The objective of this study was to examine the effect of reducing
rumen-degradable protein (RDP) and rumen-undegradable protein (RUP)
proportions on feed intake, milk production, and N-use efficiency in
primiparous and multiparous cows exposed to warm climates. Eighteen
primiparous and 30 multiparous mid-lactation Holstein cows were used in a
completely randomized design with a 2 × 2 factorial arrangement of
treatments. Cows were randomly assigned to 1 of 4 dietary treatments
formulated to contain 2 proportions of RDP (10 and 8%) and 2 proportions
RUP (8 and 6%) of dry matter (DM) indicated as follows: (1) 10% RDP, 8%
RUP; (2) 8% RDP, 8% RUP; (3) 10% RDP, 6% RUP; and (4) 8% RDP, 6% RUP.
Protein sources were manipulated to obtain desired RDP and RUP
proportions. Diets were isoenergetic and contained 50% forage and 50%
concentrate (DM basis). Cows were individually fed the 10% RDP, 8% RUP
diet 3 wk before treatment allocation. Cows were exposed to the
prevailing Tennessee July and August temperature and humidity in a
freestall barn with no supplemental cooling. Main effects and their
interaction were tested using the Mixed procedure of SAS (least squares
means ± standard error of the mean; SAS Institute Inc., Cary, NC).
Observed values of nutrient intake and milk production were used to
obtain NRC (2001) model predictions. Cows showed signs of heat stress
throughout the study. Reducing from 10 to 8% RDP decreased dry matter
intake (DMI; 0.9 kg/d) at 8% RUP, but increased DMI (2.6 kg/d) at 6% RUP
in primiparous cows. Reducing from 10 to 8% RDP decreased milk yield
(10%) at 8% RUP, but increased yield (14%) at 6% RUP. Treatments did not
affect yield of energy-corrected milk. For multiparous cows, treatments
did not affect DMI. Reducing from 10 to 8% RDP decreased yield of
energy-corrected milk (3.4%) at 8% RUP, but increased yield (8.8%) at 6%
RUP. Reducing from 10 to 8% RDP and 8 to 6% RUP both increased N-use
efficiency for primiparous and multiparous cows. The NRC model
underestimated metabolizable protein and RUP supply, and overestimated
RUP requirements, resulting in predictive losses of milk yield 1.4 to
5.8 times greater than observed values. In summary, the reduction of RDP
and RUP proportions did not affect DMI, whereas the RUP reduction at
10% RDP had a small negative effect on energy-corrected milk yield.
However, reduction of RDP and RUP consistently improved N-use efficiency
of heat-stressed multiparous cows. The reduction of RDP and RUP
proportions reduced DMI and milk yield but did not affect
energy-corrected milk yield in primiparous cows, indicating a limited
supply of nutrients.
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