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• Glutamine is an amino acid used extensively by intestinal tissues for energy, protein synthesis, and nucleic acid synthesis. We postulated that lower supply of glutamine in soy proteins than in milk proteins might be responsible for poorer performance of calves fed milk replacers containing soy protein concentrate.
• Calves fed a milk replacer in which 60% of the milk protein was replaced by soy protein concentrate had lower average daily gains, lower gain:feed, and altered intestinal morphology.
• Supplementation of glutamine to the milk replacer containing soy protein concentrate did not improve growth or intestinal morphology.

INTRODUCTION

Glutamine is an amino acid found in proteins that serves important functions for intestinal tissues. Glutamine is oxidized extensively for fuel by the mucosal tissue of the small intestine. Furthermore, rapid development and turnover of the intestinal tissues result in large requirements of glutamine for synthesis of proteins and nucleic acids.

Use of lower-cost soy protein sources in milk replacers for young calves results in decreased growth rates and evidence of adverse effects on the intestinal tissues. The reasons for these effects are not entirely understood. Glutamine is less abundant in soy proteins than in milk proteins. Supplementation of glutamine to corn-soy diets for weaning pigs has been shown to lessen the growth slump at weaning, perhaps by improving intestinal morphology and function. We tested the hypothesis that glutamine supplementation would overcome the decreased average daily gain (ADG) and altered intestinal morphology caused by milk replacer containing soy protein concentrate (SPC).

MATERIALS AND METHODS

Holstein calves (9 male and 1 freemartin female per treatment) born at the University of Illinois dairy farm were assigned to one of three diets: 1) all-milk-protein milk replacer; 2) SPC milk replacer; or 3) SPC milk replacer plus 1% (dry basis) L-glutamine. The SPC replaced 60% of the milk protein. Glutamine was added at each feeding to the reconstituted milk replacer. All-milk and SPC replacers contained 20.9% and 19.0% CP, and 15.0% and 15.1% fat, respectively. Methionine content of milk replacers was equalized by the manufacturer by use of crystalline methionine. Milk replacers were reconstituted to 12.5% solids and were fed at 10% of body weight from day 3 to day 10 of age and at 12% of body weight (adjusted weekly) from day 10 through 4 weeks of age. No starter was fed but water was available at all times. Calves were housed in hutches and bedded with straw.

Calves were weighed and measured weekly. Five calves from each treatment were slaughtered at the end of week 4. The digestive tract was removed and segments from the duodenum, jejunum, and ileum were removed and prepared for histological evaluation. Villus height and crypt depth were determined by a pathologist. Statistical contrasts compared the all-milk control versus the replacers containing SPC, and the glutamine-supplemented SPC milk replacer versus the unsupplemented SPC milk replacer.

RESULTS

Initial body weights, days on trial, and dry matter intakes did not differ among groups. The ADG was greater (P < 0.05) for calves fed the all-milk control versus those fed SPC, and glutamine did not improve ADG (Table 1). Gain:feed was lower for SPC-fed calves than for those fed the all-milk control. Intake of CP was lower for the SPC-fed calves but exceeded National Research Council (2001) requirements for all groups and so did not explain the lower growth for calves fed SPC. Villus height and crypt depth did not differ among treatments for duodenum. In jejunum, villus height (713, 506, and 464 mm) and crypt depth (300, 209, and 229 mm) were greater (P < 0.05) for calves fed the all-milk control than for either SPC group. In ileum, villus height was greater (P < 0.05) for all milk than for either SPC group (532, 458, and 456 mm), whereas crypt depth was greater for calves fed glutamine-supplemented SPC than for those fed SPC alone (352, 301, and 383 mm). The plasma urea concentration at week 4 was greater (P < 0.01) for calves supplemented with glutamine than for those fed SPC alone (5.7, 4.0, and 6.6 mg/dl), indicating that glutamine was catabolized. Supplemental L-glutamine did not improve growth or intestinal morphology of calves fed milk replacer containing SPC.

Table 1. Growth of calves fed an all-milk control milk replacer, a milk replacer containing soy protein concentrate (SPC), or the SPC-containing milk replacer supplemented with L-glutamine.