Sulfur amino acid utilization by growing steers

Two studies were conducted to evaluate sulfur amino acid requirements of growing steers. In trial 1, six ruminally cannulated steers (352 lb) were used to determine methionine requirements . Treatments were abomasal infusions of 0, 2, 4, 6, 8, or 10 g/day of L-methionine. Steers were fed 5.8 lb of a soyhull and wheat straw based diet. Continuous infusion s of acetate, propionate, and butyrate into the rumen and of dextrose into the abomasum were made to increase energy. Amino acids other than methionine were infused into the abomasum to ensure that they did not limit steer performance. Nitrogen retention increased dramatically as methionine supplementation increased and, in the presence of excess cysteine, predicted a requirement of 4 g/day of supplemental L-methionine. Plasma methionine rose with methionine supplementation and predicted a supplemental methionine requirement of 2 g/day. In trial 2, five ruminally cannulated steers (429 lb) were used to determine how efficiently methionine is converted to cysteine in growing cattle. The experimental procedures were similar to those of trial 1, except steers received a basal supplement of amino acids containing 4 g/day L-methionine (requirement in the presence of excess cysteine established in trial 1) and no cysteine. Treatments consisted of abomasal infusions of 0, 1.62, or 3.25 g/day of L-cysteine or 2 or 4 g/day of additional L-methionine. Nitrogen retention was increased by methionine, but not by cysteine, suggesting that cysteine could supply less than one-half of the total sulfur amino acid requirement (methionine + cysteine) of growing steers.


Summary
Two studies were conducted to evaluate sulfur amino acid requirements of growing steers.In trial 1, six ruminally cannulated steers (352 lb) were used to determine methionine requirements .Treatments were abomasal infusi ons of 0, 2, 4, 6, 8, or 10 g/day of Lmethion ine.Steers were fed 5.8 lb of a soyhull and wheat straw based diet.Continuous infusion s of acetate, propionate, and butyrate into the rumen and of dextrose into the abomasum were made to increase energy.Amino acids other than methionine were infused into the abomasum to ensure that they did not limit steer performance.Nitrogen retention increased dramatically as methionine supplementation increased and, in the presence of excess cystei ne, predicted a requirement of 4 g/day of supplemental L-methionine.Plasma methionin e rose with methionine supplementatio n and predicted a supplemental methionine requirement of 2 g/day.In trial 2, five ruminally cannulated steers (429 lb) were used to det ermine how efficiently methionine is converte d to cysteine in growing cattle.The experimenta l procedures were similar to those of trial 1, except steers received a basal supplemen t of amino acids containing 4 g/day L-methionin e (requirement in the presence of excess cysteine established in trial 1) and no cysteine.Treatments consisted of abomasal infusions of 0, 1.62, or 3.25 g/day of L-cysteine or 2 or 4 g/day of additional L-methionine.Nitrogen retention was increa s ed by methionine, but not by cysteine, suggesting that cysteine could supply less than one-half of the total sulfur amino acid requirement (methionine + cysteine) of growing steers.

Introduction
Methionine currently is t hought to be potentially limiting for growth in cattle, but much previous research has relied on plasma amino acid levels as the sole response criteria.That research needs verification, because several factors other than amino acid status can affect plasma amino acid levels.Nitrogen retention provide s a more precise estimate of the methionin e requirement of growing cattle and also allows extrapolation to given levels of performance.
Cystein e is considered nutritionally nonessential , because the cysteine requirement of most specie s can be supplied by methionine via transsulfuratio n (methionine conversion to cysteine).It i s important to know the efficiency of transsulfuration in growing cattle, so that we can better predict total sulfur amino acid (methionine plus cysteine) requirements.For most species, the cysteine requirement is about one-half of the total sulfur amino acid requirement and transsulfuration is assumed to be efficient, but little information exists for cattle.Our objective was to quantitate sulfur amino acid requirements of growing steers.

Experimental Procedures
Trial 1. Six ruminally cannulated Holstein steers (352 lb) were utilized in a 6 × 6 Latin Square design, with treatments being graded levels of L-methionine (0, 2, 4, 6, 8, and 10 g/day) .Each period was 6 days long, with 2 days used for treatment adaptation and 4 days for total collection of urine and feces.Blood samples were collected on the last day of each period .Steers were fed 5.8 lb of a diet containin g 83% soyhulls, 8% wheat straw, and 1% urea.Volatile fatty acids (225 g acetate, 225 g propionate, and 56 g butyate/day) were infused continuously into the rumens and dextrose (150 g/day) was infused into the abomasum s of steers to ensure that energy would not limit their ability to deposit protein.
Trial 2. Fiv e ruminally cannulated Holstein steers (429 lb) were used in a 5 × 5 Latin Square design.Conditions for this trial were similar to those used in trial 1, except the amount of su pplemental L-methionine found to maximiz e nitrogen retention i n the presence of excess cysteine in tr i al 1 (4 g/day) was provided to all steers in the basal amino acid infusions.Also, cysteine was delete d from the basal amino acid infusions.Treatments were abomasal infusions of 0, 1.62, or 3.25 g/day of L-cysteine or 2 or 4 g/day of additional L-methionine.Cysteine was supplied in equimolar amounts to methionine.Diets were similar to those in trial 1.
Rumina l infusions of 180 g acetate, 180 g propionate, and 45 g butyrate (per day) and abomasa l infusions of dextrose (300 g/day) were provided.Nonsulfur amino acids were abomasally infused in amounts similar to those used in trial 1.

Results and Discussion
Trial 1.All increases in nitrogen retention resulte d from decreased urinary nitrogen excretion .Nitrogen retention increased the most when steers were infused with up to 4 g/day of L-methionine, with some additional respons e as methionine supplementation increase d to 10 g/day (Table 1) .We conclude that the supplemental methionine requirement of these steers is 4 g/day in the presence of excess cysteine.Pl asma methionine levels were lowest when steers were infused with 0 or 2 g/day of methionine, and increased linearly when higher levels were infused, which predicted a supplemental requirement of 2 g/day, a level below that predicted by the nitrogen retention data.
Trial 2. Nitrogen retention for growing steers increased in response to methionine infusion and was maximized at a total methionin e level of 6 g/day (4 g basal plus 2 g supplemental; Table 2).Nitrogen retention was not affected by cystein e supplementation.The lack of response to cysteine supplementation sugge sts that cysteine did not spare any of the steers' requirement for methionine.Plasma methionin e levels increased with methionine supplementation , but not with cysteine supplementation .
If cysteine did spare methionine , we might expect an increase in plasma methionine levels as cysteine supplementatio n increased.Sulfur amino acid supplementatio n as methionine supported protein deposition better than did cysteine supplementation .The nonprotein functions of methionine (e.g., methyl group donor) may be quantitatively important enough to result in the higher requirement for methionine relative to cysteine .However, this conclusion may be limited to steers maintained under our experimental protocol.

Table 1 . Effects of Supplementing L-Methionine to Growing Holstein Steers
a

Table 2 . Effects of Supplementing L-Methionine or L-Cysteine to Growing Steers
Linear effect of methionine (P=.07),Quadratic effect of methionine (P=.11).