Comparison of different methods of expressing dietary protein for lactating dairy cows.

Data were from Holstein cows from 20 experiments at the University of Florida (1688 cow-period observations). Objectives were to compare milk responses to increasing dietary percent or daily intake of CP, estimated undegradable CP, and estimated metabolizable protein from Burroughs and Chalupa equations and to determine if higher producing cows (26.8 kg milk/d) responded differently to increasing dietary protein percent or intake than do lower producing cows (18.9 kg milk/d). The mathematical model included experiment, cow-within-experiment, period, body weight, and source of roughage. Intake of protein (kg/d) had a greater effect (cubic polynomial) on milk yield than protein percent of DM since CP, undegraded protein, Chalupa metabolizable protein, and Burroughs metabolizable protein intakes explained 17.2, 20.9, 23.5, and 24.1% of residual variation in milk yield compared with .6, 2.1, 1.7, and 2.1%, for percents of DM. High producing cows responded more to increasing protein intake than did low producing cows. Respective protein intake variables explained 20.6, 23.7, 30.5, and 31.2% of residual variation in milk yield in high production group compared with 17.2, 23.1, 20.6, and 20.6% in low production group.

Effects of neutral detergent fiber and roughage source on dry matter intake and milk yield and composition of dairy cows.

Data were from 20 experiments that utilized early to midlactation Holstein cows fed complete mixed diets or fed at constant forage:concentrate ratios. Within-cow diet comparisons (1688 cow-periods) were analyzed by least squares analysis of variance; mathematical model included experiment, cow in experiment, period, body weight, and source of roughage. Objectives were to determine relationships between neutral detergent fiber content of diet and milk yield and dry matter intake. Roughages and number of cow-periods were: sugarcane bagasse/silage (507), cottonseed hulls (504), corn silage (268), ground corrugated boxes (170), alfalfa/peanut hay (132), and others (107). Dry matter intake and estimated net energy intake had linear effects on milk yield and explained 21.6 and 24.0% of its residual variation; milk yield had curvilinear (quadratic) effect and explained 22.4% of dry matter intake residual variation. Interaction between neutral detergent fiber and source of roughage on milk yield, 4% fat-corrected milk, and dry matter intake resulted in reductions of 5.6, 5.6, and 13% in residual variations. Results suggest neutral detergent (% of dry matter) has greater effect on dry matter intake than on milk yield and its use in formulating diets for dairy cows will be within roughage source.