effects on sow and litter performance in one experiment (Goodband et al., 1987). This experiment was conducted to evaluate the effects of adding niacin to a corn-soybean meal-oat diet fed to sows during gestation and lactation. Sow reproductive performance, toe cracks, leg soundness, hair and skin condition, longevity, and pig performance to weaning were response criteria. Materials and Methods Sixty-seven Yorkshire x Landrace sows were signed to one of two treatments at breeding based on BW (average 204 kg) and initial parity (average 2.6 farrowings) and remained on the same dietary treatment through five gestation-lactation cycles (hereafter called periods) unless culled for structural, digestive, or reproductive problems. Two treatments were made by adding 0 or 33 mg of crystalline niacin to a basal 12.80% CP corn-soybean meal-oat diet (Table 1). Sows were fed 2.0 kg of their respective diet per day during gestation and 1.8 kg plus .45 kg for each nursing pig per day during lactation. Before this experiment all the sows were fed a 14% CP corn-soybean meal-oat diet containing 33 mg of added niacin/kg. The basal diet was analyzed for total niacin activity, proximate composition, and NDF and ADF by AOAC (1984) and for amino acids by the procedure described by Benson and Patterson (1971), with duplicate analyses on four samples (Table 1). Before amino acid analyses by automated cation-exchange chromatography, samples were hydrolyzed under N with 6 N HCl for 4 h at 145|degrees~C, except for tryptophan, which was hydrolyzed enzymatically and determined colorimetrically (Spies and Chambers, 1949). When analyzing for cystine and methionine, performic acid oxidation preceded the hydrolysis step (Moore, 1963). Total niacin activity was measured using an automatic turbidimetric microbiological system (Autoturb-2, Mitchum Schaefer, Indianapolis, IN) based on the fact that Lactobacillus plantarum ATCC 8014 requires niacin for growth. A basal medium, nutritionally complete except for niacin, was used as the diluent for the final dilutions of the samples and standards. After incubation, the growth response of the bacterial cultures was measured as percentage of transmittance. Then a dose-response line was constructed and the sample concentrations of total niacin were calculated. Table 1. Composition of air-dry basal diet fed during gestation and lactation In gredients % Corn 70.80 Soybean meal (44% CP ) 12.70 Oats 10.00 Lard 2.5 0 Dicalcium phosphate 2.40 Limestone .60 Mineral pr emix (a) .50 Vitamin premix (b)(c) .50 Analyzed composition ( d) Total niacin, ppm (e) 23.0 CP, % 12.80 Lysine, % .58 Tryptophan, % .12 Methionine + cystine, % .50 Threonine, % .48 Crude fiber, % 3.10 NDF Cell wall, % 15.70 Ash insoluble, % .50 ADF, % 4.90 Crude fat, % 5.00 Water, % 14.00 a Mineral premix provided per kilogram of diet: 4.4 g of NaCl, 125 mg of Zn as ZnC|O.sub.3~, 125 mg of Fe as FeS|O.sub.4~, 15 mg of Cu as CuO, 25 mg of Mn as MnO, .5 mg of I as Ca(I||O.sub.3~).sub.2~, and .15 mg of Se as |Na.s ub.2~Se|O.sub.3~. b Basal vitamin premix provided per kilogram of diet: 5,500 IU of vitamin A acetate, 550 IU of vitamin |D.sub.3~, 27.5 IU of vitamin E as dl-| alpha~-tocopheryl acetate, 2.2 mg of menadione as menadione sodium bisulfite com plex, 5.5 mg of riboflavin, 27.5 mg of d-pantothenic acid as calcium pantothenat e, 0 mg of niacin, 38.5 |mu~g of vitamin |B.sub.12~, 770 mg of choline chloride, 1.1 mg of folic acid, 2.2 mg of pyridoxine as pyridoxine HCl, 2.2 mg of thiamin as thiamin mononitrate, .17 mg of d-biotin, and 55 mg of ethoxyquin as a preser vative. c Vitamin premix with niacin supplied the same concentrations of vitamin s as the basal vitamin premix plus the addition of 33 mg of niacin per kilogram of diet. d Average of four samples analyzed. Calculated values for calcium and t otal phosphorus were .91 and .76%, respectively. e Calculated (NRC, 1988) total niacin value of 21 ppm in basal diet without added niacin. Diet supplemented wit h 33 ppm of niacin had an average analyzed total niacin value of 59 ppm. Sows were housed in fully enclosed gestation and farrowing buildings that were heated and ventilated. From weaning until d 107 of