nutrient digestibility, rumen fermentation parameters, and production performance in response to dietary grain source and oil supplement of holstein dairy cows

introduction high-producing dairy cows require large amounts of concentrates that are rich in energy and crude protein to meet their nutrient requirements. cereal grains and oil supplements are commonly used for increasing energy density of diets fed to high-producing dairy cows. dietary grain source (barley vs. corn) and oil supplement (soybean- vs. fish oil) resulted in varied dry matter intake and milk production responses in different research studies based on effects on nutrient digestibility and rumen fermentation characteristics. therefore, the main purpose of this study was to determine the effects of, and interactions between, grain source and oil supplement on the feed intake, rumen fermentation characteristics, nutrient digestibility and lactational performance of holstein cows. materials and methods eight lactating multiparous holstein cows (parity = 3.3 ± 1.3 and days in milk = 77 ± 22.1; mean ± sd), were used in a replicated 4 × 4 latin square design with 25-d periods. each experimental period consisted of an 18-d diet adaptation period and a 7-d collection period. cows within a square were assigned randomly to dietary treatments. cows were blocked into 2 squares of 4 cows each based upon milk production, and days in milk, and within blocks were assigned to 1 of the 4 experimental diets with a 2 × 2 factorial arrangement: 1) bf = barley-based diet supplemented with fish oil at 2% of dietary dm, 2) bs = barley-based diet supplemented with soybean oil at 2% of dietary dm, 3) cf = corn-based diet supplemented with fish oil at 2% of dietary dm, and 4) cs = corn-based diet supplemented with soybean oil at 2% of dietary dm. the tmr amounts offered and refused were measured daily for each cow and dmi determined daily for each cow. cows were milked three times daily at 0200, 1000, and 1800 h in a herringbone milking parlor. milk yield for all cows was recorded and sampled at each milking during the last 7 d of each period. milk samples were composited in proportion to milk yield, preserved with potassium dichromate, stored at 4°c, and analyzed for fat, protein, lactose, and total solids using an infrared analyzer (milkoscan 134 bn; foss electric, hillerød, denmark). at the end of each experimental period, rumen samples were obtained at 4 h after the morning feeding using the stomach tube technique. rumen ph was determined immediately after the samples were collected using a mobile ph meter (hi 8314 membrane ph meter, hanna instruments, villafranca, italy). rumen fluid samples were acidified by sulfuric acid and analyzed for volatile fatty acid by gas chromatography (model no. cp-9002 vulcanusweg 259 a.m., chrompack, delft, the netherlands). two fecal grab samples per cow were taken from the rectum twice daily across day 19 to 23 of each period and frozen at −20°c until analyzed. acid detergent insoluble ash was used as an internal marker to determine apparent total-tract nutrient digestibility. data were composited within period and analyzed with the mixed model procedure of sas (sas institute, 2003) to account for effects of square, period within square, cow within square, treatments (grain source and oil supplement), and the interaction between grain source (barley vs. corn) and oil supplement (fish oil vs. soybean oil). results and discussion apparent total-tract digestibility of dry matter (p = 0.05) and ether extract (p < 0.01) were greater in the corn- vs. barley-based diets. fish oil tended (p = 0.07) to decrease and decreased (p = 0.03) apparent total-tract digestibility of non-fibrous carbohydrate and ether extract as compared to soybean oil, respectively. an interaction of main treatment effects tended to occur for molar concentration of propionate (p = 0.09). barley-based diets increased molar concentration of propionate compared to corn-based diets for cows fed soybean oil, but not for cows fed fish oil. dry matter intake tended (p = 0.09) to be greater for barley- vs. corn-based diets, but was reduced for the fish oil compared to soybean oil supplemented diets (p < 0.01). grain source did not affect milk yield or milk composition. compared to soybean oil, fish oil negatively affected milk yield and milk composition. feed efficiency remained unchanged among treatments. conclusion results indicated that grain source and oil supplement do not interact to affect productive performance of lactating cows. due to lowering dm intake, feeding fish- vs. soybean-oil, but not changing diets fermentability, did not influence production performance of lactating cows.