Response of lactating goats to low phosphorus intake 1. Milk yield and faecal excretion of P and Ca

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Abstract
In weeks 1–6 of lactation, 11 goats were fed diets adequate in phosphorus content, supplying a basal intake of 2·8 g P/day plus 1·4 g P/kg milk produced. In group 1 (three goats), this scheme was continued until week 16. In group 2 (four goats), P supply was changed from an adequate one to a reduced amount by restricting the variable intake to 1·0 g P/kg milk during weeks 7–16 and the basal intake to 0·8 g P/day during weeks 12–16. In group 3 (four goats), P deficiency was caused during weeks 7–11 by a basal intake of 0·8 g P/day plus only 0·4 g P/kg milk. During weeks 12–16 these goats received the same supply of P provided in weeks 1–6.Milk yield was depressed in group 3, but not in group 2. Composition of milk was not affected by P intake in either group. Digestibility of organic matter was reduced from 0·73 to 0·64 by P deficiency treatment. Reduction of faecal P excretion in group 2 appeared to enable P homoeostasis even at the reduced level of P intake. The amounts of P and Ca secreted in milk during P deficiency in group 3 clearly exceeded the corresponding net absorptions. During this period, daily faecal P excretion was reduced to 17 mg/kg live weight. P concentrations in blood plasma, saliva and particle-free rumen fluid were also drastically reduced. Faecal excretion of P increased after the animals returned to the diet supplying adequate P. The increase occurred before the pre-depletion P status had been restored.
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