Milk Yield, Nitrogen Use Efficiency, and Rumen Fermentation in Barki Ewes, and the Performance of Their Offspring in Response to Increasing Dietary Protein Levels.

This study investigated the effects of increasing dietary crude protein (CP) levels on milk yield, nutrient utilization, nitrogen use efficiency, rumen fermentation, and lamb growth in lactating Barki ewes. Twenty-one multiparous ewes (mean body weight (BW) of 40.5 ± 0.41 kg), each nursing one lamb (mean BW: 6.22 ± 0.16 kg), were randomly assigned to three total mixed rations containing 13.0%, 15.1%, and 17.1% CP on a dry matter (DM) basis, respectively, in a randomized complete block design over a 70-day lactation period. Results indicated a linear increase (p < 0.05) in milk yield, alongside higher milk fat, protein, and lactose yields, as dietary CP levels rose. Crude protein intake increased linearly (p = 0.001) as dietary CP levels elevated. Apparent digestibility of organic matter, CP, neutral detergent fiber (NDF), and acid detergent fiber (ADF) also increased linearly (p < 0.05). Diets with 15.1% and 17.1% CP demonstrated higher (p < 0.05) CP, NDF, and ADF digestibility compared to the 13.0% CP diet. Nitrogen (N) intake, milk N, urinary N, urea-N, fecal N, and plasma urea concentrations increased linearly (p < 0.05) with increasing dietary CP levels. Ewes receiving 17.1% CP excreted higher (p < 0.05) amounts of urinary N, urea-N, and fecal N. Ruminal ammonia-N concentrations increased linearly (p = 0.001) with dietary CP levels, peaking (p = 0.001) in ewes on the 17.1% CP diet, followed by those on 15.1% and 13.0% CP diets. Molar proportions of isobutyrate and isovalerate increased linearly (p < 0.05) with higher dietary CP. Microbial protein content increased linearly (p = 0.001), attaining higher values with 15.1% and 17.1% CP diets (p = 0.010) compared to 13.0% CP. Lambs final BW and average daily gain (ADG) improved linearly (p = 0.001) as dietary CP levels increased, with 15.1% and 17.1% CP diets yielding superior outcomes over 13.0% CP diet. In conclusion, increasing dietary CP from 13.0% to 17.1% linearly improved digestibility, milk yield, and lamb growth. The 15.1% and 17.1% CP diets enhanced ruminal fermentation, but 17.1% CP maximized nitrogen excretion. Thus, 15.1% CP is recommended to optimize productivity and nutrient utilization in semi-intensive Barki ewe production systems.