DE = 163.5 wo*73 a9) where, DE is digestible energy required for maintenance (kcal/day), and Wis the body weight (kg). The additional DE requirement for milk production in the range from 20 to 35 kg (milk)/day at 5 percent butterfat is 1,850 kcal DE per kg milk, The additional DE requirement for growth ranges from about 8,600 to about 19,800 keal/kg gained, depending also on body weight. According to McKell (1975), the average digestibility of desert vegetation is about 36 percent compared to 52 percent for good alfalfa hay and up to 80 percent for some concentrates (Siegmund, 1967). The average energy content of most plant materials is about 4.5 keal/g dry weight (Golley, 1961}, and the digestible energy content of desert vegetation is about 1.6 kcal/g dry weight. Based on these considerations, the vegetation ingestion rate for a cow grazing desert vegetation can be estimated as follows: vy, ing = 163.5 W973 Keal/day 0.36 x 4.5 kcal/g where, V is the vegetation ingestion rate (g/day) for maintenance of a mature cow°’grazing desert vegetation. For cows which are gaining weight, producing milk, or pregnant, the energy requirement and thus the vegetation ingestion rate would be higher than estimated by Equation (20). (20) Based on the methods outlined above, we would estimate this cow's plutonium ingestion rate as follows: 8139 g(veg)/day x 0.1 x 5.5 x 1074 uCi/g(soll) 0.448 pci/day 250 g(soil)/day x 5.5 x 10° uci/g(soit) 0.138 pei/day I,+1, = 0,585 wCi/day. where I and I_ are the plutonium ingestion rates through vegetation and soil, respectively. ®In this calculation, 8,139 g/day is the vegetation ingestion rate, based on Equation (20) for a 409-kg cow; 0.1 is the assumed average vegetation/soil ratio; 5.5 x 10°" uCi/g is the average soil concentration of the Area 13 inner compound (Gilbert and Eberhardt, 1976); and 250 g/day is the assumed soil ingestion rate. The resulting estimate of the cow's total plutonium ingestion rate, 0.585 uCi/day, is comparable to the independent, siteSpecific estimates of Smith et al., 1976 (0.565 wCi/day), and of Gilbert et al., 1976 (0.620 uCi/day}. Considering the probable variability of the Méasurementé and parameters involved, the agreement of results is remarkably good. 646 " Kleiber (1961) shows that the basal metabolic rate of mammals (heat production by a fasting animal) £s proportional to the three-fourths power of body weight and that the feeding capacity (maximum energy intake) of domestic animals such as the cow is about 5 times the food intake required for basal metabolism, Data given in the Merk Veterinary Manual (Siegmund, 1967) for the digestible energy (DE) requirements for maintenance of mature cows are based on: Smith et al. (1976) estimate that a 409 kg cow which grazed for 177 days in the inner compound of Area 13 ingested a total of 100 yCi of 33°240py or 0.565 pCi/day. This estimate was based on plutonium concentrations in the rumen contents of fistulated steers allowed to graze in the same compound. Gilbert et al. (1976) made an independent estimate of 0.620 uCi/day based plutonium concentrations in Eurotta lanata (34 percent of cow's diet) and Atriplex canescens (64 percent of cow's diet) as reported for Area 13 by Romney ¢t al. (1975). The average wet weight of vegetation ingested by the fistulated steers was 30 kg/day and the average dry/wet ratio was about 0.2 (Gilbert, personal communication}. In other words, it is estimated that the cow's vegetation ingestion rate was about 6 kg/day. Based on Equation (20), 2 409 kg cow would have to ingest about 8 kg/day to meet its energy requirements for maintenance. Neither Smith e¢ al, (1976) nor Gilbert et al. (1976) include soil ingestion in their estimates of the plutonium ingestion rate for the Area 13 cow. Lin Plutonium Ingestion Rates " If one assumes that the parameter values given in Figure 2 are reasonably close to the true values, the only parameters remaining to be determined are the plutonium ingestion and inhalation rates and the rate at which plutonium enters the blood (r,)- Grazing cattle also ingest soil. In earlier papers (Martin et al., 1974; Martin and Bloom, 1976), we assumed that the soil ingestion rate might be as high as 2,000 g/day. Data recently reported by Smith (1977) indicate that this value is probably too high. The amounts of sediment (soil) recovered from the reticulum and rumen of three cows which had been grazing in Area 13 prior to sacrifice were 8.5, 57.3, and 278 g, respectively. As Smith points out: "These data suggest that the total amount of soil ingested is much less than 2 kg per day, and that a reasonable estimate would be between 0.25 and 0.5 kg." = welght of muscle and liver; and the effective half-life in milk are based on experimental results reported by Stanley et ai. (1974). The other values were assumed (Martin and Bloom, 1976) for purposes of estimation. Equation (18) ignores the retention of plutonium in the lungs of cattle and assumes that all inhaled plutonium is transferred to the gastrointestinal tract or the blood within 24 hours.