Modification of In Vitro and In Situ Dry Matter Digestibility of Forages with Chemical Treatment

Three experiments were conducted to compare the effects of anhydrous ammonia (NH3) or sodium hydroxide (NaOH) on in vitro dry matter digestibility (IVDMD) of several forage types and to determine the fraction of increase in digestibility associated with additional N from ammoniation. Forages used for hay preparation were alfalfa (Medicago sativa L.), ryegrass (Lolium multiflourm Lam.), stored (1‐yr‐old) coastal bermudagrass [Cynodon dactylon (L.) Pers.], high‐moisture (51%) coastal bermudagrass, medium‐moisture (33%) coastal bermudagrass, and low‐moisture (15%) coastal bermudagrass hays, and oat (Avena sativa L.)‐wheat (Triticum aestivum L.) straw. Hays were subjected to an application rate of 7.5% NH3 on a dry matter basis. Sodium hydroxide treated samples were prepared by adding 0.10 oz of a 3% (w/v) solution of NaOH for 48 h, and then samples were neutralized with 0.10 oz of a 3% (w/v) solution of hydrochloric acid (HC1) just prior to use in in vitro fermentations. Buffers used for in vitro fermentations contained either supplemental N in the form of casein or urea, or no supplemental N. Retention of NH3 varied between hays, with a mean increase of 55.7% N. In vitro dry matter digestibility increased with NH3 and NaOH treatments when no supplemental N was included in the in vitro buffer. However, casein reduced IVDMD of some lower quality control hays, and reduced (P < 0.05) the degree of response to NH3 treatment. Of the NaOH treated forages, only stored coastal bermudagrass and medium‐moisture coastal bermudagrass hays had decreased IVDMD when casein was added in the buffer. Addition of urea to the buffer did not affect digestibility of the NaOH treated hays. The increase in IVDMD of the control hays with urea supplementation represented 12% of the increase in IVDMD found with NH3 treatment. In situ dry matter digestibility (DMD) responses paralleled in vitro fermentation results; however, there was a limited response to NH3 in ryegrass hay and oat‐wheat straw. In situ DMD of ammoniated coastal bermudagrass hays increased nearly 100%. These data indicate supplemental N produced only a small fraction of the increase in IVDMD observed with NH3 treatment, indicating another mechanism was responsible for increased IVDMD of NH3 forages. The degree to which in vitro and in situ DMD increased with ammoniation depended on forage type, with the least digestible forages producing the greatest relative response to ammoniation.