The effects of dietary phosphorus inadequacy during pregnancy and lactation on the voluntary intake and digestibility of oat straw by beef cows and the performance of their calves

Abstract

SUMMARYTwo groups of nine in-calf beef cows were given daily for the last 16 weeks of pregnancy and the first 6 weeks of lactation 1·35 increasing to 3·15 kg molassed sugarbeet pulp dry matter containing added urea together with oat strawad libitum. Additionally, one group of cows received supplementary dicalcium phosphate which increased their total phosphorus intake from about 12 to 28 g P/day.In the absence of phosphorus supplementation, there was a marked and significant reduction in the concentration of phosphorus in the blood, and by the 4th week of lactation this was reflected in a decrease in radiographic density of the tail bones but not in a depletion of rib bone ash.During pregnancy a severely reduced phosphorus intake did not reduce either calf birth weight or the digestibility and voluntary intake of straw. An inadequate phosphorus intake during lactation resulted in a significant decline in voluntary straw consumption and digestibility. There was an accompanying decrease in metabolizable energy intake from about 78 to 55 MJ/day, an increased weight loss in the cows and a depression in their milk yield such as to significantly reduce calf live-weight gain.The long period of phosphorus inadequacy did not affect the subsequent satisfactory reproductive performance of the cows when both groups were transferred to grass and mated with a bull.