INTRODUCTION
Commonly, in the Southwestern of United States and in Northern of Mexico, the Holstein and other breeds calves are fed during growing and finishing with corn-based diets, either rolled or steam flaked. These diets contain about 12% to 13% of crude protein, and adding urea as non-protein nitrogen source (NNP) [
1,
2]. Theoretically, these diets satisfy the requirements of metabolizable amino acids during the total feeding (around 350 days) [
3], but these diets do not fulfill those requirements during the early stages of growth (first 112 to 140 days) [
4].
Deficiencies in essential amino acids in early stages of growing phase have shown negatively influence on average daily gain (ADG) and dry matter intake causing economic losses [
5,
6]. Research on the estimation of amino acids requirement and nitrogen retention in growing cattle have methionine as the primary limiting amino acid, while the second is lysine [
1,
5]. With few exceptions (ie. fishmeal), native protein supplements commonly fed to feedlot cattle (ie. oilseed meals, distillers grains) are not good sources of metabolizable methionine, and/or are deficient in lysine [
6]. Very little research has been reported that evaluates optimal levels of methionine supplementation as a single ruminally protected amino acid in practical feedlot diets that are expected to otherwise meet amino acid requirements. The objective of the present study was to evaluate the influence of rumen-protected methionine supplementation levels on early growth-performance of calf-fed Holstein steers fed a conventional steam-flaked corn-based growing diet containing urea as the only source of supplemental nitrogen.
RESULTS AND DISCUSSION
Treatment effects on growth performance and dietary net energy (NE) are shown in
Table 2. During the initial 56-d period, methionine supplementation did not affect ADG (p = 0.27), however, it increased gain efficiency (quadratic effect, p = 0.03) and estimated dietary NE (linear effect, p = 0.05). As expected, during the subsequent 56-d period, when all calves received the same basal diet (no amino acid supplementation), growth performance was not different (p>0.20) among experimental group. However, the effects of methionine supplementation during the initial 56-period carried over, so that following a 56-d withdrawal of supplementation, the overall 112-d effects on gain efficiency (quadratic effect, p = 0.05) and dietary NE (linear effect, p≤0.05) remained appreciable. Improved gain efficiency and dietary NE are the more consistent responses in growth performance with improved metabolizable amino acid nutrition [
1].
During this early feedlot growing phase, metabolizable methionine is expected to be the first limiting amino acid in calves fed a steam-flaked corn-based diets with urea as the sole source of supplemental N. Based on NRC [
3] the metabolizable methionine and lysine requirements of a steer with an average weight of 190 kg and an ADG of 1.24 kg are 9.6 and 30.6 g/d, respectively. These estimates correspond closely to requirements of 10.2 and 32.7 g/d, respectively, using empirically derived equations of Zinn and Shen [
4]. Based on observed DMI (5.31 kg/d), the corresponding expected metabolizable methionine and lysine supply [
3] to steers in this study are 8.2 and 27.0 g/d, respectively.
As has been demonstrated in previous studies [
1,
17], DMI by feedlot calves reflects and attempt to consume not only sufficient energy, but also sufficient amounts of essential amino acids to sustain their genetic potential for growth. Indeed, the low observed dietary NEm and NEg (87% and 84% of expected, respectively) is consistent with previous studies where the metabolizable amino acid supplies were inadequate. Zinn et al. [
1] observed that with a conventional steam-flaked corn-based growing-finishing diet (urea as sole source of supplemental N), supplementation of the diet (using fish meal) to provide estimated metabolizable amino acid requirements increased observed vs expected dietary NE from 0.87 to 0.97.
Treatment effects on plasma amino acid concentrations are shown in
Table 3. On d 56, methionine supplementation increased (linear effect, p<0.01) plasma methionine and arginine. As a matter of convention in plasma amino acid analysis, a breakpoint is expected where level of limiting amino acid intake exceeds requirements [
18,
19]. That is, the inflection point at which plasma concentration begins to increase (breakpoint) is taken to indicate where metabolizable supply begins to exceed requirements. However, sampling convention may play a role in this generalization, as blood collection in these studies was conducted prefeeding. In contrast, Campbell et al. [
20], with continual infusion of methionine into abomasum of steers fed a methionine deficient diet, observed that both plasma methionine and N retention increased linearly with methionine infusion. There was also a numerical, but not statistically significant (p = 0.22) increase in plasma lysine, possibly reflecting poor ruminal escape, as will be discussed later. There were no differences (p>0.20) in plasma amino acid concentrations at day 112, as expected, as all calves had received the same basal diet during the second feeding period (d 56 to d 112).
Treatment effects on characteristics of ruminal and total tract digestion are shown in
Table 4. Surprisingly, there was a quadratic effect (p = 0.03) of amino acid supplementation on ruminal OM digestion, and a tendency for a quadratic effect on total tract OM digestion (p = 0.10) and digestible energy (p = 0.12). The basis for this effect is not certain. All treatments used the same basal diet, with amino acid treatments incorporated by top-dressing at time of feeding. There were no treatment effects (p>0.20) on flow of dietary and microbial N to the small intestine. Ruminal degradation of feed N averaged 69%, in good agreement (109%) with expected [
3] based on diet formulation (63.3%). In contrast, flow of microbial N to the small intestine was less (20%) than expected based on NRC [
3]. Postruminal N digestion increased (linear effect, p = 0. 04) as dietary methionine level increased. Assuming an intestinal digestibility of 88% for methionine [
21], the estimated contribution of the digestible N by the supplementary methionine can explain about 45% of the variation in postruminal N digestion.
Treatment effects on essential amino acid flow to the small intestine and plasma amino acid profiles are shown in
Table 5 and
6. Methionine supplementation increased (linear effect, p<0.01) duodenal flow of methionine. Likewise, lysine supplementation appreciably increased (4.6%, p = 0.04) duodenal flow of lysine, although the magnitude of the change was much smaller than anticipated. With respect to the basal non-supplemented diet, observed intestinal amino acid supply of methionine, histidine, phenylalanine, threonine, leucine, isoleucine, and valine were in good agreement with expected [
3] averaging 98%, 107%, 110%, 105%, 103%, 100%, and 90%, respectively. In contrast, lysine flow to the small intestine was 120% of expected. By difference, intestinal supply of supplemental methionine (Smartamine) and lysine (Aminosure) were 36% and 9% of intake, respectively. As expected, methionine supplementation increased plasma methionine concentration (linear effect, p = 0.01). However, consistent with the low ruminal escape of supplemental lysine, the small increase in plasma lysine with supplementation was not appreciable (p = 0.47).
Metabolizable amino acid supply along with theoretical requirement [
3] based on average growth rate of steers in Trial 1 are shown in
Table 7. Metabolizable amino acid supplies were estimated according to Zinn and Owens [22]. Consistent with previous studies involving calf-fed Holstein steers [
1], estimated metabolizable methionine supply in Trial 1 was closely associated (R
2 = 0.95) with the efficiency of utilization of metabolizable energy for maintenance and gain (observed vs expected dietary NE). During the subsequent 56-d period estimated metabolizable amino acid supply was similar for individual amino acids across treatments. This was expected, as all steers received the same non-supplemented basal diet during that 56-d period.