Supplementation of grazing cattle

Beef production in many parts of the world depends almost exclusively on the extensive natural grazing found in the major beef producing countries. These areas are more often than not unsuitable for arable use because of topography, rainfall and poor soils.

Animal production in such environs is, however, seriously limited because of nutrient deficiencies at different times of the year. The role of nutritionists and farmers has been to identify these shortcomings and supplement accordingly, in order to exploit the animals production potential. It is necessary to supplement only those nutrients which are shown to limit performance of various classes of cattle during the different seasons.

The nutritive value of the grass in a given area varies considerably from season to season, as well as in a given season. Following the first rains in Spring the nutritive value of the grass increases dramatically which allows animals to gain weight for the next 4-5 months. However with the onset of the dry season and particularly after the first frost the nutritive value of the grazing drops off precipitously. This variation in nutritive value is less marked in the warm, drier and low lying (so called "sweet veld") areas, but is very pronounced in the higher lying (so called "sour veld") areas.

Since the loss of weight during the dry season was such an obvious cause of productive and reproductive failure a lot of research was directed toward it. However it was the extreme deficiency of phosphorus in South African soils and grasses that caused cattle to eat the bones of dead animals with its resultant incidence of botulism that first stimulated interest in supplementary feeding in South Africa. The first studies by Thieler et al., 1924 showed that bonemeal fed as a supplement throughout the year prevented botulism and also resulted in improved growth rates, milk production, weaning weights and calving percentages. What Theiler's work did not show was that phosphorus elicited no liveweight response when fed during the dry months of the year when the animals were normally losing weight. This was confirmed by various studies, (Bisschop & du Toit, 1929; Murray et al., 1936; Murray & Romyn, 1937; and many others).

Thus in spite of the poor status of phosphorus in dry grassveld, it is not the primary limiting nutrient in Winter grazing. It is essential that under conditions of maintenance sufficient phosphorus must reach the rumen so that microbial digestion is not impaired by shortages! Hence the need for phosphorus supplementation in Summer.

Table 1. Average weight change (kg) and total weight gain (kg) of cattle fed phosphorus (P) supplement during different times of the year

Period	Season	Negative control	"Winter" P	"Summer" P	"Winter" & "Summer" P
Jan - Mar	"Summer"	+35.2	+41.6	+61.6	+57.1
Apr - June	"Winter"	+5.3	+18.1	+14.1	+6.8
July - Sep	"Winter"	-29.8	-30.3	-36.3	-30.6
Oct - Nov	"Summer"	+45.7	+61.6	+77.5	+88.1
Initial weight (kg)		314.9	307.7	305.4	311.8
Total weight gain		+56.3	+91.8	+116.8	+121.4

Source: van Schalkwyk & Lombard (1969)

Work published by De Brouwer (1990) giving O, 4 and 8g Phosphorus per animal/day gave the following results:-

Cows achievements with differing Phosphorus levels during the Summer
Phosphoprus intake /cow/day	0	4	8
Weight gain	40	59	61
Condition score (1-5) Start End	2.6 2.7	2.7 2.9	2.8 3.0
P content of bone (mg/cm³)
Start End	139 113	138 131	156 142

These results should however not be taken in isolation. They should be correlated with the other nutrients , namely protein and energy.

The following table represents the results obtained in an experiment over a 5 year period. This trial by Mr Chris de Brouwer over both Summer and Winter shows once again the importance of correct supplementation.

Phosphorus supplementation (De Brouwer 1990)
Winter supplementation (g/animal/day)	Summer Supplementation (g/animal/day)	Final weight	Final Condition score (1-5)	Final Bone P content (mg/cm³)
10 10 10	8 4 0	485 483 467	2.25 2.38 1.88	175.9 140.6 126.5
5 5 5	8 4 0	502 482 486	2.44 1.94 1.94	164.3 141.1 86.3
0 0 0	8 4