Effects of tallow and urea on in vitro rumen fermentation and nutrient digestion in sheep.

An in vitro study was conducted using a 4 • 2 • 2 factorial design with RAC (readily available carbohydrate) levels of 30, 40, 50 and 60%, tallow levels of 0 and 3%, and urea levels of 0 and 2%. Acetic and propionic acid production and ammonia disappearance increased as the level of RAC increased from 30 to 50%. Tallow reduced acetic acid production and increased the amount of propionic acid produced. Urea increased VFA production and ammonia disappearance. There was a tallow • urea interaction for acetic, propionic and butyric acid production as well as for ammonia disappearance. The interactions resulted in a significant decrease in acetic and butyric acid production and ammonia disappearance when tallow and urea were added simultaneously. Three metabolism studies were conducted to observe the effects of various combinations of tallow and urea as well as the effects of tallow in combination with biuret or cottonseed meal. Data from trial 1 indicated that the addition of urea resulted in higher crude protein digestibility and nitrogen retention. Lambs receiving 3% tallow plus urea retained more nitrogen than lambs receiving 5% tallow plus urea or the negative control. Data from trial 2 indicated that crude protein from urea or biuret was digested and retained more efficiently than that from cottonseed meal when the rations contained 5% tallow. In trial 3, 2.17% urea, in the presence of 5% tallow, resulted in an improved nitrogen balance as compared to rations with 1.5 or 1.16% urea. Digestibility of crude protein was not affected, but plasma urea increased as urea levels increased.