Effect of dietary protein quality and rumen protected amino acid supplementation on performance and carcass characteristics of feedlot cattle.

Abstract

The objective of this study was to evaluate the effect of rumen undegradable protein (RUP) and rumen protected lysine (RPLys) and methionine (RPMet) supplementation on growth performance, blood parameters and carcass characteristics of feedlot cattle over a 135-day feeding period. One hundred and twenty Bonsmara type steers were blocked by body weight and randomly allocated to one of four treatment diets in a complete randomised block design. Each treatment consisted of 5 pens with 6 animals per pen. The treatments were 1) Basal diet supplemented with urea (CON), 2) Basal diet supplemented with RUP (RUP), 3) Basal diet supplemented with RUP and RPLys (RUP+L) and 4) Basal diet supplemented with RUP and RPLys and RPMet (RUP+L+M). Data was statistically analysed using the PROC MIXED model (SAS, 2021). The average daily gain (ADG) and feed conversion ratio (FCR) did not differ between treatments and ranged from 1.84 to 1.90 kg/d and 4.84 to 5.04 kg feed / kg gain respectively (P>0.05). Treatments RUP+L and RUP+L+M resulted in lower serum blood urea nitrogen (BUN) concentrations on three sampling days when compared to the CON treatment which is indicative of better N utilisation (P<0.05).Serum blood AA were reduced for Ala, Glu, Pro and Tyr on sampling day 99 and reduced for Ileu, Lys, Leu, Val , Pro and Tyr when treatment RUP+L+M is compared to CON suggesting an increased uptake and utilisation of these AA for protein and muscle deposition (P<0.05). The 13th rib subcutaneous fatness was lower for treatment RUP+L+M compared to the other treatments (P < 0.05) and accordingly the channel fat mass of carcasses from animals supplemented with both RPLys and RPMet was lower than the CON group (P<0.05). This suggest that Lys and Met supplementation as well as the ratio of Lys to Met may be important factors to consider if the consumer demands leaner carcasses. In conclusion, a diet formulated according to NRC standards with sufficient RDP, energy and effective NDF that promotes optimal rumen fermentation can achieve above average growth performance and do not need to be supplemented with additional RUP or RPAA to meet MP requirements. Supplementation of RPLys and Met has shown potential for production of leaner carcasses.