Effects of feeding oscillating dietary ruminally-degradable protein levels on production, ruminal function, omasal nutrient flow, and N utilization in dairy cows
Date
2020-04-09
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
0000-0001-9645-9390
Type
Thesis
Degree Level
Masters
Abstract
The objectives of this experiment were to determine the effects of feeding oscillating dietary ruminally-degradable protein (RDP) levels on: 1) feed intake, milk production and composition; and 2) ruminal fermentation characteristics, extent of ruminal nutrient digestion, omasal outflow of nutrients, and nitrogen (N) balance in lactating dairy cows. Eight Holstein dairy cows (days-in-milk = 110 ± 40; mean bodyweight = 734 ± 72 kg) were used in a replicated 4 × 4 Latin square design experiment with 4 dietary treatments. Each experimental period was 28 d long (14 d of dietary adaptation and 14 d of data and sample collection). Four cows in one Latin square were ruminally-cannulated and were used to study dietary effects on ruminal fermentation, nutrient digestion, and N balance. Heat-treated soybean meal (SoyPlus; Landus Cooperative, Ames, Iowa) and untreated soybean meal were used to manipulate dietary RDP levels of the experimental diets. The diets were initially formulated to be isonitrogenous at 16.5% CP; however, actual dietary CP contents ranged from 18.1 to 18.6% CP. Three diets were formulated to contain 9.34% (as % of dietary DM) RDP, 11.3% RDP, and 12.6% RDP. These three diets were then combined into 4 dietary treatments as follows: 1) feeding the 9.34% RDP diet on a continuous basis (designated LRDP); 2) feeding the 11.3% RDP diet on a continuous basis (MRDP); 3) feeding the 9.34 and 11.3% RDP diets on an oscillating (48-h) basis (LRDP/MRDP); and 4) feeding the 9.34 and 12.6% RDP diets on an oscillating (48-h) basis (LRDP/HRDP). Treatments 1 and 2 were designated STATIC RDP diets, whereas treatments 3 and 4 were designated OSC RDP diets. Experimental diets were offered to cows as total mixed rations (TMR) twice a day for ad-libitum intake. Dry matter intake (mean = 29 kg/d) was unaffected by diet (P ≥ 0.31). Milk yield was greater (P = 0.02) in cows fed the LRDP/ MRDP diet compared to those fed the LRDP/HRDP diet. Milk protein content was greater (P < 0.01) in cows fed the OSC RDP diets compared to those fed the STATIC RDP diets. Milk protein yield was greater (P = 0.02) in cows fed the OSC RDP diets compared to those fed the STATIC RDP diets. Milk urea-N content was greater (P < 0.01) in cows fed the MRDP diet compared to those fed the OSC RDP diets. Milk urea-N content was greater (P < 0.01) in cows fed the LRDP/HRDP diet compared to those fed the LRDP/MRDP diet. Plasma urea-N concentration tended to be greater (P = 0.10) in cows fed the OSC RDP diets compared to those fed the STATIC RDP diets. Nitrogen intake tended to be greater (P = 0.09) in cows fed the OSC RDP diets compared to those fed the STATIC RDP diets. Total urinary excretion was unaffected by diet (P ≥ 0.15). Urinary N excretion (expressed as g/d) was greater (P = 0.04) in cows fed the MRDP diet compared to those fed the OSC RDP diets. Also, urinary N
excretion (expressed as g/d) was greater (P = 0.03) in cows fed the LRDP/MRDP diet compared to those fed the LRDP/HRDP diet. Urinary N excretion (expressed as % of N intake) was unaffected by diet (P ≥ 0.30). Urinary urea-N excretion (expressed as g/d) was unaffected by diet (P ≥ 0.11); however, urinary urea-N excretion (expressed as % of N intake) was greater (P = 0.05) in cows fed the LRDP/HRDP diet compared to those fed the LRDP/MRDP diet. Total N excretion (expressed as % of N intake) was unaffected by diet (P ≥ 0.23). Milk N secretion (expressed as g/d or % of N intake) was unaffected by diet (P ≥ 0.19). Apparent N balance (expressed as g/d) was unaffected by diet (P ≥ 0.13. Ruminal acetate concentration was greater (P < 0.01) in cows fed the OSC RDP diets compared to those fed the STATIC RDP diets. Ruminal concentrations of propionate were greater (P = 0.03) in cows fed the OSC RDP diets compared to those fed the STATIC RDP diets. Ruminal concentrations of total SCFA were greater (P = 0.01) in cows fed the OSC RDP diets compared to those fed the STATIC RDP diets. Ruminal ammonia-N concentration, acetate: propionate ratio and ruminal pH were not affected by diet (P ≥ 0.20). Intake, omasal flow and apparent digestion of DM were not affected by diet (P ≥ 0.14). Omasal flow of NH₃-N (expressed as g/d) was unaffected by diet (P ≥ 0.52). In conclusion, oscillating RDP levels of dairy cow diets improved milk protein yield and content and tended to increase plasma urea-N concentration and N intake. Total N excretion, milk N secretion, and apparent N balance were unaffected by feeding OSC RDP diet. Generally, the dietary changes did not improve NUE as expected.
Description
Keywords
RDP, oscillating
Citation
Degree
Master of Science (M.Sc.)
Department
Animal and Poultry Science
Program
Animal Science