Metabolizable Amino Acid Requirements of Feedlot Calves

One hundred ninety-two crossbred steers (229 kg) were used to evaluate the influence of metabolizable amino acid intake on growth performance and health during the initial 42-d receiving period. Treatments consisted of four levels of metabolizable lysine (23, 24, 25 and 26 g/kg diet DM). Morbidity averaged 36%, and was not affected (P > 0.20) by treatments. No steers died during the study. Increasing the metabolizable lysine supply increased DMI, ADG, gain efficiency, and dietary NE (linear effect, P < 0.01). Metabolizable amino acid supply of the basal diet was determined using 6 steers (214 kg) with cannulas in the rumen and proximal duodenum. Metabolizable amino acid supply of the diet was in close agreement with expected (R2 = 0.99; P < 0.0001) based on NRC (2000) [12] Level 1 model. The metabolizable amino acid supplies for treatments in Trial 1 were estimated by adding tabular metabolizable amino acid values for the respective supplemental proteins (cottonseed meal and fish meal) to the observed metabolizable amino acid supply of the basal diet. Treatment effects on metabolizable lysine supply explained 99% of the variation (P < 0.01) in ADG, and 91% of the variation in observed versus expected dietary NE. The biological value for the intestinal chyme was determined based on chemical score technique, using bovine tissue as the reference protein. Accordingly, methionine and lysine were closely co-limiting amino acids having ratios of 77% and 79%, respectively. We conclude that current NRC standards reliably predict both requirements and supplies of metabolizable amino acids for feedlot calves. Diet formulations that do not meet the metabolizable amino acid requirements may depress both ADG and the partial efficiency of utilization of metabolizable energy for maintenance and gain.


Introduction
Formulating diets to meet protein requirements of feedlot cattle enhance ADG and energetic efficiency [1]- [3].Zinn and Shen (1998) observed that NRC (2000) provided reliable estimates of amino acid supply to the small intestine based on diet formulation as well as metabolizable amino acid requirements of feedlot calves.However, very little research has been conducted which validates the practicality of such diet formulations for lightweight calves during the initial receiving period.The objective of this study was to further evaluate the practicality of current standards (NRC, 2000) in making diet formulations to meet metabolizable amino acid requirements.The study specifically addresses two important issues: 1) reliability of current standards for estimation of amino acid supply to the small intestine of feedlot steers fed a conventional receiving diet; and 2) the relationship between limiting amino acid supply and growth performance responses (metabolizable amino acid requirements) during the initial receiving period.

Materials and Methods
Trial 1.One hundred ninety-two crossbred steers (229 kg) were used in 42-d receiving trial to evaluate the influence of metabolizable amino acid intake on health and performance.Steers were blocked by weight and randomly allocated to 24 pens (8 steers/pen).Pens were 43 m 2 with 22 m 2 overhead shade, equipped with automatic waterers and fence-line feed bunks.Processing on arrival includes branding, ear-tagging, castration (elastration), vaccination for IBR-PI 3 (TSV-2, Zoetis Inc., Kalamazoo, MI), Clostridials/Haemophilus (Ultrabac, TSV-2, Zoetis Inc., Kalamazoo, MI), treatment for internal and external parasites (Ivomec plus, Merck, Rahway, NJ), and injection with 1 mL vitamin A & D (Vita-jec A & D, Agripharm, Greely, CO).Horns, if present, were tipped.Calves visually diagnosed as sick received medication until rectal temperature remained below 39.4˚C for two consecutive days.Composition of experimental diets is shown in Table 1.Treatments consisted of a steamflaked corn-based receiving diet formulated based on NRC (2000, level 1) to provide four levels of metabolizable lysine (23, 24, 25 and 26 g/kg diet DM) using combinations of fish meal and cottonseed meal.Diets were prepared at weekly intervals and stored in plywood boxes located in front of each pen.Steers were allowed ad libitum access to their experimental diets.Fresh feed was provided twice daily.Steers were implanted with Synovex-S ® (Zoetis Inc., Kalamazoo, MI).For calculating steer performance, initial is the off truck arrival weight.Final BW was reduced 4% to account for digestive tract fill.Estimates of steers' performance were based on pen means.Energy gain (EG) was calculated by the equation: EG = ADG 1.097 × 0.0557W 0.75 , where EG is the daily energy deposited (Mcal/d) and W is the mean shrunk body weight [4].Maintenance energy (EM) was calculated by the equation: EM = 0.077W 0.75 [4].The NE m and NE g value of the diets were obtained by means of the quadratic formula: (−b ± (b 2 -4ac) 0.5 )/2a, where a = −0.41EM,b = 0.877EM + 0.41DMI + EG, c = −0.877DMI,and NE g = 0.877NE m −0.41 [1].The experimental data were analyzed as a randomized complete block design according to the following statistical model: Y ij = μ + B i + T j + ε ij , where μ is the common experimental effect; B i represents blocks (df = 5); T j represents dietary treatment effect (df = 3); and ε ij represents the residual error (df = 15).Treatments effects were tested by means of orthogonal polynomials (Statistix-10, Analytical Software, Tallahassee, FL).
Trial 2. Six steers (214 kg) with cannulas in the rumen and proximal duodenum [5] were used to determine metabolizable amino acid supplies for the basal diet (treatment 1, 23 g lysine/kg, Table 1).Chromic oxide (0.4%) was added as a digesta marker.Steers were maintained in individual pens with access to water at all times.Diets were fed at 08:00 and 20:00 hours daily.Dry matter intake was restricted to 6.3 kg/d.Steers were given 10 d for diet adjustment and 4 d for collection.During collection, duodenal and fecal samples were taken twice daily as follows: d 1, 07:50 and 13:50 hours; d 2, 09:00 and 15:00 hours; d 3, 10:50 and 16:50 hours, and d 4, 12:00 and 18:00 hours.Upon completion of the trial, approximately 500 mL of ruminal fluid were obtained from each steer, composited across diets; bacteria were isolated via differential centrifugation [6].The microbial isolates were prepared for analysis by oven drying at 70˚C and grinding with mortar and pestle.Feed, duodenal and fecal samples were prepared for analysis by oven drying at 70˚C and grinding in a lab mill (Micro-Mill, Bel-Arts Products, Pequannock, NJ).Samples were oven dried at 105˚C until no further weight was lost and stored in tightly sealed glass jars.Samples were subjected to all or part of the following analysis: ash, ammonia N, Kjeldahl N [7], chromic oxide [8]; and purines [9].Microbial organic matter (MOM) and N (MN) leaving the abomasum were calculated using purines as a microbial marker [9]; and amino acids (hydrolysis under N in sealed ampules with 6 NHcl overnight at 110˚C.Organic matter fermented in the rumen was considered equal to OM intake minus the difference between the amount of total OM reaching the duodenum and MOM reaching the duodenum.Feed N escape to the small intestine was considered equal to total N leaving the abomasum minus ammonia-N, MN, and estimated endogenous N (0.195 g/kg BW0.75) [10].The metabolizable amino acid supplies for the other three treatments were estimated by adding tabular metabolizable amino acid values for the supplemental proteins (fishmeal, cottonseed meal, and corn used in the replacements) to the observed metabolizable amino acid supply of the basal diet.The metabolizable amino acid supply for treatments in Trial 1 were obtained by multiplying respective metabolizable amino acid supply in Trial 2 by corresponding DMI for that treatment in Trial 1 and then dividing by 6.25.

