Nutritional quality of mesquite (Prosopis glandulosa) and potential toxicosis in sheep

Mesquite leaves are emergency forage during dry seasons but they have low palatability. An in vivo digestion trial was completed with lambs (n = 15) assigned to diets of 0, 5, 10, 15, or 20% mesquite leaves mixed with alfalfa hay to measure effects of mesquite on digestion parameters. Proportion of mesquite leaves in the diet negatively affected dry matter (DM) intake, nitrogen (N) balance, gross energy (GE) intake, retained N, retained GE, and weight gain at levels > 5% of the diet (P < 0.01). Mesquite intake was the highest at the 5% level (1.81 g/kg BW; P < 0.01) and averaged 0.78 g/kg BW for the remaining diets. Coefficient of apparent digestibility (COD) was not affected by level of mesquite in the diet (P = 0.58). An in situ Dacron bag trial revealed that pure alfalfa hay was more digestible than mesquite leaves (P = 0.01). However, %N, acid and neutral detergent fiber (ADF and NDF) did not differ between mesquite and alfalfa. Low levels of enzymes bilirubin, aspartate aminotransferase, and gammaglutamyl transferase suggested no liver damage (P > 0.05). Allelochemicals in mesquite were presumably strong intake inhibitors.

A conditioned flavor aversion (CFA) trial tested the effect of postingestive feedback from mesquite on the intake of a novel feed (rye). On day 1, lambs were offered rye and then ground mesquite was infused into their rumens by tube. Lambs (n=21) were assigned to dosing treatments 0 (control), 3.0 (low), and 4.5 (high) g/kg BW of mesquite leaves. On day 3, lambs dosed with mesquite ate less than controls (P < 0.01) showing a strong CFA. Aversion to rye persisted for at least 2 days (P < 0.01). The high dose of mesquite decreased the intake of alfalfa ration for at least 3 days (P <0.01). Persistent diahrrea in lambs receiving the high mesquite dose could be a result of toxins in mesquite.

Examination of nutrients in mesquite leaves collected from May to November 1995, showed an increasing trend for DM from 42% (May) to 58% (November). Content of N decreased with season (P < 0.01), from a maximum of 2.73% in May to 1.58% in November. Analysis of ADF and NDF indicated similar effect of months (P <0.01). Minimum content of fiber was found in May, (23.7% ADF; 32.0% NDF) and maximum levels were reached in June (32.9% ADF; 43.1% NDF). Values of fiber decreased again in November (29% ADF; 38.8% NDF). In vitro digestibility was similar for May and June (79%), and decreased significantly (P <0.01) for the remaining months, from 74% in July to 69% in November.