gestation, the sows were in individual gestation stalls (2.1 m x .6 m) that had 1.0 m of solid concrete in front and 1.1 m of slatted floor (12.7-cm concrete slat and 2.5-cm slot) in the back. On d 107 of gestation, the sows were moved to the farrowing building and placed in farrowing crates (2.1 m x .6 m) that had an area for newborn pigs (2.1 m x .6 m) on each side of the stall. The floor was completely slatted (12.7-cm slat and 1.3-cm slot except for a 3.8-cm slot behind the sow). At birth, the pigs were weighed, ear-notched, tail-docked, injected with iron dextran (100 mg of Fe), and needle teeth were clipped. The baby pigs had access to a rubber mat (1.0 m x .4 m) below an electric radiant heater on one side of the farrowing crate. Criteria were sow weights at breeding, weight gain during gestation, weight loss during lactation, interval from weaning to first estrus, culling rate and reasons for culling, pig weights at birth and weaning, and the number of pigs per litter born alive, born dead, and weaned. Also, at breeding and d 60 of gestation each sow was subjectively evaluated by two people using three separate criteria: hair and skin condition, lameness, and incidence of toe cracks. Each area had a 3-point scoring system, where a score of 1 represented no defects (i.e., normal appearing skin and hair, no lameness, or no toe cracks). For hair and skin condition, a score of 2 represented a mild dermatitis or rough haircoat, and a score of 3 represented a severe dermatitis or an extremely rough haircoat. Soundness scores of 2 and 3 represented slight to mild lameness and extreme lameness (inability to walk), respectively. In the assessment of toe cracks, a score of 2 represented one or two toe cracks, and a score of 3 represented three or more toe cracks. A score of 3 occurred so infrequently for each area evaluated that the scores of 2 and 3 were combined for statistical analysis. Interval to first estrus was for all sows that exhibited estrus within 20 d of weaning. Sows were bred at first estrus by artificial insemination, using semen pooled from two or more boars. Sows were inseminated once daily while in estrus, usually for three consecutive days. Sows that did not exhibit estrus within 60 d of weaning were culled. Sow and litter data were analyzed as a repeated measure experiment, in which period represented the number of gestation-lactation cycles each sow completed during the experiment (Gill and Hafs, 1971). The statistical model contained treatment (0 or 33 mg of added niacin), sow within treatment, period, and the treatment x period interaction. The overall treatment effects were analyzed using sow within treatment as the error term. Discrete variables, such as the numbers of pigs born alive, born dead, and weaned, and the interval from weaning to first estrus were transformed using an |(x + 1).sup..5~ transformation to stabilize the variances, although actual data are reported. All treatment comparisons of performance variables were made, as described for a split-plot design (Snedecor and Cochran, 1980), using Fisher's F-protected LSD test. Subjective evaluation scores, culling rate, and reasons for culling were analyzed as a linear model For categorical data as described by Grizzle et al. (19 69). Treatment differences among the frequencies of subjective scores of 1 and 2 (scores of 2 and 3 combined), culling rate, and reasons for culling were determined using the CATMOD|R~ procedure of SAS (1986). The model statement used to determine treatment and period differences contained treatment, period, and treatment x period. Overall treatment differences were determined using only treatment in the model statement. Results and Discussion The analyzed value for total niacin in the basal diet without supplemental niacin averaged 23 mg/kg, similar to the calculated value of 21 mg/kg (NRC, 1988). The analyzed value for total niacin in the diet supplemented with 33 mg/kg averaged 59 mg/kg. There were no treatment x period (gestation-lactation cycle) interactions (P|is greater than~ .20) for any of the variables evaluated in this experiment; thus, the main effects of niacin are reported. Supplemental niacin (33 mg/kg) did