Results and Discussion
Treatment effects on feedlot performance are shown in Table 2. Morbidity averaged 36%, and was not affected (P > 0.20) by treatments.No steers died during the study.Increasing the metabolizable lysine increased DMI, ADG (Figure 1), gain efficiency, and dietary NE (linear effect, P < 0.01).Similarly, [11] observed that supplementation with rumen-stable Lysine (RS-Lys) improved (P < 0.01) ADG in growing cattle.Characteristics of digestion and metabolizable amino acid supply of treatment 1 (23 g lysine/kg; (Table 1) is shown in Table 3. Proportionately, observed and expected amino acid concentrations were in very close agreement (r 2 = 0.99; P < 0.0001).However, observed metabolizable amino acid supply was greater (16%) then predicted based on [12] Level 1 model.
Treatment effects on biological value of duodenal chyme for the four dietary treatments based on chemical score (amino acid in duodenal chyme × 100/amino acid in reference protein), using "ideal protein" for growing  swine [13], and bovine tissue [12], are shown in Table 4.Some researchers have suggested that amino acid requirements cannot be logically related to amino acid composition of deposited protein because amino acids are used for various functions in addition to deposition, and because protein turnover and recycling rates differ.However, research with non-ruminants does not support that contention.Indeed, amino acid requirements for non-ruminants are closely related to and have been extensively calculated from amino acid composition of deposited carcass tissue or of secreted milk protein [14].Carcass amino acid composition served as one basis for developing the "ideal protein" concept, which is used extensively to calculate amino acid requirements of swine [13].As has been shown previously using similar techniques [2] [14], histidine would appear to be the first  limiting amino acid.No research has been reported that directly evaluates the histidinerequiermentsof cattle.In light of this fact, and the fact that supplementation with individual amino acids such as lysine and methionine enhanced growth performance [2] reasoned that it is more likely that current standards overestimate the histidine requirements.More research is needed in this area.Methionine and lysine were closely co-limiting amino acids.Indeed, metabolizable lysine supply explains 99% of the variation (P < 0.01) in ADG (Table 5).Metabolizable lysine also explained 91% of the variation in (P < 0.01) in dietary NE.This finding is consistent with [2] who observed that changes in dietary NE with protein supplementation are a very sensitive indicator metabolizable amino acid adequacy.

Conclusion
It is concluded that the Level 1 model [12] is a reliable tool for predicting both requirements and supplies of metabolizable amino acids for feedlot calves during the receiving period.Diet formulations that do not meet metabolizable amino acid requirements may depress both ADG and the partial efficiency of energy utilization.

Figure 1 .
Figure 1.Relationship between metabolizable lysine intake (mLysine) and the ratio of expected/observed DMI and ADG.

Table 1 .
Dry mater composition of experimental diets fed to steers in Trials 1 a and 2.

Table 2 .
Effects of the treatment on 42-d performance of crossbred steers and net energy value of the receiving diet.
a Initial and final BW reduced 4% to account for fill.b Linear effect, P < 0.05.c Linear effect <0.001.

Table 3 .
Characteristics of ruminal and total tract digestion and observed vs expected metabolizable amino acid supply of treatment 1 (basal diet,