This proof-of-concept study was conducted on HAW Farms (Belen, NM) to test the hypothesis that l-arginine (Arg) can enhance milk production by lactating cows. A rumen-protected Arg product (RPA, which contained a by-pass matrix) and an unprotected Arg product (UPA) that had the same composition as RPA were manufactured by Biotechnology Services & Consulting Inc. (Coppell, TX). Before parturition, 54 healthy Holstein cows (parities 1 to 4), weighing 550–600 kg, were assigned randomly to receive either no dietary supplementation (non-handling control) or dietary supplementation of 500 g RPA, 500 g UPA or an isonitrogenous amount of L-alanine (Ala) per day beginning on d 1 to 4 after parturition (the initial day of supplementation = d 0 of the trial). Each cow was fed twice daily a typical silage- and alfalfa hay-based lactation diet containing 17% crude protein (25 kg DM/d) to meet the NRC-recommended requirements of nutrients, and had free access to drinking water. A supplement was administered to cows twice daily (equally divided doses at 8 a.m. and 6 p.m.) by gavaging immediately after consumption of their regular meals. On d 0 and 7 of the trial, milk yields of cows were determined using an auto-milking system. On either day, the composition of nutrients did not differ (P > 0.05) among the 4 groups of cows. Changes in milk yields between d 7 and 0 (analyzed by the paired t-test) were significant (P = 0.016) for the RPA group but not significant (P > 0.05; NS) for the other groups (Table 1). Interestingly, variation in milk yield change was much less for RPA-supplemented cows, compared with the other groups of cows. These results indicate that cows fed RPA had more consistent lactation performances and produced more milk. Table 1. Effects of dietary supplementation with RPA on milk production by dairy cows (mean ± SEM; *P = 0.016)

Treatment group	No. of cows	Milk yield (MY)		Change in MY (kg/cow/7d): d 7 − d 0	CV of MY change (%)
d 0	d 7
Non-handling control	14	27.7 ± 2.3	31.5 ± 3.4	3.77 ± 3.22NS	321
Isonitrogenous control (Ala)	13	26.3 ± 3.5	30.9 ± 3.0	4.58 ± 3.36NS	264
Unprotected Arg product	14	26.0 ± 3.3	31.3 ± 2.3	5.36 ± 3.59NS	259
Rumen-protected Arg product	13	26.2 ± 2.6	33.6 ± 2.1	7.35 ± 2.70*	132