**Background:** In Mediterranean dairy sheep systems, ewe lambs are often bred at over 1 year of age despite being capable of reaching puberty at 60–65% of mature live weight. Traditional management joins ewe lambs with rams in a single flock without direct reproductive control, resulting in poor pregnancy synchronization, prolonged lambing seasons, and reduced milk production. While the ram effect can advance puberty by approximately 2 weeks, responses are variable. Hormonal protocols using intravaginal progestagen-releasing devices have been used but show lower efficacy in ewe lambs compared to adult ewes. This study investigated whether a protocol combining repeated ovarian ultrasound scanning with GnRH administration could advance pregnancy onset, narrow the lambing season, and improve economic outcomes in ewe lambs of varying body weights entering their first reproductive season.
**Methods:** The experiment was conducted from mid-July to early October 2019 at the Bonassai research station of Agris Sardegna (40° N, 8° E). A total of 133 prepubertal Sarda ewe lambs born in the previous autumn-winter were selected from a single flock. Ovarian status was confirmed as non-cyclic via two ultrasound scans 8 days apart. On Day 1, lambs were weighed and divided into three weight groups: high weight (HW, 39.1±0.3 kg, n=35), medium weight (MW, 33.8±0.2 kg, n=65), and low weight (LW, 28.2±0.3 kg, n=33). Within each weight group, animals were randomly allocated to either GnRH treatment or control (CTR, ram exposure only). CTR groups were joined with fertile rams (1:10 ratio) as a single flock. GnRH groups were kept separate from rams and received a single dose of gonadorelin (40 μg/head, 0.8 mL). After one week (Day 8), GnRH lambs underwent ultrasound scanning; those with corpora lutea received PGF2α analog (cloprostenol 100 μg/head) and were joined with rams. Non-responding lambs received a second GnRH dose. After another week (Day 15), scanning was repeated: those with corpora lutea received PGF2α, while remaining lambs received a third GnRH dose. All GnRH lambs were then joined with rams. Mating behavior was monitored daily for 120 days, and pregnancy was confirmed within 30 days by transrectal ultrasound. Pregnancy length was assumed at 150 days. Milk production was recorded via automatic milking machine records until dry-off on August 15. Economic evaluation included costs from birth to first dry-off across five phases: suckling/weaning (phase I, fixed at 48.80 €/lamb), growing (phase II, 46.5 € for HW, 40.71 € for MW, 27.98 € for LW), open phase (phase III, feed costs of 0.26–0.31 €/day depending on weight), pregnancy (phase IV, 0.34–0.38 €/day), and lactation (phase V, with individual DMI estimation). Drug costs were 0.72 € per GnRH dose and 0.37 € per PGF2α dose; ultrasound cost was 1.00 € per scan. Milk price was set at 1.30 €/liter. Gross margin was calculated as milk income minus feed and reproduction costs. Statistical analysis used a general linear model with fixed effects of weight class, treatment, and their interaction, with Tukey test for comparisons (significance at p<0.05).
**Key Results:** At the 25% pregnancy threshold, the GnRH-MW group required only 14.0 days compared to 26.0 days for CTR-MW (p<0.01 for interaction), representing a 46% reduction. The GnRH-LW group also showed advancement (15.5 vs. 23.6 days). However, at the 50% threshold, only weight class was significant (p=0.01), with LW groups requiring 34.88 days vs. 23.22 for HW and 24.27 for MW. At the 75% threshold, weight class remained significant (p<0.01): LW required 49.88 days vs. 31.19 for HW and 34.78 for MW. No significant treatment effect was observed at 50% or 75% thresholds. In HW groups, GnRH treatment did not advance pregnancy compared to CTR. Milk yield was significantly higher in HW (270 kg/year) and MW (258 kg/year) vs. LW (212 kg/year) (p=0.001). GnRH-treated groups tended to have higher milk yield than CTR (260 vs. 240 kg/year, p=0.10). Lactation length was longer in HW (229 days) and MW (230 days) vs. LW (211 days) (p=0.006). Protocol costs were higher for GnRH groups (4.91 €/ewe) vs. CTR (2.00 €/ewe) (p=0.001). Gross margin from birth to first dry-off was numerically higher in GnRH-MW (55.0 €/ewe) compared to CTR-MW (28.4 €/ewe) and other groups, though not statistically significant (p=0.30 for treatment). When adjusted for empty ewes, GnRH-MW showed a positive net income of 2.95 €/milking ewe, while all other groups showed negative values. Empty ewe rates were 0% in CTR-HW and CTR-LW, 9.7% in CTR-MW, 5.6% in GnRH-HW, 3.0% in GnRH-MW, and 6.7% in GnRH-LW.
**Clinical Implications:** The echo-synch protocol combining ultrasound scanning with GnRH administration is technically and economically effective for advancing pregnancy in ewe lambs that have not reached optimal weight at first breeding season, particularly those in the medium weight range. The protocol's value lies in its targeted approach: ultrasound allows identification of animals that respond to GnRH (those developing corpora lutea) and those that do not, enabling timely management decisions. For high-weight lambs that have already reached optimal weight, ram exposure alone is sufficient and more economical. Low-weight lambs require nutritional management to reach adequate weight before reproductive protocols can be effective, as they showed poor responses regardless of treatment. The study confirms that live weight measurement is critical for reproductive management decisions in ewe lambs. The economic analysis demonstrates that while GnRH treatment adds costs (approximately 2.91 €/ewe more than control), the return from earlier pregnancy onset, longer lactation, and higher milk yield can offset these costs in appropriately selected animals. The findings support a stratified management approach where ewe lambs are grouped by weight, with hormonal treatment reserved for those near but not at optimal weight, while underweight lambs receive nutritional intervention before breeding.