Two experiments were conducted to determine the available P requirements of grower and grower/finisher pigs and to define the conditions for conducting a growth assay for P availability. In the first experiment, diets with four levels of calculated available P (1–4 g/kg) and four Ca: available P ratios (1·7–2·9) were used to determine the available P requirements of grower pigs. The diets were formulated by substituting the required amounts of limestone and sodium tripolyphosphate for sugar in a soya-bean meal and sugar-based diet. In addition to measuring growth responses, a range of bones were examined to determine the most suitable criteria for assessing the response to available P. There was a small quadratic response of feed intake and growth rate of the pigs to level of available P, with maximum responses occurring to approximately 3 g available P/kg (P < 0·05). There were linear depressing effects of increasing Ca:available P ratios on carcass gain and feed conversion ratio (P < 0·01) but most of these effects occurred when the ratio exceeded 2·5:1. All bone variables examined increased linearly (P < 0·05) or curvilinearly (P < 0·01) with increasing available P concentration. In general, these variables were not affected by the Ca: available P ratio. The results of the growth responses and bone development indicate that the grower pig requires approximately 3 g available P/kg. However, for availability assays, where linearity of response is needed, the dietary concentration of available P should be a maximum of approximately 2 g/kg. In the second experiment four levels of calculated available P (1–4 g/kg) with a Ca: available P ratio of 2·5:1 were used to determine the available P requirements of grower/finisher pigs from 20 to 90 kg live weight. At 50 kg live weight the dietary available P concentration for half the pigs fed at 2, 3 and 4 g available P/kg was reduced to 1, 2 and 3 g/kg respectively. The pigs were fed ad lib. and growth performance, bone characteristics, P retention and ash concentration in the empty body were taken as response criteria to assess P adequacy. Among the variables tested, the ash concentration in the radius/ulna bone and P and ash concentrations in the empty body appeared to be more responsive than other variables to the changes in dietary P levels. Based on these variables, the P requirements for growth and bone development of growing pigs from 20 to 50 kg live weight was 3 g/kg and reduced to 2 g/kg for finisher pigs from 50 to 90 kg live weight.

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Phosphorus studies in pigs. 1. Available phosphorus requirements of grower/finisher pigs.

Effect of cultivar and environment on physicochemical and cooking characteristics of field pea (Pisum sativum)

Experiments were conducted with pigs and rats to determine the availability of P in feeds. Initially, the availability of P in a soya-bean meal and field peas (Pisum sativum cultivar Early Dun) was assessed using a slope–ratio assay for grower pigs. Three different levels of either monosodium phosphate (MSP), soya-bean meal or field peas were added to a basal sucrose–soya-bean meal diet (2·5 g/kg P) to give three levels of P (3·0, 3·5 and 4 g/kg) for each source. The diets were offered for 35 d at three times maintenance energy requirements to female pigs initially weighing 20 kg live weight. Several bone variables and the ash and P concentration and retention levels in the empty body were used as criteria of availability. The responses to MSP were linear for all variables. However, responses to P in the test proteins, particularly soya-bean meal, were mostly non-linear, except for ash and P concentrations and retentions in the empty body. The estimates of P availability in the soya-bean meal and in the field peas were dependent on the criteria used to assess availability. Using bone variables as the criterion, the availabilities of P in soya-bean meal and field peas were approximately 0·17 and 0·38 respectively. Using ash or P concentrations or retentions in the empty body as the response criterion, the availabilities of P in soya-bean meal and field peas were 0·61 and 0·38 respectively. The mean retention values for P from MSP, soya-bean meal and field peas were 0·74, 0·33 and 0·21 respectively. Experiments were then conducted to define the conditions necessary for a slope–ratio assay for P availability with growing rats to determine if they could be used as a model to assess P availability for pigs. These experiments examined level of P in the diet, the effect of Ca:P ratio and criteria for assessing response (growth rate, feed intake, feed conversion ratio, bone ash and bone bending moment). The results indicated that a suitable range of dietary P for a slope–ratio assay in rats was 1·8–3·5 g/kg, with Ca: P ratio of between 1·3 and 6·2, and that ash content or bone bending moment of the femur bone were suitable criteria of response. However, using bone bending moment as the criterion of response, the availability of P in soya- bean meal was 0·81, which was considerably higher than the estimate with pigs. Overall the results indicated that the estimates of the availability of P in feeds for pigs were dependent on the criteria used to assess availability and that the value for rats led to an overestimate of the value for pigs.

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Phosphorus studies in pigs. 2. Assessing phosphorus availability for pigs and rats.

Variation in physico-chemical properties of field peas (Pisum sativum)

Whole-crop peas. I. Changes in botanical and chemical composition and rumen in vitro degradability during maturation

Ninety-one samples of field peas (Pisum sativum, cv. Early Dun) were taken from commercial crops grown in South Australia. Crude protein (N x 6 25) ranged from 2 1 .3 to 26.2% (92% dry matter basis) with mean 23.8 and standard deviation 0.8%. Seed mass was a poor indicator of protein level. Changes in the amino acid profile which accompanied changes in crude protein level appeared to be of only minor nutritional significance. The mean gross energy level was 16.8 ± 0.1 MJ/kg; an indirect estimate suggested a digestible energy level for pigs of around 14 MJ/kg. Deficiencies in calcium and phosphorus for the growing pig were confirmed and it is suggested that selenium deficiency could occur on grain and pea diets in which both ingredients had been grown on soils deficient in selenium.

RL Davies

Papers

Field peas (Pisum sativum) as a feed for growing and finishing pigs. 1. Nutrient levels in commercial crops