not (P|is greater than~ .20) Improve any of the sow reproductive or sow and litter performance criteria measured compared with the basal diet without added niacin (Tables 2 and 3). Biotin, another B vitamin that functions as a coenzyme, is also required for normal hair, skin, and feet (NRC, 1988). The addition of biotin to sow diets (Bryant et al. 1985a,b) had no effect on sow or litter performance through four parities, similar to the results obtained with niacin in the present experiment. However, biotin did reduce the incidence of toe lesions and shortened the interval from weaning to estrus, which did not occur with the addition of niacin in the present experiment. Thus, our results indicate that the sows were able to obtain sufficient niacin from the basal diet fed in this experiment. Niacin was provided by the dietary ingredients (corn, soybean meal, and oats) and from the conversion of excess tryptophan to niacin. Niacin in corn is largely unavailable to the pig (Luce et al. 1967). The chick, however, is able to utilize 30% of the niacin in corn and 100% of the niacin in soybean meal (Yen et al., 1977). The availabilities of niacin in oats and soybean meal have not been determined for the pig. TABULAR DATA OMITTED If we assume that the availabilities of niacin in corn and soybean meal are similar for pigs and chicks, and that the availability of niacin in oats is the same as that for corn, our basal diet would provide approximately 9 mg of available niacin/kg, using NRC (1988) values for niacin in feeds. During gestation our sows were fed 2.0 kg of diet daily. This amount of basal diet (no niacin added) provided an estimated 18 mg of available niacin/d (9 mg of available niacin/kg times 2.0 kg of diet/d), close to the NRC (1988) requirement of 19 mg/d. The 2.0 kg of basal diet also provided 2.4 g of tryptophan, .7 g in excess of the requirement (NRC, 1988). Assuming that the efficiency of converting tryptophan to niacin is the same for sows as for young pigs (50 mg of tryptophan required to synthesize 1 mg of niacin; Luecke et al., 1947; Firth and Johnson, 1956), this excess tryptophan would yield an additional 14 mg of niacin/d. Thus, the estimated available niacin in the diet plus that which could be synthesized from excess tryptophan totals 32 mg/d, 1.7-times the NRC (1988) gestation requirement. Using the same approach, the diet supplemented with crystalline niacin would provide an estimated 98 mg of available niacin/d (2.0 kg times 33 mg of added niacin/kg pins 32 mg/d from the basal diet). During lactation, feed consumption would have averaged 5.85 kg/d when the sow was nursing nine pigs. This amount of diet would provide 7.02 g of tryptophan/d, which is .72 g more than the tryptophan requirement for lactating sows determined by Lewis and Speer (1974). This excess tryptophan could be converted to 14 mg of niacin/d. The estimated available niacin provided by the basal diet is approximately 53 mg/d (9 mg of available niacin/kg times 5.85 kg of diet/d), which meets the NRC (1988) lactation requirement of 53 mg. Thus, any niacin converted from tryptophan would be in excess of the niacin requirement. The diet supplemented with 33 mg of crystalline niacin would provide an estimated 260 mg of available niacin/d (5.85 kg times 33 mg of added niacin/kg plus 67 mg/d from the basal diet). The lack of response to supplemental niacin indicates that sows are able to meet their daily niacin requirement when fed a practical, 12.80% CP diet during gestation and lactation without supplementation with crystalline niacin. When niacin synthesis from excess tryptophan is considered in calculating total daily niacin intake, the recommended (NRC, 1988) available niacin intakes for sows during gestation and lactation seem to be adequate. Manoukas et al. (1968) also found that the available niacin in practical diets for laying and breeding hens was greater than the minimum requirement, concluding that supplementation with crystalline niacin was unnecessary. Implications A 12.80% CP corn-soybean meal-oat diet fed to sows during gestation and lactation does not require supplemental niacin. 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