Thursday, March 14, 2013

Is it important to balance AA in the diets fed to growing Holstein Steers?

On February 5th and 6th, the 24th Florida Ruminant Nutrition Symposium was held in Gainesville, FL. Several important topics were covered for both dairy and beef production. Presenters talked about immunity, environment, economics and nutrition. Dr. Richard Zinn challenged the audience to consider balancing the AA of the receiving diets fed to lightweight (particularly Holstein) steers for the first 52 days in the feed yard.

In his presentation, Dr. Zinn covered the basic work that lead to the current metabolizable protein requirements for steers the in the Nutrient Requirements of Beef Cattle (NRC, 2000). The metabolizable protein requirements are the sum of the requirements for maintenance plus gain. Once the amount of metabolizable protein required by the steer is known, you can estimate the metabolizable AA requirements based on the average AA composition of bovine tissue.

The author summarized data from 11 trials showing the AA profile of the chime entering the small intestine of beef cattle. The AA profile was very consistent and according to Dr. Zinn, during the latter growing and finishing phases, microbial AA will play a dominant role in satisfying the requirements. However, feedlot cattle may experience a deficiency in metabolizable AA during the early growth period. This may happen due to the relatively lower DMI compared with the genetic potential for growth.

A research trial was conducted by Carrasco et al. (2012, unpublished) to evaluate the effects of feeding increasing grams of a commercial RP-Methionine source (Smartamine® M) to a basal diet enriched with a commercial RP-Lysine source (Reashure-L™). The underlying objective of the research was to find the precise grams of methionine required to maximize the growing Holstein steers. The hypothesis was that feeding an AA balanced diet during the first phase in the feed yard to growing Holstein steers would prove beneficial.

The steers were grouped in 30 pens (5 steers/pen) and allocated five treatments. The diet for treatment one was not enriched with either commercial source of RP-AAs. Diets for treatments 2 to 5 were all enriched with the RP-Lys at 1.01% of the DM. Diets 2 to 5 were also enriched with the RP-Met at 0.032%, 0.064%, 0.096 and 1.28% of the DM. The steers received the treatment diets for the first 56 days in the feedlot, and from day 57 to day 112 all the steers received the treatment 1 diet (no RP-AA added). The calves fed the RP-AA enriched diets had slightly (but not significantly) higher ADG; however, they had a significantly higher linear and quadratic DMI efficiency (ADG/DMI). The steers fed the RP-AA diets had a highly linear increase in plasma Met; but not an increase in the plasma Lys concentration. This may have been due to a lower than expected rumen protection of the commercial RP-Lys product and may also explain the lower than expected ADGs of treatments 2 to 5.

The author stated that supplementing diets to meet the metabolizable AA requirements, particularly during the early stages in the feedlot, will enhance ADG and that several AA, namely, methionine, lysine, histidine, threonine and arginine may be co-limiting growth performance.