42. Effects of Limit-feeding Beef Cows with Either High-forage or High-concentrate Diets Pre- and Post-calving on Colostrum and Milk Yield and Composition, Nutrient Utilization, and Calf Performance.

Abstract Our objectives were to evaluate the effects of limit-feeding high forage and high concentrate diets pre- and post- calving on colostrum and milk yield and composition, nutrient utilization, and calf performance. Forty-nine pregnant (236 ± 10 d of gestation) Angus and Simmental-Angus cows (BW = 606±68kg) were blocked by breed, age, and BW and assigned to one of two treatments: 1) forage-based diet with intake limited to 1.5% BW (HFOR; n = 25); and 2) high concentrate corn-based diet, with intake limited to 1.2% BW (HCON; n = 24). Diets were limit-fed to maintain an equal intake of metabolizable energy. Cows were housed in a group-pen equipped with electronic feeders and waterers. Treatments started ∼47 d pre-calving, and continued until d 70 ± 10 d post-calving. Total colostrum from the left-rear quarter was collected pre-suckling on d 0 (calving). Colostrum weight and volume were recorded and samples were collected and analyzed for triglycerides, protein, lactose, and IgG concentration. Total milk was collected utilizing portable milking machines on days 14, 28, 42, 56, 70, 114, 145, and 186 post-calving. Milk weight and volume were recorded, and samples were analyzed for fat, protein, lactose, and milk urea nitrogen (MUN). Milk yield and composition were analyzed as repeated measures using the GLIMMIX procedure of SAS. The model included treatment, day, and their interaction as fixed effects, and the subject was cow nested within block, treatment, and breed. Although MEI was similar between groups (P = 0.36; 21.56 and 22.08±0.43 Mcal/d for HFOR and HCON respectively), HFOR had lower (P < 0.01) dry matter, organic matter, crude protein, ether extract, and non-fiber carbohydrate digestibility, suggesting that dietary substrate plays a major role in nutrient utilization compared with energy intake. Despite this, colostrum yield, composition, and IgG concentration were unaffected by treatment (P > 0.09). Treatment by day interactions (P < 0.01) were observed for milk production and MUN, with HFOR yielding more than HCON on d14, 42, 56, and 70, and having greater MUN from d 14 through 56. Moreover, treatment by day interactions (P < 0.01) were observed for milk protein yield, lactose yield, and energy corrected milk yield, with the HFOR cows having a greater protein and energy corrected milk yield on d42 through 70, and greater lactose yield on d14, 42, 56, and 70. Lastly, milk fat was greater for HCON-fed cows (P = 0.02; 3.75 vs. 3.41%, for HCON and HFOR, respectively). Lastly, calf BW was similar between groups (P = 0.27). In conclusion, although the diet of HFOR cows was less digestible, those cows were more efficient in nutrient utilization, as evidenced by the improved milk yield and composition. However, this did not translate into improved offspring performance.