# DIETARY PROTEIN TO METABOLIZABLE ENERGY RATIO'S EFFECTS ON GROWTH AND NUTRIENT UTILIZATION OF PREPUBERTAL HOLSTEIN HEIFERS

Open Access

- Graduate Program:
- Animal Science
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- March 27, 2002
- Committee Members:
- James George Beierlein, Committee Member
- Peter R Tozer, Committee Member
- Arlyn Judson Heinrichs, Committee Chair
- Lawrence Dean Muller, Committee Member
- Lisa Holden, Committee Member

- Keywords:
- growth
- digestibility
- protein to energy ratios
- dairy heifers

- Abstract:
- Three experiments were conducted to evaluate the ratio of dietary protein to metabolizable energy on growth, efficiency of growth, and nutrient utilization of prepubertal Holstein heifers between 125 and 234 kg of body weight (BW). In the first experiment, sixty Holstein heifers (125 to 234 kg BW) were used to evaluate increasing dietary protein to metabolizable energy ratios (CP:ME) on feed efficiency, structural growth, and body condition score. Treatment rations contained a specific CP:ME ratio, 48.3, 59.1, 67.5, and 76.5 g of crude protein (CP) per Mcal of metabolizable energy (ME) with non-structural carbohydrate (NSC) to rumen degradable protein (RDP) ratios (NSC:RDP; g of NSC per g of RDP) of 4.15 for the 48.3 CP:ME ratio, 3.30 for the 59.1 CP:ME ratio, 2.75 for the 67.5 CP:ME ratio, and 2.32 for the 76.5 CP:ME ratio. Treatment rations were assigned to heifers in a complete randomized block design with treatment periods lasting 20 wk. Body weight was recorded weekly on two consecutive days and used to adjust dry matter intake (DMI), which averaged approximately 2.0% of BW, to achieve a desired 0.80 kg/d rate of gain. By design, average daily gain was not influenced while DMI showed an increasing quadratic effect with increasing CP:ME ratios. Feed efficiency improved linearly with increasing CP:ME ratios. A trend toward linear increases in wither and hip height growth occurred with increasing CP:ME ratios. Observed linear effects in feed efficiency and some structural growth measurements demonstrate positive results when feeding CP:ME ratios above 48.3 to Holstein heifers gaining 0.80 kg/d. Linear decreases in NSC:RDP ratios of treatment rations may have increased RDP utilization and maximized the efficiency that protein was used toward growth. In the second experiment, four prepubertal Holstein heifers (153 to 196 kg BW) fitted with rumen cannulae were used to evaluate increasing CP:ME ratios on nutrient utilization in a 4×4 Latin square design. Heifers were fed treatment rations at 2.0% BW as DMI that contained a CP:ME ratio of 45.0, 63.3, 69.4, or 77.3 with NSC:RDP ratios of 4.25 for 45.0 CP:ME ratio, 3.05 for the 63.3 CP:ME ratio, 2.66 for the 69.4 CP:ME ratio, and 2.33 for the 77.3 CP:ME ratio. A linear increase was observed for rumen ammonia as CP:ME ratios increased. Nitrogen utilization tended to improve quadratically with the 63.3 CP:ME ratio (3.05 NSC:RDP ratio) possessing the highest N retained as a percent of N intake, resulting in 20, 14, and 41% greater N retained as a percent of N intake than the 45.0, 69.4, and 77.3 CP:ME ratios, respectively. Dry matter (DM) utilization also improved quadratically with the 63.3 CP:ME ratio (3.05 NSC:RDP ratio) having the highest total tract apparent DM digestibility. Microbial N calculated from urinary excretion of purine derivatives increased linearly with increasing CP:ME ratios. Feeding a CP:ME ratio of 63.3 at 2.0% BW as DMI to Holstein heifers produced less calculated microbial N than higher CP:ME ratios (69.4 and 77.3), but achieved a better synergistic relationship of dietary protein to energy (CP:ME and NSC:RDP ratios) resulting in greater total tract apparent DM digestibility. From the results of the second experiment, it was hypothesized that greater calculated microbial N received from higher CP:ME ratios (69.4 and 77.3) than the 63.3 CP:ME ratio occurred due to increased quantities of RDP or specific RDP fractions. In the third experiment, four prepubertal Holstein heifers (148 to 190 kg BW) fitted with rumen cannulae were used to evaluate a specific CP:ME ratio, with altered soluble (AB1) and potentially rumen degradable (B2B3) protein fractions on nutrient utilization. A 4×4 Latin square design with a 2×2 factorial arrangement of treatments was used. Heifers were fed treatment rations at 2.0% BW as DMI containing a CP:ME ratio of 62.1 g of CP per Mcal of ME with altered AB1 fractions (33.6 or 40.6% AB1 as a percent of CP) and B2B3 fractions (20.9 or 28.2% B2B3 as a percent of CP). Non-structural carbohydrate to RDP ratios ranged from 3.92 to 3.12 for treatment rations. Increased intakes of AB1 increased rumen ammonia but decreased total volatile fatty acid concentrations. Increased intakes of B2B3 tended to increase total volatile fatty acid concentrations while increasing molar proportions of propionate. Nitrogen utilization was not affected by increased intakes of AB1 or B2B3. Feeding a CP:ME ratio of 62.1:1 at 2.0% BW as DMI to Holstein heifers with increased potentially rumen degradable protein (28.2 versus 20.9 % B2B3 as a percent of CP) affects rumen fermentation but has no overall effect on N utilization. In summary, results from three experiments indicate that Holstein heifers, between 125 and 234 kg BW, receiving a diet at 2.0% BW as DMI containing a CP:ME ratio between 60:1 to 69:1 achieved improved feed efficiency, structural growth, and nutrient utilization. Optimum performance resulted when a 63:1 CP:ME ratio, in comparison to lower and higher CP:ME ratios evaluated, contained 16% dietary CP and 2.6 Mcal of ME per kg of DM. The 16% dietary CP of the 63.1 CP:ME ratio should consist of 34% AB1 and 31% B2B3, resulting in a RDP of 65% as a percent of dietary CP. It also appears NSC:RDP ratios should range between 3.30 and 2.66, but is contingent on protein fractions of the RDP.