|Abstract:||With increasing feed prices and decreasing profit margins, livestock producers are constantly searching for ways to increase feed nutritive value and consequently improve production per unit of feedstuff. Research on the use of foliar fungicide has shown to increase corn grain yield in recent years. Fungicides can increase forage quality and by changing corn composition and nutritive value. However, little is known on what effects this may have on quality of corn ensiled and production when the silage is fed to cattle. Thus, the objectives of the present study were to explore the associations of corn plant foliar fungicide application on harvested whole plant corn silage quality, aerobic stability, digestibility, and cow performance. Treatments were control (CON), corn received no foliar fungicide application; treatment 1 (1X), corn received one application of pyraclostrobin foliar fungicide (PYR; Headline; BASF Corp. Florham Park, New Jersey) at corn stage V5; treatment 2 (2X), in which corn received 2 applications of foliar fungicides, PYR at corn stage V5, and a mixture of pyraclostrobin and metconazole (PYR+MET; Headline AMP; BASF Corp. Florham Park, New Jersey) at corn stage R1; and treatment 3 (3X), in which corn received 3 applications of foliar fungicides, PYR at corn stage V5, PYR+MET at corn stage R1, and PYR+MET at corn stage R3. Corn was harvested at the ¾ milk line and then ensiled for 7 mo. The first study utilized 64 Holstein cows with parity 2.53 ± 1.5, BW 653 ± 80 kg, and 161 ± 51 days in milk (DIM). Cows were blocked and randomly assigned to one of four treatments to be included in diet (35 % of the DM as corn silage). The trial was conducted in two consecutive periods each consisting of 1 wk for adaptation (covariate) followed by 5 wk of measurements where cows received assigned treatments. Body weight, BCS, and lame scores were assessed weekly. Milk production, and DMI were measured on a daily basis. Milk samples for milk composition analysis were collected during wk 5. Activity was measured using the Hobo Pendant® G logger. Blood samples were taken on d 1 (covariate) and d 29 to assess blood metabolites. Corn silage was analyzed for nutrients and density weekly, and mold and yeast for each period. All data was analyzed using a MIXED procedure in SAS (v9.4 Institute Inc., Cary, NC). In the second study digestibility of corn silages were then estimated using an in situ procedure. Three rumen-cannulated lactating multiparous Holstein cows (376 ± 28 DIM) were used. Dried unground corn silage was put into 288 (3 per time points/treatment/cow) 10 × 20 cm bags and incubated for 8 different times (0, 2, 4, 8, 12, 48, 72, and 96 h). A sample of unground dried corn silage was also placed into 20 × 40 cm bag and incubated for 48 h. Corn silage treated with fungicide had more sugar concentration (1.2%) than CON (0.75%). Dry matter intake was 23.78, 22.95, 19.54, and 21.33 kg for CON, 1X, 2X and 3X, respectively. There was a linear tendency for DMI. Milk yield (34.5, 34.5, 34.2, 34.4 kg/d) and milk components did not differ among treatments. However, there were trends for increased FCM/DMI (1.65 vs. 1.47) and ECM/DMI (1.60 vs. 1.43) for cows fed corn silage with fungicide compared with CON. Serum glucose was lower for cows receiving treated corn silage with fungicide when compared with CON (51.1 vs. 63.4 mg/dL.). The digestible portion of DM was greater for all corn silage treated with fungicide when compared with CON (0.36, 0.42, 0.40, and 0.47 for CON, 1X, 2X, and 3X, respectively). There was a linear effect of treatment frequency on the proportion of DM digestibility. The two different sizes of Dacron bags used (10 × 20 vs 20 × 40 cm) for the in situ digestibility technique were different for DM, NDF, ADF, CP, and starch for 48 h digestibility. Larger bags had greater digestibility for DM (33 vs 35%), and lower digestibility for NDF (42 vs 35%), and ADF (47 vs 39%) than smaller bags. In conclusion, cows receiving corn silage treated with a foliar fungicide had better feed conversion than CON and corn silage that had fungicide application had higher in situ DM digestibility and a trend for a lower fractional rate of digestion as well as linear effects for decreasing DM solubility, increasing DM digestibility, and a decreasing fractional rate of DM digestion. The aforementioned changes led to increased predicted and actual milk per ton of silage produced for 2X and 3X, and increases income over feed cost for corn silage treated with fungicide when compared with CON. Treatment 2X had higher income over feed cost than other treatments. Corn treated with foliar fungicide increased corn silage quality, and milk production efficiency and thus seems to be a valuable tool to increase profitability in dairy farms.