Stocking rate, calving season and post-weaning growth effects for Angus cattle on annual pasture

Abstract

The effects of stocking rate, calving season and post-weaning growth were investigated over 3 years for Angus cattle on annual pastures. Angus cows, mated to Angus bulls for 8 weeks to start calving early summer (ES), early autumn (EA) or early winter (EW), were stocked at 0.6, 0.8 and 1.0 cows/ha and calves/ha until their fourth calving ended. Weaning, at average age 10.5 months, was at the end of October, January or April. Some calves were slaughtered at weaning and others after a year at 1.4 calves/ha. Late autumn-winter pasture growth [2500-3500 kg dry matter (DM)/ha] was below average in 2 of the 3 years of the study, and spring growth did not vary (overall mean 4400 kg DM/ha). Unseasonable pasture growth during late summer-early autumn occurred between weaning in EA and EW for years 2 and 3. Milk consumption was an overall average of 4.8 kg/ca1f.day. The only variation was a higher value in EW in year 2 than in years 1 and 3. Winter calf growth in ES and EA was sometimes affected by year and stocking rate, and overall average spring growth was 1.23, 1.33 and 1.13 kg/calf.day in ES, EA and EW, respectively. Weaning weight occasionally differed with stocking rate in ES and EA, and tended to differ between years in EW in proportion to the level of unseasonable pasture growth. Overall, carcasses tended to be slightly heavier and fatter in ES but otherwise followed the weaning weight patterns. Minimum cow weight tended to vary with stocking rate in years 2 and 3 and overall increased from ES to EW. Maximum weight also differed in these years. Yearly treatment means for average daily weight change during mating were -1.47 to +1.45 kg/cow. Weight gain declined from ES to EW except for reversal of the ES and EA situations in year 3 when poor winter pasture conditions followed unseasonable pasture growth. Average mating weight was greater with the lightest stocking rate in year 3 and with EW in years 1 and 2. Condition score generally followed weight patterns. In the 3 years, calf birth weight tended to be higher in EW, except for increased birth weight in EA in a year of unseasonable pasture growth. Average calving percentage and interval between successive calvings were 87% and 365 days, respectively, with no clear treatment trends. Overall average gross margins/ha ($A) for the lightest to heaviest stocking rates, respectively, were 126, 141 and 142 in ES, 93,109 and 118 in EA, and 54, 70 and 84 in EW. In general, ES enabled mainly heavier carcasses to be sold at seasonally favourable prices but resulted in some calf scours. Relative to ES and EA, EW calving tended to result in less cow stress in most years but heavier birth weights, more dystocia and lighter weaning weights if prolonged unseasonable pasture growth did not occur. Calf response to pasture conditions was generally proportional to calf age, and spring growth was probably affected by the level of milk consumption and calf growth potential relative to age. Post-weaning calf growth did not differ consistently with calving season. The relationship between animal performance and pasture condition may be useful for assessing different calving seasons in other environments.