Materials and Methods
| Table 1. Ingredient composition of diets. | |
| Ingredient | Amount |
| (% of DM) | |
| Alfalfa hay | 24.63 |
| Corn silage | 17.03 |
| Corn, dry ground | 17.74 |
| Blood meal | 0.42 |
| Soybeans, cracked roasted | 5.44 |
| Brewers grains, wet | 11.85 |
| Soybean hulls | 18.14 |
| Calcium soaps | 1.99 |
| Urea | 0.40 |
| Limestone | 0.27 |
| Magnesium oxide | 0.18 |
| Monosodium phosphate | 0.62 |
| Sodium bicarbonate | 0.74 |
| TM salt | 0.45 |
| Vitamin premix | 0.10 |
Thirty Holstein, multiparious cows were allocated into 15 blocks based on current milk yield and projected 305-day mature equivalent for milk yield, and cows within a block were assigned to one of two fat supplements fed at 1.99% of the diet: 1) Megalac®(Church and Dwight Co., Inc., Princeton, NJ) or 2) Rumolac® (Robt Morgan, Inc., Paris, IL). Cows fed Megalac averaged 146 + 50 days in milk and a parity of 2.9 + 1.2, and those fed Rumolac averaged 155 + 34 days in milk and a parity of 3.9 + 1.5. Cows were fed a common diet for three days for covariate adjustment of data, and then cows were fed experimental diets for 30 days. The trial was conducted during May and June 2000. The experimental diets consisted of 24.6% alfalfa hay, 17.0% corn silage, and 58.4% concentrates (Table 1) that were mixed as a TMR. Diets were formulated to contain 18.7% CP, 5.58% fatty acids, 34.7% NDF, and 23.3% ADF and to meet recommendations for minerals and vitamins (NRC, 1989). Cows were milked and fed twice daily, housed in tie-stalls, and injected with bovine somatotrophin (Posilac®; Monsanto, St. Louis, MO).
Dry matter intake and milk yield were measured daily. Milk was sampled twice weekly on two consecutive days for analysis of fat and protein by infrared spectroscopy, milk urea nitrogen by using a Skalar SAN Plus segmented flow analyzer (Skalar, Inc., Norcross, GA), and somatic cells at the Dairy Herd Improvement Laboratory (DHI Cooperative, Inc., Powell, OH). Forages were sampled weekly for DM analysis and adjustment of the TMR. Diets were sampled weekly and analyzed for DM, CP, NDF, ADF, and fatty acids.
Data were analyzed as a randomized complete block design. Dry matter intake, milk yield and body condition score were analyzed with covariant adjustment of data using the general linear model procedure of SAS (1997). Milk composition and change in body condition were not covariately adjusted.