The effects of supplementary phosphorus on the voluntary consumption and digestibility of a low phosphorus straw-based diet given to beef cows during pregnancy and early lactation

SUMMARYTwo similar experiments were conducted in consecutive years using a total of 26 pregnant beef cows. The two groups of 13 cows were given 2 kg molassed sugar-beet pulp (SBP)/day for the last 16 weeks of pregnancy and 3 kg SBP/day for the first 6 weeks of lactation with oat straw ad libitum. One group was given 250 g of a fully soluble liquid supplement (LS) containing urea, phosphoric acid, calcium and sodium chloride, trace elements and vitamins poured on to the SBP. The other group received no supplementary phosphorus but was given the same amounts of supplementary nitrogen (as crystalline urea) and calcium (as calcium carbonate) together with the same amounts of sodium chloride, trace elements and vitamins as were present in the LS. The LS provided 3.7 g P/day. The overall mean phosphorus intakes of the two groups were about 5.5 g (unsupplemented) and 10.5 g P (LS)/day.During pregnancy, reduced phosphorus intakes did not affect either the voluntary intake or digestibility of the straw. There was, however, a reduction in the blood phosphorus concentration for the cows which did not receive LS.After calving, the voluntary straw intake, digestibility of straw organic matter, metabolizablo energy intakes and blood phosphorus concentrations of the cows which received no phosphorus supplement were severely reduced. Using the present data and that from an earlier, similar experiment, a highly significant relationship was established between blood phosphorus concentration and voluntary straw intake during the period 5.6 weeks after calving for those cows with a blood phosphorus concentration below 1.0 mmol P/l. This relationship was voluntary straw intake (kg D.M./day) = 1.55 + 5.01 × blood phosphorus concentration (mmol/1).These results, obtained with individually fed, housed cattle, tend to suggest that a total phosphorus intake of only about 10.12 g P/day (of which 3.7 g was in the form of phosphoric acid) was adequate to maintain normal blood phosphorus concentration and voluntary straw intake and digestibility by these beef cows over the last 16 weeks of pregnancy and the first 6 weeks of lactation. This should be contrasted with the results of a similar experiment conducted earlier which clearly indicated that a daily intake of about 12 g P/day derived solely from sugar-beet pulp and oat straw was markedly inadequate.