Abstract
The objective of this experiment was to investigate the effect of dietary levels of digestible histidine (dHis) and MP on lactational performance and plasma and muscle concentrations of free AA in dairy cows. A randomized block design experiment was conducted with 48 Holstein cows, including 20 primiparous, averaging (±SD) 103 ± 22 d in milk and 45 ± 9 kg/d milk yield at the beginning of the experiment. A 2-wk covariate period preceded 12 experimental wk, of which 10 wk were for data and sample collection. Experimental treatments were (1) MP-adequate (MPA) diet with 2.1% dHis of MP (MPA2.1), (2) MPA with 3.0% dHis (MPA3.0), (3) MP-deficient (MPD) diet with 2.1% dHis (MPD2.1), and (4) MPD with 3.0% dHis (MPD3.0). Actual dHis supply was estimated at 64, 97, 57, and 88 g/d, respectively. Diets supplied MP at 110% (MPA) and 96% (MPD) of NRC 2001 dairy model requirements calculated based on DMI and production data during the experiment. Dry matter intake and milk yield data were collected daily, milk samples for composition and blood samples for AA analysis were collected every other week, and muscle biopsies at the end of covariate period, and during wk 12 of the experiment. The overall DMI was not affected by dHis or MP level. Milk yield tended to be increased by 3.0% dHis compared with 2.1% dHis. Milk true protein concentration and yield were not affected by treatments, whereas milk urea nitrogen concentration was lower for MPD versus the MPA diet. Milk fat concentration was lower for MPD versus MPA. There was a MP × dHis interaction for milk fat yield and energy-corrected milk; milk fat was lower for MPD3.0 versus MPD2.1, but similar for cows fed the MPA diet regardless of dHis level whereas energy-corrected milk was greater for MPA3.0 versus MPA2.1 but tended to be lower for MPD3.0 versus MPD2.1. Plasma His concentration was greater for cows fed dHis3.0, and concentration of sum of essential AA was greater, whereas carnosine, 1-Methyl-His and 3-Methyl-His concentrations were lower for cows fed MPA versus MPD diet. Muscle concentration of His was greater for cows fed dHis3.0 treatment. The apparent efficiency of His utilization was increased at lower MP and His levels. Overall, cows fed a corn silage-based diet supplying MP at 110% of NRC (2001) requirements tended to have increased ECM yield and similar milk protein yield to cows fed a diet supplying MP at 96% of requirements. Supplying dHis at 3.0% of MP (or 86 and 96 g/d, for MPD3.0 and MPA3.0, respectively) tended to increase milk yield and increased plasma and muscle concentrations of His but had minor or no effects on other production variables in dairy cows.
Highlights
In dairy farming, excess N, originating from high protein diets, is an environmental burden in terms of N leakage and ammonia and nitrous oxides emissions, as well as an economic loss for the farmer, as protein is the most expensive major nutrient in dairy rations (Hristov et al, 2011a)
All treatments provided adequate NEL (Table 2), whereas MPA2.1 and MPA3.0 supplied 110 and 109%, and MPD2.1 and MPD3.0 supplied 97 and 96% of MP requirements according to NRC (2001), respectively
Milk yield was not affected by MP but tended to be greater (P = 0.10) for dHis3.0 compared with dHis2.1
Summary
Excess N, originating from high protein diets, is an environmental burden in terms of N leakage and ammonia and nitrous oxides emissions, as well as an economic loss for the farmer, as protein is the most expensive major nutrient in dairy rations (Hristov et al, 2011a). Only when MP supply from the basal diet was deficient did milk protein yield positively respond to RPHis supplementation (Räisänen et al, 2021b). These data further highlighted the fact that His is especially limiting in situations where the cow is more reliant on rumen microbial protein due to its lower His concentration compared with milk protein (Kim et al, 1999). In a 12-wk continuous experiment with MPA basal diet, His became limiting for milk and milk protein production (Giallongo et al, 2017), indicating that a long-term His deficiency can compromise lactational performance even when MP supply is adequate
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