Abstract
We evaluated the effects of supplemental calcium ammonium nitrate (CAN) fed to dairy cows on dry matter (DM) intake, nutrient digestibility, milk quality, microbial protein synthesis, and ruminal fermentation. Six multiparous Holstein cows at 106 ± 14.8 d in milk, with 551 ± 21.8 kg of body weight were used in a replicated 3 × 3 Latin square design. Experimental period lasted 21 d, with 14 d for an adaptation phase and 7 d for sampling and data collection. Cows were randomly assigned to receive the following treatments: URE, 12 g of urea/kg of DM as a control group; CAN15, 15 g of CAN/kg of DM; and CAN30, 30 g of CAN/kg of DM. Supplemental CAN reduced DM intake (URE 19.0 vs. CAN15 18.9 vs. CAN30 16.5 kg/d). No treatment effects were observed for apparent digestibility of DM, organic matter, crude protein, ether extract, and neutral detergent fiber; however, CAN supplementation linearly increased nonfiber carbohydrate digestibility. Milk yield was not affected by treatments (average = 23.1 kg/d), whereas energy-corrected milk (ECM) and 3.5% fat-corrected milk (FCM) decreased as the levels of CAN increased. Nitrate residue in milk increased linearly (URE 0.30 vs. CAN15 0.33 vs. CAN30 0.38 mg/L); however, treatments did not affect nitrite concentration (average: 0.042 mg/L). Milk fat concentration was decreased (URE 3.39 vs. CAN15 3.35 vs. CAN30 2.94%), and the proportion of saturated fatty acids was suppressed by CAN supplementation. No treatment effects were observed on the reducing power and thiobarbituric acid reactive substances of milk, whereas conjugated dienes increased linearly (URE 47.6 vs. CAN15 52.7 vs. CAN30 63.4 mmol/g of fat) with CAN supplementation. Treatments had no effect on microbial protein synthesis; however, molar proportion of ruminal acetate and acetate-to-propionate ratio increased with CAN supplementation. Based on the results observed, supplementing CAN at 30 g/kg of DM should not be recommended as an optimal dose because it lowered DM intake along with ECM and 3.5% FCM, although no major changes were observed on milk quality and ruminal fermentation.
Highlights
Nitrate (NO3−) compounds have been explored as feed additives to decrease methane (CH4) production and as dietary NPN sources in ruminants
This study was designed as replicated Latin squares balanced for carryover effects; considering the short adaptation period, we analyzed data for carryover effects, and no significant differences (Supplemental Tables S1 and S2, https://doi.org/10.6084/m9.figshare.17159195.v2, https://doi.org/10.6084/m9.figshare.17159225.v2; Almeida, 2021a,b) were observed, except tendencies for allantoin (P = 0.06; Table S1) and microbial protein synthesis (P = 0.07; Table S1)
Milk protein responses might be attributable to the effects of NO3− supplementation on DMI, which may have affected ruminal and post-ruminal N availability and subsequently milk protein synthesis, no effects were observed on microbial protein synthesis, which makes it difficult to explain
Summary
Nitrate (NO3−) compounds have been explored as feed additives to decrease methane (CH4) production and as dietary NPN sources in ruminants. Feng et al (2020), using a meta-analytical approach, reported the efficacy of NO3− supplementation (mean = 16.7 g/kg of DM) at reducing CH4 yield (g/DMI) in both dairy cows (15.5 ± 1.15%) and beef cattle (8.95 ± 1.76%). Nitrite (NO2−), an intermediate formed during reduction from NO3− to NH3, is toxic to rumen methanogens (Latham et al, 2016; Popova et al, 2019). The efficacy of NO3− at reducing the metabolic activity of methanogens (Patra and Yu, 2013; Popova et al, 2019) is consistent, effects of dietary NO3− on ruminal fermentation parameters remain inconsistent.
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