Water Dynamics of Dairy Cattle as Affected by Initiation of Lactation and Feed Intake

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Abstract
Effects of lactation and feed intake on sizes and half-lives of body water pools were investigated in 4 Holstein cows in a thermoneutral environment. Various markers were administered, on average, 24 days prepartum (period 1, feed intake fixed to an average of consumption after offered at a rate ensuring 10% refusal), 24 days postpartum (period 2, restricted to intake in period 1) and 42 days postpartum (period 3, feed again offered at a rate ensuring 10% refusal). Total body water decreased from period 1-2; it did not change as a percent of body weight. Estimated body water half-lives and fluxes for the 3 periods were 7.5, 3.7 and 2.9 days; and 42, 66 and 87 l/day, respectively. Increased water intake during lactation closely matched water secreted in milk. Empty body water, estimated by a 2-compartment, 4-parameter model, as a percent of body weight did not change across periods. Lactation and increased feed intake decreased half-life of body water. Plasma volume increased as a percent body weight because of loss of body weight in early lactation; extracellular volume tended to decrease with body weight. Initiation of lactation and feed intake during lactation affected water dynamics of the dairy cow.