Relationship between linear type traits and fertility in Nguni cows.
The broad objective of the study was to determine the relationship between linear type traits and fertility in Nguni cow herds. Data collected from 300 Nguni cattle owning households from two municipalities (150 each) were used to compare trait preferences of Nguni cattle owners located in semi-arid and sub-humid production environments. A total of 1017 records from 339 cows of Venda, Pedi, Swazi and Makhatini ecotypes were used to investigate sources of variation of linear traits in Nguni cows of different ecotypes. A total of 1559 Nguni cows kept under thornveld, succulent karoo, grassland and bushveld vegetation types were used to determine the relationship between six linear type traits (body stature, body length, heart girth, navel height, body depth and flank circumference) and fertility traits (calving interval and age at first calving) in Nguni cow herds under natural rangelands. Relationships between the linear type traits and incidences of still births and abortions in Nguni cow herds were determined using 250 Nguni cows from two sites experiencing sub-humid and semi-arid environments (125 cows each). Cows with at least Parity 3 were used in the study. Nguni cattle owners located in sub-humid areas mostly preferred fertility traits (calving interval and age at first calving) whilst those from semi-arid regions preferred traits reflective of adaptation to harsh conditions. In sub-humid areas, calving interval (CI) and age at first calving (AFC) were ranked first and second, respectively. Although lowly ranked, linear traits were iv considered by communal farmers in selecting Nguni cows for breeding stock. Cow fertility problems were mainly experienced in semi-arid areas compared to sub-humid areas. Semi-arid areas had more households (32.7 %) with cows with extended CI (2 and 3 years) than sub-humid areas (19.1 %). Body depth, flank circumference and heart girth were influenced (P < 0.05) by parity of cow, season of measurement and body condition score (BCS). Body depth, flank circumference and heart girth increased with increase in parity of cow. Cows in Parity 7 had the deepest bodies and navels hanging closest to the ground. Venda cows had the same flank circumference and heart girth across all seasons (P > 0.05). Body stature, body length, heart girth, navel height, body depth and flank circumference varied with ecotype of cow (P < 0.05). Venda cows had significantly higher body depths. Cows with deeper bodies had navels near the ground (r = -0.32) and longer bodies (r = 0.46; P < 0.05). Cows raised on the succulent karoo rangelands had shortest calving interval, calved earliest, deepest bodies, widest chests and flanks. Linear type traits under study can be grouped into two distinct factors, one linked to body capacity (body depth, flank circumference and heart girth) and the other to the frame size of the cows (body stature, body length and navel height). Calving interval and age at first calving decreased linearly with increase of body capacity (P < 0.05). There was a quadratic increase in age at first calving as frame size of cows increased (P < 0.05). As the body depth increased the likelihood of the incidence of still births and abortions in cows decreased (odds ratios 1.15 and 1.15, respectively). It was concluded that small-framed cows with large body capacities had short calving intervals, calved early and were less likely to abort or experience still births.